Showing papers in "Ecological Monographs in 1999"

Distance‐based redundancy analysis: testing multispecies responses in multifactorial ecological experiments

Abstract: We present a new multivariate technique for testing the significance of individual terms in a multifactorial analysis-of-variance model for multispecies response variables. The technique will allow researchers to base analyses on measures of association (distance measures) that are ecologically relevant. In addition, unlike other distance-based hypothesis-testing techniques, this method allows tests of significance of interaction terms in a linear model. The technique uses the existing method of redundancy analysis (RDA) but allows the analysis to be based on Bray-Curtis or other ecologically meaningful measures through the use of principal coordinate analysis (PCoA). Steps in the procedure include: (1) calculating a matrix of distances among replicates using a distance measure of choice (e.g., Bray-Curtis); (2) determining the principal coordinates (including a correction for negative eigenvalues, if necessary), which preserve these distances; (3) creating a matrix of dummy variables corresponding to the design of the experiment (i.e., individual terms in a linear model); (4) analyzing the relationship between the principal coordinates (species data) and the dummy variables (model) using RDA; and (5) implementing a test by permutation for particular statistics corresponding to the particular terms in the model. This method has certain advantages not shared by other multivariate testing procedures. We demonstrate the use of this technique with experimental ecological data from intertidal assemblages and show how the presence of significant multivariate interactions can be interpreted. It is our view that distance-based RDA will be extremely useful to ecologists measuring multispecies responses to structured multifactorial experimental designs.

Exotic plant species invade hot spots of native plant diversity

Abstract: Some theories and experimental studies suggest that areas of low plant spe- cies richness may be invaded more easily than areas of high plant species richness. We gathered nested-scale vegetation data on plant species richness, foliar cover, and frequency from 200 1-m 2 subplots (20 1000-m 2 modified-Whittaker plots) in the Colorado Rockies (USA), and 160 1-m 2 subplots (16 1000-m 2 plots) in the Central Grasslands in Colorado, Wyoming, South Dakota, and Minnesota (USA) to test the generality of this paradigm. At the 1-m 2 scale, the paradigm was supported in four prairie types in the Central Grasslands, where exotic species richness declined with increasing plant species richness and cover. At the 1-m 2 scale, five forest and meadow vegetation types in the Colorado Rockies contradicted the paradigm; exotic species richness increased with native-plant species richness and foliar cover. At the 1000-m 2 plot scale (among vegetation types), 83% of the variance in exotic species richness in the Central Grasslands was explained by the total percentage of nitrogen in the soil and the cover of native plant species. In the Colorado Rockies, 69% of the variance in exotic species richness in 1000-m 2 plots was explained by the number of native plant species and the total percentage of soil carbon. At landscape and biome scales, exotic species primarily invaded areas of high species richness in the four Central Grasslands sites and in the five Colorado Rockies vegetation types. For the nine vegetation types in both biomes, exotic species cover was positively correlated with mean foliar cover, mean soil percentage N, and the total number of exotic species. These patterns of invasibility depend on spatial scale, biome and vegetation type, spatial autocorrelation effects, availability of resources, and species-specific responses to grazing and other disturbances. We conclude that: (1) sites high in herbaceous foliar cover and soil fertility, and hot spots of plant diversity (and biodiversity), are invasible in many landscapes; and (2) this pattern may be more closely related to the degree resources are available in native plant communities, independent of species richness. Exotic plant in- vasions in rare habitats and distinctive plant communities pose a significant challenge to land managers and conservation biologists.

Responses of tundra plants to experimental warming:meta‐analysis of the international tundra experiment

