Showing papers in "Ecological Monographs in 2002"

The global biogeography of roots

Abstract: Studies in global plant biogeography have almost exclusively analyzed re- lationships of abiotic and biotic factors with the distribution and structure of vegetation aboveground. The goal of this study was to extend such analyses to the belowground structure of vegetation by determining the biotic and abiotic factors that influence vertical root distributions in the soil, including soil, climate, and plant properties. The analysis used a database of vertical root profiles from the literature with 475 profiles from 209 geographic locations. Since most profiles were not sampled to the maximum rooting depth, several techniques were used to estimate the amount of roots at greater depths, to a maximum of 3 m in some systems. The accuracy of extrapolations was tested using a subset of deeply ( .2 m) sampled or completely sampled profiles. Vertical root distributions for each profile were characterized by the interpolated 50% and 95% rooting depths (the depths above which 50% or 95% of all roots were located). General linear models incorporating plant life-form dominance, climate, and soil var- iables explained as much as 50% of the variance in rooting depths for various biomes and life-forms. Annual potential evapotranspiration (PET) and precipitation together accounted for the largest proportion of the variance (12-16% globally and 38% in some systems). Mean 95% rooting depths increased with decreasing latitude from 808 to 308 but showed no clear trend in the tropics. Annual PET, annual precipitation, and length of the warm season were all positively correlated with rooting depths. Rooting depths in tropical veg- etation were only weakly correlated with climatic variables but were strongly correlated with sampling depths, suggesting that even after extrapolation, sampling depths there were often insufficient to characterize root profiles. Globally,.90% of all profiles had at least 50% of all roots in the upper 0.3 m of the soil profile (including organic horizons) and 95% of all roots in the upper 2 m. Deeper rooting depths were mainly found in water- limited ecosystems. Deeper 95% rooting depths were also found for shrublands compared to grasslands, in sandy soils vs. clay or loam soils, and in systems with relatively shallow

Spatial autocorrelation and autoregressive models in ecology

Abstract: Recognition and analysis of spatial autocorrelation has defined a new par- adigm in ecology. Attention to spatial pattern can lead to insights that would have been otherwise overlooked, while ignoring space may lead to false conclusions about ecological relationships. We used Gaussian spatial autoregressive models, fit with widely available software, to examine breeding habitat relationships for three common Neotropical migrant songbirds in the southern Appalachian Mountains of North Carolina and Tennessee, USA. In preliminary models that ignored space, the abundance of all three species was cor- related with both local- and landscape-scale habitat variables. These models were then modified to account for broadscale spatial trend (via trend surface analysis) and fine-scale autocorrelation (via an autoregressive spatial covariance matrix). Residuals from ordinary least squares regression models were autocorrelated, indicating that the assumption of independent errors was violated. In contrast, residuals from autoregressive models showed little spatial pattern, suggesting that these models were appropriate. The magnitude of habitat effects tended to decrease, and the relative importance of different habitat variables shifted when we incorporated broadscale and then fine-scale space into the analysis. The degree to which habitat effects changed when space was added to the models was roughly correlated with the amount of spatial structure in the habitat variables. Spatial pattern in the residuals from ordinary least squares models may result from failure to include or adequately measure autocorrelated habitat variables. In addition, con- tagious processes, such as conspecific attraction, may generate spatial patterns in species abundance that cannot be explained by habitat models. For our study species, spatial patterns in the ordinary least squares residuals suggest that a scale of 500-1000 m would be ap- propriate for investigating possible contagious processes.

Fine root architecture of nine north american trees

Abstract: The fine roots of trees are concentrated on lateral branches that arise from perennial roots. They are important in the acquisition of water and essential nutrients, and at the ecosystem level, they make a significant contribution to biogeochemical cycling. Fine roots have often been studied according to arbitrary size classes, e.g., all roots less than 1 or 2 mm in diameter. Because of the size class approach, the position of an individual root on the complex lateral branching system has often been ignored, and relationships between the form of the branching root system and its function are poorly understood. The fine roots of both gymnosperms and angiosperms, which formed ectomycorrhizae (EM) and arbuscular mycorrhizae (AM) fungal associations, were sampled in 1998 and 1999. Study sites were chosen to encompass a wide variety of environments in four regions of North America. Intact lateral branches were collected from each species and 18 561 individual roots were dissected by order, with distal roots numbered as first-order roots. This scheme is similar to the one commonly used to number the order of streams. Fine root diameter, length, specific root length (SRL; m/g), and nitrogen (N) concentration of nine North American tree species (Acer saccharunz, Juniperus monosperma, Liriodendron tulipifera, Picea glauca, Pinus edulis, Pinus elliottii, Pinus resinosa, Populus balsamifera, and Quercus alba) were then compared and contrasted. Lateral roots 75% of the total number and length of individual roots sampled in all species except Liriodendron tulipifera. Both SRL and N concentration decreased with increasing root order in all nine species, and this pattern appears to be universal in all temperate and boreal trees. Nitrogen concentrations ranged from 8.5 to 30.9 g/kg and were highest in the first-order "root tips." On a mass basis, first- order roots are expensive to maintain per unit time (high tissue N concentration). Tissue N appears to be a key factor in understanding the C cost of maintaining first- and second- order roots, which dominate the display of absorbing root length. There were many sig- nificant differences among species in diameter, length, SRL, and N concentration. For example, two different species can have similar SRL but very different tissue N concen- trations. Our findings run contrary to the common idea that all roots of a given size class function the same way and that a common size class for fine roots works well for all species. Interestingly, fine root lateral branches are apparently deciduous, with a distinct lateral branch scar. The position of an individual root on the branching root system appears to be important in understanding the function of fine roots.

Predator functional responses: discriminating between handling and digesting prey

Abstract: We present a handy mechanistic functional response model that realistically incorporates handling (i.e., attacking and eating) and digesting prey. We briefly review current functional response theory and thereby demonstrate that such a model has been lacking so far. In our model, we treat digestion as a background process that does not prevent further foraging activities (i.e., searching and handling). Instead, we let the hunger level determine the probability that the predator searches for new prey. Additionally, our model takes into account time wasted through unsuccessful attacks. Since a main assumption of our model is that the predator's hunger is in a steady state, we term it the steady-state satiation (SSS) equation. The SSS equation yields a new formula for the asymptotic maximum predation rate (i.e., asymptotic maximum number of prey eaten per unit time, for prey density approaching infinity). According to this formula, maximum predation rate is determined not by the sum of the time spent for handling and digesting prey, but solely by the larger of these two terms. As a consequence, predators can be categorized into two types: handling-limited predators (where maximum predation rate is limited by handling time) and digestion-limited predators (where maximum predation rate is limited by digestion time). We give examples of both predator types. Based on available data, we suggest that most predators are digestion limited. The SSS equation is a conceptual mechanistic model. Two possible applications of this model are that (1) it can be used to calculate the effects of changing predator or prey characteristics (e.g., defenses) on predation rate and (2) optimal foraging models based on the SSS equation are testable alternatives to other approaches. This may improve optimal foraging theory, since one of its major problems has been the lack of alternative models.

Dynamics of measles epidemics: Estimating scaling of transmission rates using a time series sir model

Abstract: Before the development of mass-vaccination campaigns, measles exhibited persistent fluctuations (endemic dynamics) in large British cities, and recurrent outbreaks (episodic dynamics) in smaller communities. The critical community size separating the two regimes was ;300 000-500 000. We develop a model, the TSIR (Time-series Suscep- tible-Infected-Recovered) model, that can capture both endemic cycles and episodic out- breaks in measles. The model includes the stochasticity inherent in the disease transmission (giving rise to a negative binomial conditional distribution) and random immigration. It is thus a doubly stochastic model for disease dynamics. It further includes seasonality in the transmission rates. All parameters of the model are estimated on the basis of time series data on reported cases and reconstructed susceptible numbers from a set of cities in England and Wales in the prevaccination era (1944-1966). The 60 cities analyzed span a size range from London (3.3 3 10 6 inhabitants) to Teignmouth (10 500 inhabitants). The dynamics of all cities fit the model well. Transmission rates scale with community size, as expected from dynamics adhering closely to frequency dependent transmission (''true mass action''). These rates are further found to reveal strong seasonal variation, corresponding to high transmission during school terms and lower transmission during the school holidays. The basic reproductive ratio, R0, is found to be invariant across the observed range of host community size, and the mean proportion of susceptible individuals also appears to be constant. Through the epidemic cycle, the susceptible population is kept within a 3% interval. The disease is, thus, efficient in ''regulating'' the susceptible population—even in small cities that undergo recurrent epidemics with frequent extinction of the disease agent. Recolonization is highly sensitive to the random immigration process. The initial phase of the epidemic is also stochastic (due to demographic stochasticity and random immigration). However, the epidemic is nearly ''deterministic'' through most of the growth and decline phase.

Fitting population models incorporating process noise and observation error

Abstract: We evaluate a method for fitting models to time series of population abun- dances that incorporates both process noise and observation error in a likelihood framework. The method follows the probability logic of the Kalman filter, but whereas the Kalman filter applies to linear, Gaussian systems, we implement the full probability calculations numerically so that any nonlinear, non-Gaussian model can be used. We refer to the method as the "numerically integrated state-space (NISS) method" and compare it to two common methods used to analyze nonlinear time series in ecology: least squares with only process noise (LSPN) and least squares with only observation error (LSOE). We compare all three methods by fitting Beverton-Holt and Ricker models to many replicate model-generated time series of length 20 with several parameter choices. For the Ricker model we chose parameters for which the deterministic part of the model produces a stable equilibrium, a two-cycle, or a four-cycle. For each set of parameters we used three process-noise and observation-error scenarios: large standard deviation (0.2) for both, and large for one but small (0.05) for the other. The NISS method had lower estimator bias and variance than the other methods in nearly all cases. The only exceptions were for the Ricker model with stable-equilibrium parameters, in which case the LSPN and LSOE methods has lower bias when noise variances most closely met their assumptions. For the Beverton-Holt model, the NISS method was much less biased and more precise than the other methods. We also evaluated the utility of each method for model selection by fitting simulated data to both models and using information criteria for selection. The NISS and LSOE methods showed a strong bias toward selecting the Ricker over the Beverton-Holt, even when data were generated with the Beverton-Holt. It remains unclear whether the LSPN method is generally superior for model selection or has fortuitously better biases in this particular case. These results suggest that information criteria are best used with caution for nonlinear population models with short time series. Finally we evaluated the convergence of likelihood ratios to theoretical asymptotic distributions. Agreement with asymptotic distributions was very good for stable-point Rick- er parameters, less accurate for two-cycle and four-cycle Ricker parameters, and least accurate for the Beverton-Holt model. The numerically integrated state-space method has a number of advantages over least squares methods and offers a useful tool for connecting models and data and ecology.

The effects of prey availability on the numerical response of wading birds

Abstract: Reduced prey availability has emerged as a primary hypothesis to explain population constraints on wading birds in numerous wetlands around the world. However, there is almost no understanding of which component of prey availability (i.e., prey density or vulnerability of prey to capture) is affecting populations and whether the relative effects of each component differ among species. In this study, I manipulated prey density and water depth (i.e., prey availability) in 12 0.2-ha ponds to determine their relative effects on the numeric response of eight species of free-ranging wading birds (White Ibis, Eudocimus albus; Wood Stork, Mycteria americana; Snowy Egret, Egretta thula; Glossy Ibis, Plegadis falcinellus; Great Egret, Ardea alba; Tricolored Heron, Egretta tricolor; Great Blue Heron, Ardea herodias; and Little Blue Heron, Egretta caerulea). The experiment was conducted in a constructed wetland adjacent to, and west of, the northern tip of the remnant Everglades, in Palm Beach County, Florida, USA. Each pond was set to a water depth of 10 cm, 19 cm, or 28 cm, and was stocked with golden shiners (Notemigonus crysoleucas) at a density of either 3 fish/m2 or 10 fish/m2. Total bird use (all treatments pooled) increased from day 1 (day after stocking) to day 6, stabilized for several days at ∼280 birds, and then decreased until day 16, when bird use nearly ceased. Fish were depleted most rapidly in the shallow treatment and least rapidly in the deep treatment. The giving-up-density (GUD) of prey increased with increasing water depth. There was no significant difference among species in the slope of that relationship; however, a visual inspection of the data showed that differences in GUDs were becoming more apparent in the deepest treatment. At that depth, the White Ibis, Wood Stork, and Snowy Egret had higher GUDs than did the Glossy Ibis, Great Egret, Tricolored Heron, Great Blue Heron, and Little Blue Heron. Also, the first three species were affected significantly by both prey density and water depth, whereas the latter five species showed a decidedly weaker response to one or the other component of prey availability. The first three species were more abundant in the shallow treatments and the high prey density treatments, and they abandoned the study site before other species reached their maximum density. The feeding strategy of the first group appeared to be one of searching for new high-quality food patches rather than staying and exploiting food patches that were declining in quality. Species that employed a searching strategy also have shown the most severe population declines, suggesting that factors affecting bird density at feeding sites may also have affected population size.

Dynamics of measles epidemics: scaling noise, determinism, and predictability with the tsir model

Abstract: Two key linked questions in population dynamics are the relative importance of noise vs. density-dependent nonlinearities and the limits on temporal predictability of population abundance. We propose that childhood microparasitic infections, notably mea- sles, provide an unusually suitable empirical and theoretical test bed for addressing these issues. We base our analysis on a new mechanistic time series model for measles, the TSIR model, which captures the mechanistic essence of epidemic dynamics. The model, and parameter estimates based on short-term fits to prevaccination measles time series for 60 towns and cities in England and Wales, is introduced in a companion paper. Here, we explore how well the model predicts the long-term dynamics of measles and the balance between noise and determinism, as a function of population size. The TSIR model captures the basic dynamical features of the long-term pattern of measles epidemics in large cities remarkably well (based on time and frequency domain analyses). In particular, the model illustrates the impact of secular increases in birth rates, which cause a transition from biennial to annual dynamics. The model also captures the observed increase in epidemic irregularity with decreasing population size and the onset of local extinction below a critical community size. Decreased host population size is shown to be associated with an increased impact of demographic stochasticity. The interaction between nonlinearity and noise is explored using local Lyapunov exponents (LLE). These testify to the high level of stability of the biennial attractor in large cities. Irregularities are due to the limit cycle evolving with changing human birth rates and not due to complex dynamics. The geometry of the dynamics (sign and magnitude of the LLEs across phase space) is similar in the cities and the smaller urban areas. The qualitative difference in dynamics between small and large host communities is that demographic and extinction-recolonization stochasticities are much more influential in the former. The regional dynamics can therefore only be understood in terms of a core-satellite metapopulation structure for this host-enemy system. We also make a preliminary exploration of the model's ability to predict the dynamic consequences of measles vaccination.