Abstract: The International Tundra Experiment (ITEX) is a collaborative, multisite experiment using a common temperature manipulation to examine variability in species response across climatic and geographic gradients of tundra ecosystems. ITEX was designed specifically to examine variability in arctic and alpine species response to increased temperature. We compiled from one to four years of experimental data from 13 different ITEX sites and used meta-analysis to analyze responses of plant phenology, growth, and reproduction to experimental warming. Results indicate that key phenological events such as leaf bud burst and flowering occurred earlier in warmed plots throughout the study period; however, there was little impact on growth cessation at the end of the season. Quantitative measures of vegetative growth were greatest in warmed plots in the early years of the experiment, whereas reproductive effort and success increased in later years. A shift away from vegetative growth and toward reproductive effort and success in the fourth treatment year suggests a shift from the initial response to a secondary response. The change in vegetative response may be due to depletion of stored plant reserves, whereas the lag in reproductive response may be due to the formation of flower buds one to several seasons prior to flowering. Both vegetative and reproductive responses varied among life-forms; herbaceous forms had stronger and more consistent vegetative growth responses than did woody forms. The greater responsiveness of the herbaceous forms may be attributed to their more flexible morphology and to their relatively greater proportion of stored plant reserves. Finally, warmer, low arctic sites produced the strongest growth responses, but colder sites produced a greater reproductive response. Greater resource investment in vegetative growth may be a conservative strategy in the Low Arctic, where there is more competition for light, nutrients, or water, and there may be little opportunity for successful germination or seedling development. In contrast, in the High Arctic, heavy investment in producing seed under a higher temperature scenario may provide an opportunity for species to colonize patches of unvegetated ground. The observed differential response to warming suggests that the primary forces driving the response vary across climatic zones, functional groups, and through time.

Effects of resource limitation on a detrital‐based ecosystem

Abstract: We examined the importance of terrestrial detrital inputs to secondary productivity of a headwater stream. Following a year of pretreatment studies on two headwater streams, we excluded terrestrial litter inputs (=treatment) to one stream while using the other as a reference. We excluded litter for 3 yr followed by 1 yr of small woody debris (≤10 cm diameter) removal and litter exclusion. Monthly benthic samples were collected from dominant mixed substrate (cobble, pebble, and sand-silt) as well as from moss-covered bedrock outcrop substrates. We used randomized intervention analysis (RIA) to test the null hypotheses that no change in abundance or biomass of functional feeding groups or specific taxa occurred in the treatment stream relative to the reference stream. Benthic organic matter was significantly lower in mixed substrate habitats of the treatment stream; however, small woody debris did not show a significant reduction prior to manual removal during year 4. At the end of the treatment period, tot...

Genetic responses to climate in pinus contorta: niche breadth, climate change, and reforestation

Abstract: Fundamental plant–environment relationships were revealed by analyses of 20-yr height and survival of 118 populations representing two subspecies of Pinus contorta growing in common gardens at 60 environmentally disparate test sites in British Columbia. The approach involved (1) preparing models that described the general climate of British Columbia, (2) developing population-specific response functions driven by predicted climate variables, (3) developing general transfer functions that predict performance from the climatic distances over which populations were transferred, and (4) interpreting the results in terms of niche breadth, effects of climate change on adaptedness of populations, and reforestation in a changing environment. Polynomial regression models used physiographic descriptors to predict seven climate variables from normalized records of 513 weather stations. Values of R2 ranged over 0.80–0.97 for thermal variables and 0.54–0.61 for precipitation variables. Validations with independent data from 45 stations were strong and suggested that the models were generally free of bias within the limits of the original data. Response functions describing the height or survival of each population were developed from quadratic regressions using predicted climate variables for each test site. Mean annual temperature and mean temperature in the coldest month were the most effective variables for predicting population height, while the ratio of summer temperature to summer moisture was the best predictor of survival. Validation of the response functions with independent data from two additional test sites produced values of R2 between actual and predicted values that were as high as 0.93 for height and 0.73 for survival. The results demonstrated that natural populations have different climatic optima but tend to occupy suboptimal environments. Nevertheless, the general transfer functions showed that optimal growth and survival of the species as a whole is associated with the null transfer distance. These seemingly anomalous results suggest that the same processes thought to determine the distribution of species control the distribution of genotypes within species: (1) environmental selection to produce a broad fundamental niche, and (2) density-dependent selection to produce a relatively narrow realized niche within which most populations are relegated to suboptimal environments. Consequently, the steep geographic clines typical of P. contorta seem to be driven more by density-dependent selection than by environmental selection. Asymmetric gene flow from the center of distribution toward the periphery is viewed as a primary regulator that provides the fuel for both environmental and density-dependent selection and thereby indirectly perpetuates suboptimality. The response functions predict that small changes in climate will greatly affect growth and survival of forest tree populations and, therefore, that maintaining contemporary forest productivities during global warming will require a wholesale redistribution of genotypes across the landscape. The response functions also provide the climatic bases to current reforestation guidelines and quantify the adjustments necessary for maintaining adaptedness in planted trees during periods of small (∼1°C) temporal temperature shifts.