Competitor-induced plasticity in tadpoles: consequences, cues, and connections to predator-induced plasticity

Abstract: Phenotypically plastic responses to environmental change are typically com- partmentalized by the type of environmental cues that cause the induction (e.g., temperature, light), but different types of environmentally induced responses might very well be related to each other. I used a number of experiments to document competitor-induced behavior and morphology in wood frog tadpoles (Rana sylvatica), to quantify how competitor-induced changes affected subsequent competitive ability, and to investigate the cues responsible for the competitor-induced response. Competitors induced wood frogs to increase their activity and generally develop larger bodies and smaller tails. Further, wood frogs were able to discriminate between intraspecific and interspecific competitors. Whereas behavioral re- sponses to competitors are well documented, the widespread morphological responses are a new discovery; these responses are particularly intriguing because they are in the opposite direction of predator-induced responses in tadpoles. The competitor-induced phenotypes are not simply an allometric effect, but appear to be adaptive responses to competitors; competitor-induced wood frogs experienced higher relative growth rates than noninduced wood frogs in subsequent performance trials, but past studies suggest that the competitor- induced phenotype should experience higher predation risk. When responding to compe- tition, wood frogs were not responding only to reduced food. They were able to sense changes in both the per capita food levels and changes in density (per unit volume) that were independent of per capita food level. Thus, these animals have an amazing ability to sense changes in their environments and respond in very precise ways. When one considers competitor-induced responses in light of past studies on predator- induced phenotypes in larval anurans, one sees that predator-induced traits provide indi- viduals with increased predator resistance but decreased competitive ability, whereas com- petitor-induced traits provide individuals with. increased competitive ability but decreased predator resistance. This suggests that predator- and competitor-induced plasticity have evolved an intricate link that presents a trade-off between competitive ability and predator resistance ability. This trade-off is common across species and makes sense in light of the environments in which wood frogs live, ranging from ponds with high densities of predators and few tadpoles competing to ponds with few predators and extremely high densities of tadpoles competing. Further, there is evidence that this type of plasticity trade-off might be quite common in other phenotypically plastic taxa.

Effects of forest fragmentation on bird assemblages in a novel landscape context

Abstract: We report findings of a large-scale study in a 100 000-ha subsection of the Tumut region in southern New South Wales, southeastern Australia. The study was designed to measure the effects of landscape context and habitat fragmentation on forest birds. The study region consisted of a forest mosaic characterized by different landscape contexts: large, continuous areas of native Eucalyptus forest, extensive stands of exotic softwood (radiata pine, Pinus radiata) plantation, and remnant patches of native Eucalyptus forest scattered throughout the extensive areas of radiata pine plantation. A set of 85 eucalypt remnants was randomly selected across several stratifying variables: four patch size classes (1-3 ha, 4-10 ha, 11-20 ha, and .20 ha), two isolation age classes (,20 years and .20 years since fragmentation), and five dominant eucalypt forest type classes. In addition to the 85 eucalypt remnants, a further 80 3-ha sites were selected for study: 40 in large, continuous areas of eucalypt forest and 40 in radiata pine stands. Point-interval counts of forest birds at the 165 sites were conducted in 1996 and 1997. Of 90 species recorded, 23.1 species (95% confidence interval, 22.0, 24.2 species), on average, were present in continuous eucalypt forest, 20.6 (19.5, 21.7) species in patch- shaped eucalypt remnants, 20.6 (19.5, 21.7) species in strip-shaped eucalypt remnants, and 16.7 (15.6, 17.8) species in radiata pine. Strong gradients in bird assemblages were found. These gradients were governed by a combination of landscape context, remnant size, and remnant shape effects, and, in the case of radiata pine sites, the extent of native forest surrounding the pine. These gradients could, in part, be explained by bird life history attributes such as foraging guild and nesting height. For example, birds more often detected in patch-shaped remnants were smaller, produced smaller clutches, were more likely to be migratory, and typically had cup nests or burrows. The results of our study showed that eucalypt fragments of all sizes and shapes have significant conservation value. This is because they contain many native species of birds, some of which are more abundant in fragments than they are in continuous eucalypt forests, and also because they increase native bird populations in nearby non-native pine plantations.

Parasite (Ribeiroia ondatrae) infection linked to amphibian malformations in the western United States

Abstract: Parasites and pathogens can influence the survivorship, behavior, and very structure of their host species. For example, experimental studies have shown that trematode parasites can cause high frequencies of severe limb malformations in amphibians. In a broad-scale field survey covering parts of California, Oregon, Washington, Idaho, and Montana, we examined relationships between the frequency and types of morphological abnormalities in amphibians and the abundance of trematode parasite infection, pH, con- centrations of 61 pesticides, and levels of orthophosphate and total nitrate. We recorded severe malformations at frequencies ranging from 1% to 90% in nine amphibian species from 53 aquatic systems. Infection of larvae by the trematode Ribeiroia ondatrae was associated with, and functionally related to, higher frequencies of amphibian limb malfor- mations than found in uninfected populations (#5%). Parasites were concentrated around the basal tissue of hind limbs in infected anurans, and malformations associated with infection included skin webbings, supernumerary limbs and digits, and missing or mal- formed hind limbs. In the absence of Ribeiroia, amphibian populations exhibited low (0- 5%) frequencies of abnormalities involving missing digits or distal portions of a hind limb. Species were affected differentially by the parasite, and Ambystoma macrodactylum , Hyla regilla, Rana aurora, R. luteiventris, and Taricha torosa typically exhibited the highest frequencies of abnormalities. None of the water-quality variables measured was associated with malformed amphibians, but aquatic snail hosts (Planorbella spp.) were significant predictors of the presence and abundance of Ribeiroia infection. Morphological comparisons of adult specimens of Ribeiroia collected from different sites and raised in experimental definitive hosts suggested that all samples represented the same species— R. ondatrae. These field results, coupled with experimental research on the effects of Ribeiroia on amphibians, demonstrate that Ribeiroia infection is an important and widespread cause of amphibian limb malformations in the western United States. The relevance of trematode infection to declines of amphibian populations and the influence of habitat modification on the pathology and life cycle of Ribeiroia are emphasized as areas requiring further research.

Multivariate sexual dimorphism, sexual selection, and adaptation in greater antillean anolis lizards

Abstract: Sexual variation in body form is a common phenomenon in the natural world. Although most research has focused on dimorphism in size, examination of differences in shape can provide insight into ecological factors that may differ in importance to the sexes. In this study, we investigated the patterns of body shape dimorphism in 15 species of Greater Antillean Anolis lizards and investigated whether these patterns can be explained by allometry, phylogenetic effect, or sexual differences in habitat use. We found extensive shape and ecological variation between males and females. Previous studies have been conducted on males only; we found that females have also evolved morphologies to match their habitats. However, we concluded that adaptive patterns differ for the sexes and that interspecific ecological variation is related more strongly to shape than to size for each sex. Previous studies on males have revealed repeated convergent evolution of morphology to habitat types (termed ''ecomorphs''). Here, we found that ecomorphs also differ in the magnitude and direction of shape dimorphism. These results cannot be accounted for by allometric scaling or by phylogenetic similarity (regardless of assumptions regarding evo- lutionary process); they support previous studies that have found important life-history differences for species of different habitat types. We found some evidence for independent adaptation of the sexes, but with more complex ecological patterning occurring between sexes than can be explained by sexual selection alone. Consequently, some combination of functional differences and sexual selection is required.

Extreme floods, channel change, and riparian forests along ephemeral streams

Abstract: The geomorphic effectiveness of extreme floods increases with aridity and decreasing watershed size. Therefore, in small dry watersheds extreme floods should control the age structure and spatial distribution of populations of disturbance-dependent riparian trees. We examined the influence of extreme floods on the bottomland morphology and forest of ephemeral streams in a semiarid region. Along six stream reaches on the Colorado Piedmont we examined channel changes by analyzing a rectified sequence of aerial photographs spanning 56 yr, and we investigated the spatial distribution of different-aged patches of forest by aging 189 randomly sampled cottonwood trees. Channel change in these ephemeral sand-bed streams is dominated by widening, which occurs over a span of hours during infrequent floods, and postflood narrowing, which occurs over decades between floods. Narrowing is accelerated where reliable moisture increases the density and growth rate of vegetation on the former bed. Reproduction of cottonwood trees has occurred mostly in former channel bed during periods of channel narrowing beginning after floods in 1935 and 1965 and continuing for as long as two decades. Thus cottonwood establishment is related to low flows at the time scale of a year, but to high flows at the time scale of decades. At sites that have not experienced major floods in the last 80 yr, little channel change has occurred, cottonwood reproduction has been limited, tree density has declined, and succession to grassland is occurring. Because channel change and tree reproduction in this region are driven by infrequent local events, channel width and tree age distributions vary greatly over time and among sites. For the same reason, riparian forests along these ephemeral streams can be as wide as forests along perennial rivers with much higher mean discharge.

Influences of established trees on mycorrhizas, nutrition, and growth of quercus rubra seedlings

Abstract: Established plants may facilitate the regeneration of closely related seedlings if they increase populations of mutualistic symbionts that would otherwise be limiting. In this study we examined the influence of ectomycorrhizal and vesicular-arbuscular mycor- rhizal (VAM) trees on Quercus rubra seedlings to determine how trees influence mycorrhizal infection, nutrient uptake, and growth of seedlings. In two related experiments, we planted Q. rubra acorns adjacent to stump sprouts of Q. montana (=Near-Quercus) and Acer rubrum (=Near-Acer), and, in the second experiment, near Quercus spp. stumps that had not re- sprouted (=Near-Dead-Quercus). Congeneric Quercus were used to prevent root grafting; using stump sprouts minimized aboveground differences between treatments by limiting canopy size. In both experiments, Near-Quercus seedlings were infected by ectomycorrhizal fungi to a significantly greater extent than Near-Acer or Near-Dead-Quercus seedlings. Near-Quercus seedlings were also infected by a different and more diverse community of ectomycorrhizal fungi. Cenococcum geophilum, the only ectomycorrhizal fungus more abundant on Near-Acer seedlings than on Near-Quercus seedlings, appears to be relatively ineffective at increasing seedling nutrient uptake. Near-Quercus seedlings had greater con- centrations and contents of N and P than other seedlings in both experiments. In the first experiment, Near-Quercus seedlings had greater growth than Near-Acer seedlings, although it was not clear if this represented beneficial influences of Q. montana, or an undetermined negative influence of A. rubrum. No significant growth responses were found in the second experiment; severe drought may have prevented the expression of growth potential. The results demonstrate indirect facilitation of seedlings by established congeneric trees through increased seedling ectomycorrhizal infection.

The expanding scale of species turnover events on coral reefs in belize

Abstract: Beginning in the late 1980s, white-band disease nearly eliminated the stag- horn coral Acropora cervicornis from reefs in the central shelf lagoon of Belize. The lettuce coral Agaricia tenuifolia replaced Acropora cervicornis in the early 1990s, but anomalously high water temperatures in 1998 caused severe bleaching and catastrophic mortality of Agaricia tenuifolia. The short-lived transition in dominance from Acropora cervicornis to Agaricia tenuifolia left an unambiguous signature in the fossil record of these uncemented lagoonal reefs. Analysis of 38 cores, extracted from 22 sampling stations in a 375-km2 area of the central lagoon, showed that Acropora cervicornis dominated continuously for at least 3000 years prior to the recent events. Agaricia tenuifolia occasionally grew in small patches, but no coral-to-coral replacement sequence occurred over the entire area until the late 1980s. Within a decade, the scale of species turnover increased from tens of square meters or less to hundreds of square kilometers or more. This unprecedented increase in the scale of turnover events is rooted in the accelerating pace of ecological change on coral reefs at the regional level.

Local population differences in phenotypic plasticity: predator-induced changes in wood frog tadpoles

Abstract: Taxa that are divided into separate populations with low levels of interpop- ulation dispersal have the potential to evolve genetically based differences in their phe- notypes and the plasticity of those phenotypes. These differences can be due to random processes, including genetic drift and founder effects, or they can be the result of different selection pressures among populations. I investigated population-level differences in pred- ator-induced phenotypic plasticity in eight populations of larval wood frogs (Rana sylvatica) over a small geographic scale (interpopulation distances of 0.3-8 km). Using a common-garden experiment containing predator and no-predator environments, I found population differences in behavior, morphology, and life history. These responses exhibited a habitat-related pattern: the four populations from closed-canopy ponds did not differ from each other in any of their phenotypes whereas the four populations from open- canopy ponds did differ from each other in these traits. This phenotypic pattern matches the pattern of competitors and predators found in these two types of ponds. Based on two years of pond surveys, the four closed-canopy ponds contained very similar competitor and predator assemblages while the assemblages of the four open-canopy ponds were more diverse and highly variable among open-canopy ponds. When combined with past studies, which demonstrate that predators and competitors select for alternative behavioral and morphological traits, these patterns suggest that the population differences may have arisen via natural selection and not via random mutation or drift. In a second experiment, I cross-transplanted two of the populations into each other's ponds to determine if the populations were locally adapted to the conditions of their native pond (using low and high competition crossed with the presence or absence of a lethal predator). The populations continued to exhibit phenotypic differences, and one of the two populations tested exhibited superior growth in its native pond. This suggests that some wood frog populations are adapted to the local conditions of their natal pond and that localized selection by predation and competition may be the underlying mechanism. Col- lectively, these experiments indicate that taxa that are divided into discrete populations and face different predator and competitor environments can evolve different phenotypically plastic responses.

Summer inputs of riverine nutrients to the Baltic Sea: Bioavailability and eutrophication relevance

Abstract: Most nitrogen and phosphorus transported by world rivers to the oceans is associated with dissolved organic matter. However, organic matter as a potential source of N and P has hitherto been largely neglected in studies of coastal microbial food webs. We examined 50 rivers, draining a major part of the Baltic Sea watershed, with respect to summer concentrations, chemical composition, and biological availability of N and P. The broad spectrum of rivers studied enabled us to assess whether the input of terrigenous organic matter can be an important nutrient source, at various levels of anthropogenic loading of inorganic N and P. Concentrations of total N and P ranged from 9 to 220 mumol/L and from 0.14 to 5.56 mumol/L, respectively, with the highest concentrations in the southern part of the Baltic Sea drainage area and in several rivers on the Finnish western coast. Urea and dissolved combined amino acids (DCAA) each constituted 4-20% of dissolved organic nitrogen (DON), while dissolved free amino acids (DFAA) made up <3% of DON. The contribution of urea and amino acids to the DON pool was inversely correlated with DON concentration. Bacterial regrowth bioassays in selected rivers demonstrated that similar to30% of DON and similar to75% of dissolved organic phosphorus (DOP) was potentially available to the indigenous bacterial assemblage of the Baltic Sea, and hence susceptible to mineralization within the pelagic food web. Our study is among the first to demonstrate that bacterioplankton are able to utilize a major part of DON and DOP from a broad spectrum of natural waters. The C:N ratio, absorbance spectra, and fluorescence properties of the organic matter suggest that the observed high bioavailability of DON and DOP was due to a large contribution of organic matter from riverine primary production compared to the humic matter derived from terrestrial vascular plants. In addition, algal and bacterial cells dominated the transport of particulate organic material, further enhancing productivity of coastal waters. No correlations were found between DON bioavailability and the fraction of DON bound in urea and amino acids, indicating a utilization of other N compounds (e.g., amides) by the bacteria. We estimate that the input of summer riverine N to the Baltic Sea consists of 48% dissolved inorganic N, 41 % DON, and 11 % particulate N. Corresponding values for phosphorus are 46%, 18%, and 36% of dissolved inorganic P, DOP, and particulate P, respectively. During the thermal summer stratification, when freshwater inputs are trapped in the surface layer, rivers contribute similar to30% of N and similar to5% of P needed to support the export production (plankton sedimenting out of the photic layer) in the Baltic Sea. The high availability to bacteria suggests that DOP is a major stimulator of pelagic productivity in the P-limited northern part of the Baltic Sea. Based on reported concentrations in other areas, we suggest that the global contribution of riverine organic N and P to the primary production of coastal waters is comparable to the contribution of inorganic nutrients. (Less)

The scale of community structure: habitat variation and avian guilds in tropical forest understory

Abstract: Both local and regional habitat characteristics influence species richness and community structure. The scale at which communities are studied, however, affects the detection of relationships between habitat characteristics and patterns of habitat selection, species diversity, and species composition, and may obscure observation of differences in how species perceive the scale of environmental variation. To determine how environmental variation at different scales is related to species occurrence and richness, I analyze mist net sampling data on several guilds of forest understory birds. Bird capture, vegetation, and physical environment data come from 23 0.5-ha study sites in primary and secondary forest in Amazonian Ecuador. The percentages of primary forest within concentric circles around each site form forest imbeddedness measures (FIMs), which are evaluated using satellite imagery. Variation in FIM size represents different measurement scales for determining forest cover. Primary forest cover is also analyzed in successively larger tori surrounding sites and is used, after variable reduction with Principal Components Analysis, to summarize variation in forest cover around sites. Linear regression, surface trend analysis, and ordination help to quantify how variation in guild composition and species richness is explained by forest cover, vegetation structure, and physical environment. Species composition is related to variation in primary forest cover, primarily within 200-600 m of study sites. Canonical Correspondence Analysis (CCA) indicates that nectarivores, shrub-layer frugivores, and ant-following birds are captured in areas with relatively low primary forest cover. In contrast, shrub-layer insectivores, shrub-layer omnivores, and birds probing dead foliage for large insects tend to be captured in areas of relatively high primary forest cover. The species richness of insect gleaners, ant followers, and omnivores is statistically related to the percent cover of primary forest within hundreds of meters from the study sites. This suggests that some mechanisms that influence guild composition act over substantial distances. Nonetheless, the small radii of FIMs related to the species richness of dead-leaf probers suggests that local conditions and variation in forest cover over short (<200 m) distances directly or indirectly influence species richness of some primary forest birds. The significant relationship between temperature variation among capture sites and species richness of ant-following birds suggests that these species choose among habitats in a temperature range at which physiological constraints operate, either directly on the birds themselves or on the ants they follow. Species richness within the nectarivores, in contrast, shows no relationship with large-scale variation in primary forest cover. The radius of the FIM most closely associated with species richness differs among guilds, which suggests variation in the scale at which forest cover is associated with guild structure, as well as variation in the strength of the association. Differences in the scale of relationships between environment and species richness among guilds suggest that the mechanisms that influence both species' habitat use and community structure differ among guilds. A single mechanism, operating at a single scale, is inconsistent with these patterns.

Diversity, biomass and ecosystem processes in the marine benthos.

Abstract: Recent studies in terrestrial, plant-dominated systems have shown that reductions in diversity can affect essential ecosystem processes, especially productivity. However, the exact form of the relationship between diversity and ecosystem functions remains unknown, as does the relevance of these studies to other systems. We studied the relationships between macroinvertebrate species richness and ecosystem functions in a soft-bottom, intertidal system. We also considered, as a separate variable, the effects of macroinvertebrate biomass on ecosystem functions. A field experiment was conducted at Blackness, a mudflat in the Firth of Forth, Scotland, United Kingdom, using cages with different mesh sizes (195, 300, and 3000 μm) to establish low, medium, and high species richness treatments through differential colonization of defaunated sediments. Low, medium, and high biomass treatments were established by enclosing differing amounts of ambient sediment in defaunated plots. Other treatments controlled for the effects of defaunation and caging. The experiment ran for six weeks in the summer of 1999. All treatments contained species within the same five main functional groups of macroinvertebrate, but species' identity varied both within and between treatments (thus species richness was considered a random, rather than fixed, variable). A total of 27 macroinvertebrate species were sampled across all treatments; 37% of these occurred in the low, 52% in the medium, and 74% in the high diversity treatments. At the end of the experiment, the following physical variables were measured as indicators of ecosystem functions such as sediment stabilization and nutrient fluxes: sediment shear strength (a measure of sediment cohesiveness), water content, silt/clay content, organic content, redox potential (a measure of anoxia), nitrate, nitrite, phosphate and ammonium fluxes, and community respiration. Changes in biomass and species richness were found to have significant effects on oxygen consumption; these relationships were driven in particular by the presence of the largest species in our study, Nephtys hombergii. All other variables were not significantly affected by the treatments. These results support the null hypotheses of no relationship between ecosystem functions and diversity and biomass. However, our experiment was necessarily limited in both spatial and temporal scale; the implications of this when scaling up to larger scale generalizations are discussed. Our results suggest that diversity/biomass/ecosystem function relationships in the soft sediment benthos are likely to be very complex and may depend more on functional groups than species richness.

Predation structures communities at deep‐sea hydrothermal vents

Abstract: The structure and dynamics of natural communities result from the interplay of abiotic and biotic factors. We used manipulative field experiments to determine the relative roles of abiotic conditions and biotic interactions in structuring deep-sea (2500 m depth) communities along environmental gradients around hydrothermal vents of the eastern tropical Pacific Ocean (East Pacific Rise, at 9 50 N). We tested (1) whether predation by crabs and fishes affects the recruitment of benthic species and subsequent community structure and (2) whether the effects of predation vary along the steep gradients of tem- perature, oxygen, sulfide, and metal concentrations near vents. Recruitment substrates (ba- salt cubic blocks, roughly 10 cm on a side), both uncaged and caged to exclude predators (crabs, fishes, whelks, and octopi), were deployed along a decreasing vent fluid-flux gra- dient. The exclusion of predators for 8 mo increased the abundance of small mobile gas- tropods and amphipod crustaceans but decreased the abundance of sessile invertebrates, including juvenile vestimentiferan worms, tubiculous polychaetes, and mussels. Effects of predation were strongest nearest to hydrothermal vents, where abiotic environmental con- ditions were most extreme but productivity and the overall abundances of benthic inver- tebrates and mobile predators were the greatest. Additional 5-mo experiments conducted at three different locations showed similar trends at all sites, indicating that these effects of predation on benthic community structure are repeatable. Stomach-content analyses of the most abundant predators found at vents indicated that the zoarcid fish ( Thermarces cerberus) primarily feeds on the vent snail Cyathermia naticoides, the limpet Lepetodrilus elevatus, and the amphipod crustacean Ventiella sulfuris, the very species that showed the greatest increase following predator exclusion. In contrast, brachyuran ( Bythograea ther- mydron) and galatheid (Munidopsis subsquamosa ) crab stomachs did not contain small mobile grazers, and crabs presented with arrays of the most common vent invertebrate species preferred mussels and vestimentiferans over limpets. Our results indicate that pre- dation by large mobile predators influences the structure of hydrothermal vent communities, directly by reducing the abundance of gastropod prey species, and indirectly by reducing

Comparison of ecological communities: the problem of sample representativeness

Abstract: Obtaining an adequate, representative sample of ecological communities to make taxon richness (TR) or compositional comparisons among sites is a continuing challenge. Although randomization in the collection of sample units is often used to assure that sampling is representative, randomization does not convey the concept of how well samples represent the community or site from which they are drawn. In ecological surveys, how well a sample represents a community or site literally means the similarity in taxon composition and relative abundance between a sample and the community from which it is drawn. Using both field and simulated data, we show that the proportion of the total taxon richness at a site (%TTR) achieved with a fixed sample size varies across sites, which in turn causes changes in site-to-site differences in observed TR with sample size. This means that equal-sized samples may differentially represent the communities from which they are drawn. However, the similarity of a sample to the community from which it is drawn cannot be measured directly because the taxon composition and relative abundance of the community is usually unknown. We propose to estimate it by measuring the average similarity among replicate samples randomly drawn from a community, i.e., autosimilarity, which is measured with Jaccard Coefficient in this study. Using the same data sets, we found that: (1) samples of equal size from different sites or communities achieved different levels of autosimilarity, with lower levels achieved in taxon richer sites, indicating variation in how well samples of equal size represent their respective communities; (2) %TTR was positively and almost linearly correlated with autosimilarity, indicating that autosimilarity might be a good predictor of TTR; and (3) when samples were compared at the same level of autosimilarity, similar %TTRs across different sites were achieved (i.e., the relative differences in taxon richness among sites became independent of sample size). We conclude that standardization on autosimilarity, rather than on sample size, can improve the accuracy of taxon richness comparisons.

Integrated ecological economic modeling of the Patuxent River Watershed, Maryland

Abstract: Understanding the way regional landscapes operate, evolve, and change is a key area of research for ecosystem science. It is also essential to support the “place-based” management approach being advocated by the U.S. Environmental Protection Agency and other management agencies. We developed a spatially explicit, process-based model of the 2352 km2 Patuxent River watershed in Maryland to integrate data and knowledge over several spatial, temporal, and complexity scales, and to serve as an aid to regional management. In particular, the model addresses the effects of both the magnitude and spatial patterns of human settlements and agricultural practices on hydrology, plant productivity, and nutrient cycling in the landscape. The spatial resolution is variable, with a maximum of 200 × 200 m to allow adequate depiction of the pattern of ecosystems and human settlement on the landscape. The temporal resolution is different for various components of the model, ranging from hourly time steps in the hydrologic sector to yearly time steps in the economic land-use transition module. We used a modular, multiscale approach to calibrate and test the model. Model results show good agreement with data for several components of the model at several scales. A range of scenarios with the calibrated model shows the implications of past and alternative future land-use patterns and policies. We analyzed 18 scenarios including: (1) historical land-use in 1650, 1850, 1950, 1972, 1990, and 1997; (2) a “buildout” scenario based on fully developing all the land currently zoned for development; (3) four future development patterns based on an empirical economic land-use conversion model; (4) agricultural “best management practices” that lower fertilizer application; (5) four “replacement” scenarios of land-use change to analyze the relative contributions of agriculture and urban land uses; and (6) two “clustering” scenarios with significantly more and less clustered residential development than the current pattern. Results indicate the complex nature of the landscape response and the need for spatially explicit modeling.

Competition–colonization trade-offs in a guild of african acacia-ants

Abstract: In systems where disturbance creates colonization opportunities, competition–colonization trade-offs can lead to coexistence between competitors sharing a single, limiting resource. In upland East Africa, four species of obligate acacia-ants compete for possession of Acacia drepanolobium host trees but still coexist at fine spatial scales. Previous experiments revealed a hierarchy of competitive dominance among mature ant colonies in battles over occupied trees (Crematogaster sjostedti > C. mimosae > C. nigriceps > Tetraponera penzigi), and the pattern of ant species turnover on single host trees over time is consistent with this hierarchy. Here we present evidence that competition–colonization trade-offs are likely to facilitate coexistence within this guild. The population of host trees is highly dynamic. Although most suitable host trees are occupied by ants, mature A. drepanolobium trees may be vacated after fire, extensive damage, or drought. Smaller host trees with limited nesting space become available when they recruit into the population as saplings or coppices. These unoccupied trees may be colonized by expansion of nearby, mature colonies as well as by foundress queens attempting to initiate new colonies. We identified competition–colonization trade-offs between two stages of colony development: competitive colony expansion onto nearby trees, and competition among foundress queens for nest-initiation sites. All three Crematogaster species readily occupy nearby, empty trees by colony expansion. In contrast, T. penzigi colonizes empty trees almost exclusively by dispersive foundress queens. Colonies of C. sjostedti are competitively dominant over those of all other species, but foundress queens of this species rarely attempt to start independent colonies. Foundress queens of the secondary competitive dominant (C. mimosae) are restricted mostly to low-quality, small saplings. In contrast, competitively subordinate colonies of T. penzigi and C. nigriceps produce foundress queens that disproportionately colonize empty mature trees, the highest quality host plants that are available. Competition between foundress queens is intense. Queens are significantly hyper-dispersed among potential nest-initiation sites, indicating that the presence of resident foundresses deters subsequent colonization attempts, and competitive dominance among foundresses who fight during colony founding is the reverse of that seen among mature colonies. Coexistence within this guild may therefore be promoted by ontogenetic reversals in relative competitive ability between early and late stages of colony development, a pattern that is consistent with the niche hypothesis for competition–colonization trade-offs.

Old-growth northern hardwood forests: spatial autocorrelation and patterns of understory vegetation

Abstract: The goal of this research was to examine spatial patterns of forest understory vegetation at a fine resolution for future work on underlying processes. We used a cyclic, two-dimensional sampling design, sampling plants in 2100 quadrats (0.25 m2), placing one 62 × 29.5 m grid within each of four old-growth northern hardwood (Acer saccharum–Betula alleghaniensis) stands. The specific plan used was designed to maximize spatial information and sampling efficiency. The study was done in the Sylvania Wilderness Area, Ottawa National Forest, Upper Michigan, USA. Spatial patterns of ground-layer species vary with the environment, species ecological characteristics, and their interactions. Competition with maple saplings may be a strong determinant of understory spatial patterns of these forests. Based on 95% confidence intervals, spatial analysis showed that most ground-layer species were positively autocorrelated to distances of <2.5 m in stands with high sapling density, while many of these same species were autocorrelated at up to 21 m in stands with low sapling density. Most ground-layer species also had distributions indistinguishable from random at three other resolutions (9.0 × 10.5 m to 9.0 × 31.5 m blocks) in stands with high sapling density, but aggregated distributions in stands with low sapling density. Logistic regression analysis yielded direct and indirect negative correlations between ground-layer species and maple saplings. Plant species temporal guilds were autocorrelated according to the following rank distances: spring ephemerals > evergreen ≥ early summer > late summer > dimorphic. More species were autocorrelated to greater distances on loamy soils than on sandy soils. Plant species dispersal guilds were autocorrelated according to the following rank distances: ballistic ≥ spores ≥ ant > ingested > adhesive. Thus there is a general inverse relationship between autocorrelation distance and migration rates of dispersal guilds. The spatial distribution of microtopography and decayed, coarse woody debris appears to be important for the maintenance of plant diversity and heterogeneity in old-growth stands. Forest managers can help maintain biological diversity by giving preferential management to those species that are rare and, if once locally extirpated, have poor re-colonization ability. This requires maintaining regional landscape diversity, as well as within-stand microhabitats.

Interplay between energy limitation and nutritional deficiency: empirical data and food web models

Abstract: Food quality may play an important role in consumer population dynamics. The frequently large differences in elemental and biochemical composition observed be- tween autotrophs and their grazers suggest that food quality may be of particular importance for herbivores. Under nutrient-depleted conditions the carbon-to-nutrient ratios of auto- trophs can increase to such an extent that consumers become nutrient rather than energy limited. Estimating the importance of this effect in situ in pelagic food webs is complicated by the omnivory of many consumers and by rapid nutrient recycling. Isolated predator- prey studies inadequately represent this interaction; instead, an ecosystem perspective is required. We used seven years of data from large, deep Lake Constance to develop seasonally resolved flux models of the pelagic food web and analyzed the balance between energy and nutrient constraints. The carbon (C) and phosphorus (P) flows were simultaneously quantified and balanced. C represented food quantity/energy. P was taken as a surrogate of food quality, because algal C:P ratios exceeded the threshold above which P limitation of herbivores is predicted by stoichiometric theory throughout summer and autumn. Primary production exceeded bacterial C production by a factor of 3, but autotrophs and bacteria took up approximately equal amounts of P during summer and autumn. As a consequence, the C and P supplies of suspension-feeding zooplankton were decoupled: Consumer C demands were largely met by phytoplankton whereas P was mostly obtained from bacteria and their protist predators. The degree of consumer P deficiency varied according to supplementation of their algal diet with P-enriched bacteria or bacterivores. This favored the occurrence of omnivores, i.e., organisms that minimized P deficiencies at the cost of enhanced energy limitation. In contrast with previous perceptions, P reminer- alization during P-depleted summer conditions was dominated by bacterivorous flagellates, carnivorous crustaceans, and fish, which fed on prey with an elemental composition similar to their own, whereas herbivores contributed only 30% of P cycling despite their large biomass and C production. Our results suggested a co-limitation of predominantly herbiv- orous consumers by C and P and a mutual dependence of the two types of deficiency at the individual and system level. This pattern is not specific to pelagic systems but appears to be applicable across ecosystem types.

Effects of sea urchins (parechinus angulosus) on recruits and juveniles of abalone (haliotis midae)

Abstract: Much current research on community ecology concerns indirect effects, cas- cading trophic interactions, and the role of top-down vs. bottom-up processes, as well as subsidization of matter and energy between different systems. We addressed all these topics by exploring interactions between the urchin Parechinus angulosus and both recruits and juveniles of the abalone Haliotis midae, in the light of the fact that juvenile abalone shelter beneath the urchins. In field experiments at six sites, urchins were eliminated from removal plots to determine the degree to which juvenile abalone (individuals 3-35 mm in length) depend upon urchins. Comparisons were made with undisturbed and disturbance-control plots. We hypothesized that after removal of urchins: (1) there would be an immediate decline of juvenile abalone, since they are intimately associated with the urchins; (2) abalone recruits (recent settlers <3 mm in length) would be unlikely to suffer immediately or directly because, unlike juveniles, they are only weakly associated with the urchins; (3) foliar algae would proliferate due to decreased grazing by urchins; (4) there would be a consequent reduction in crustose coralline algae due to overgrowth by foliar algae; and (5) the decline of crustose corallines would reduce recruitment of abalone due to a loss of settlement and nursery habitat. In urchin-removal plots, juvenile abalone declined dramatically, becoming virtually extinct locally. Their disappearance was slowest in crevices compared with exposed habitats, but even there they failed to survive longer than five months. Contrary to expec- tations, abalone recruits also disappeared from urchin-removal plots, possibly because the cover of sediment almost doubled there. Also contrary to our hypotheses, neither foliar macroalgae nor encrusting corallines responded to urchin removal. The explanation of these results is that P. angulosus was able to gather drift kelp when this food source was available, rather than acting as an active grazer. This feeding pattern has profound consequences and explains why urchins failed to influence algal community composition. By sheltering be- neath urchins, juvenile abalone gain protection from predators and may also benefit from an enhanced food supply. Bottom-up subsidization of urchins by drift kelp is thus respon- sible for the feeding behavior of the urchins, and indirectly facilitates their role in harboring juvenile abalone. This role is of immense importance for the lucrative abalone fishery, and depletion of urchins could lead to a collapse of abalone populations. Human exploitation of top predators that consume urchins, such as rock lobsters and some fishes, has conceivably exaggerated the role of urchins in sustaining juvenile abalone.

Predation on a common anolis lizard: can the food-web effects of a devastating predator be reversed?

Abstract: We artificially invaded a set of small islands with the large lizard Leioce- phalus carinatus to determine effects on food-web elements including an intermediate predator, the lizard Anolis sagrei; the latter was previously found to have major, mostly direct effects on web spiders, as well as detectable indirect effects on aerial arthropods (including parasitoids) and leaf damage. In addition to these food-web elements, we mon- itored ground-surface arthropods; they are expected to be affected directly, as well as indirectly via A. sagrei ,b yL. carinatus. Five islands were randomly selected for invasion while six others served as controls. In addition, four islands without A. sagrei were mon- itored to determine the natural state of the sagrei-free food web. We also monitored a variety of A. sagrei's traits, from behavioral through physiological and demographic to morphological, to elucidate the mechanisms whereby L. carinatus might affect the smaller lizard and to evaluate trait-mediated as well as density-mediated effects. Comparison of unmanipulated islands with and without A. sagrei showed that A. sagrei appeared to be affecting web-spider density and diversity, numbers of small aerial arthro- pods, number of large ground-surface arthropods, and two types of leaf damage, scars and mines. All effects were negative save one: Small aerial arthropods were more abundant with A. sagrei; this variable and parasitoid abundance were negatively related to web-spider density, implicating an indirect effect pathway from A. sagrei via web spiders. Introduction of L. carinatus had major and immediate effects on A. sagrei density and perch height; effects on perch diameter, percentage hatchlings, and the width of the body- size distribution followed over time. Behavioral shifts in habitat use continued to the end of the experiment, after changes in population density had mostly leveled off. Adult-male body condition became poorer after introduction of L. carinatus. Among islands, percentage use of the ground was correlated negatively with percentage vegetated area of the island and positively with premanipulation relative hindlimb length. We hypothesize that longer- legged individuals do better on the ground because they are faster there. Conditional ev- idence for an increase in adult-male relative hindlimb length suggested that, when the ground-inhabiting L. carinatus was introduced, such individuals were differentially able to escape predation. Introduction of L. carinatus significantly reversedA. sagrei's effect on number of species and density of web spiders but had no other major effects on lower level food-web elements. Thus, a relatively short-chained (two links) effect, with a relatively strong second link ( A. sagrei on spiders), showed the greatest indirect effect of the manipulation. Another poten- tially strong indirect effect, that on large ground-surface arthropods, did not occur, possibly because of compensatory (direct) predation by L. carinatus. Thus presence of omnivory may also be relevant to whether reversal of effects occurs. Although most A. sagrei effects were not reversed in the time available for the experiment, this may be typical, as natural L. carinatus colonizations can dwindle away to extinction over about the same time span, producing at best only short-term indirect effects. Population-density estimates from other sites and comparisons to other experimental studies suggest that, in general, predators on Anolis may have erratic effects in space and time.

Ontogenetic niche shifts and flexible behavior in size-structured populations

Abstract: Flexible behavior has been shown to have substantial effects on population dynamics in unstructured models. We investigate the influence of flexible behavior on the dynamics of a size-structured population using a physiologically structured modeling approach. Individuals of the size-structured population have a choice between living in a risky but profitable habitat and living in a safer but less profitable habitat. Each of the two habitats houses its own resource population on which the individuals feed. Two types of flexible behavior are considered: discrete habitat shifts, in which individuals instantaneously and nonreversibly shift from living in the safe habitat to the more risky/profitable habitat, and continuous habitat choice, in which individuals can continuously adapt their habitat choice to current resource/mortality conditions. We study the dynamics of the model as a function of the mortality risk in the risky/profitable habitat. The model formulation and parameterization are derived using data on Eurasian perch (Perca fluviatilis) and describe reproduction as a yearly event at the beginning of summer, while all other processes are continuous in time. The presence of two habitats per se, with unique resources that are shared among all consumers, does not change model dynamics, when compared to the one-resource situation. Flexible behavior increases the range of mortality levels for which the population can persist, because it allows individuals to hide from high mortality in the risky habitat. In contrast, flexible behavior does not significantly change the dynamics for mortality risks, where the consumer population also persists without it. Discrete habitat shifts result in model dynamics that are largely similar to the dynamics observed with continuous habitat choice, as long as individuals strongly respond to small differences in habitat profitability. In these cases, consumers spend an increasing part of their first year of life in the safe habitat, when mortality risks in the risky habitat increase. Ultimately, consumers are driven out into the risky habitat by intercohort competition from their successive year class. Therefore, major mortality and rapid growth occur among 1-yr-old individuals. Younger individuals exhibit retarded growth due to intracohort competition in the safe habitat, which may also induce large-amplitude fluctuations when the mortality risk is high in the risky habitat. With continuous habitat choice and a low responsiveness to habitat profitability, consumer persistence is increased as well, but large-amplitude fluctuations are absent. In this case, consumers always spend a significant part of their first year of life in the risky habitat, even at high mortality risks. Major mortality and rapid growth occur among individuals younger than 1 yr, while the shift to the risky habitat is mainly induced by intracohort competition for resources. The high mortality and rapid growth at younger ages lead to an increase in maximum size and fecundity of surviving individuals, as well as to larger total population biomasses. We argue that the pattern of individual habitat use is mainly determined by population feedback on resource levels.

Riparian tree seedling distribution on Wisconsin River sandbars: controls at different spatial scales

Abstract: We investigated the influence of physical characteristics at the local (quadrat) and landscape (sandbar, channel, and river cross-section) scale on the distribution of pioneer tree seedlings (Acer saccharinum, Betula nigra, Populus deltoides, and Salix spp.) on 30 sandbars along a 16-km reach of the Wisconsin River in 1998. At the landscape scale, new seedlings were most frequent in side channels that were inactive (stagnant) at low river flow, older seedlings were most frequent in narrower side channels, and saplings were most frequent on higher sandbars and in narrower channels. At the local scale, seedling occurrence in individual 1 × 2 m quadrats (n = 692) was significantly related to the horizontal and topographic position and the vegetative cover on the quadrat. Landscape relationships for species that dispersed prior to a small midsummer flood (Acer and Betula) were stronger than models for later dispersers (Populus and Salix), and models for old seedlings and saplings were stronger than those for ...

Small‐mammal foraging behavior: mechanisms for coexistence and implication for population dynamics

Abstract: We investigated predation risk and competition as they affected small-mammal foraging behavior in semiarid north-central Chile. Giving-up densities (GUD) of seeds were used to measure the foraging activity of the three most common small mammals at the site: degu (Octodon degus), Darwin's leaf-eared mouse (Phyllotis darwini), and the olivaceous field mouse (Akodon olivaceus), under shrubs (cover) and in the open on predator-excluded and competitor-excluded (Octodon) plots. Experiments were conducted during both new and full moons. Monthly small-mammal censuses using standard mark–recapture techniques provided data on movement, reproduction, and long-term fluctuations in density between 1989 and 1994. Diurnal Octodon foraged more (had lower GUD) in the absence of predators (although this was confounded by a numerical increase resulting from predator exclusion), and foraged more under shrubs than in the open. However, the lack of a significant cover × predator exclusion interaction and thermoregulation studi...