Plant removals in perennial grassland: vegetation dynamics, decomposers, soil biodiversity, and ecosystem properties

Abstract: The consequences of permanent loss of species or species groups from plant communities are poorly understood, although there is increasing evidence that individual species effects are important in modifying ecosystem properties. We conducted a field experiment in a New Zealand perennial grassland ecosystem, creating artificial vegetation gaps and imposing manipulation treatments on the reestablishing vegetation. Treatments consisted of continual removal of different subsets or “functional groups” of the flora. We monitored vegetation and soil biotic and chemical properties over a 3-yr period. Plant competitive effects were clear: removal of the C3 grass Lolium perenne L. enhanced vegetative cover, biomass, and species richness of both the C4 grass and dicotyledonous weed functional groups and had either positive or negative effects on the legume Trifolium repens L., depending on season. Treatments significantly affected total plant cover and biomass; in particular, C4 grass removal reduced total plant biomass in summer, because no other species had appropriate phenology. Removal of C3 grasses reduced total root biomass and drastically enhanced overall shoot-to-root biomass ratios. Aboveground net primary productivity (NPP) was not strongly affected by any treatment, indicating strong compensatory effects between different functional components of the flora. Removing all plants often negatively affected three further trophic levels of the decomposer functional food web: microflora, microbe-feeding nematodes, and predaceous nematodes. However, as long as plants were present, we did not find strong effects of removal treatments, NPP, or plant biomass on these trophic groupings, which instead were most closely related to spatial variation in soil chemical properties across all trophic levels, soil N in particular. Larger decomposer organisms, i.e., Collembola and earthworms, were unresponsive to any factor other than removal of all plants, which reduced their populations. We also considered five functional components of the soil biota at finer taxonomic levels: three decomposer components (microflora, microbe-feeding nematodes, predaceous nematodes) and two herbivore groups (nematodes and arthropods). Taxa within these five groups responded to removal treatments, indicating that plant community composition has multitrophic effects at higher levels of taxonomic resolution. The principal ordination axes summarizing community-level data for different trophic groups in the soil food web were related to each other in several instances, but the plant ordination axes were only significantly related to those of the soil microfloral community. There were time lag effects, with ordination axes of soil-associated herbivorous arthropods and microbial-feeding nematodes being related to ordination axes representing plant community structure at earlier measurement dates. Taxonomic diversity of some soil organism groups was linked to plant removals or to plant diversity. For herbivorous arthropods, removal of C4 grasses enhanced diversity; there were negative correlations between plant and arthropod diversity, presumably because of negative influences of C4 species in the most diverse treatments. There was evidence of lag relationships between diversity of plants and that of the three decomposer groups, indicating multitrophic effects of altering plant diversity. Relatively small effects of plant removal on the decomposer food web were also apparent in soil processes regulated by this food web. Decomposition rates of substrates added to soils showed no relationship with treatment, and rates of CO2 evolution from the soil were only adversely affected when all plants were removed. Few plant functional-group effects on soil nutrient dynamics were identified. Although some treatments affected temporal variability (and thus stability) of soil biotic properties (particularly CO2 release) throughout the experiment, there was no evidence of destabilizing effects of plant removals. Our data provide evidence that permanent exclusion of plant species from the species pool can have important consequences for overall vegetation composition in addition to the direct effects of vegetation removal, and various potential effects on both the above- and belowground subsystems. The nature of many of these effects is driven by which plant species are lost from the system, which depends on the various attributes or traits of these species.

Evolutionary divergences in leaf structure and chemistry, comparing rainfall and soil nutrient gradients

Abstract: Leaf structural and chemical characteristics were compared in pairs of species that were phylogenetically independently contrasted along rainfall gradients (10 pairs) or soil nutrient gradients (9 pairs), using perennial plants in New South Wales, Australia. The objective was to test hypotheses regarding leaf attributes that should be successful in sustaining populations in vegetation at lower vs. higher rainfall or lower vs. higher soil nutrients, and especially to assess the proposition that lower rainfall and lower soil nutrients favor similar shifts in leaf attributes, and in this sense can be grouped together as sources of “stress.” Some evolutionary divergences in leaf structure occurred repeatedly toward the lower end of both rainfall and soil nutrient gradients. These include narrower leaves, lower specific leaf area (SLA), thicker lamina, and denser leaf tissue. In other respects, the different resource gradients appeared to favor different leaf attributes. In 8 of 10 contrasts, plants of low-wat...

Impact of drought on desert shrubs : Effects of seasonality and degree of resource island development

Abstract: Large areas of semiarid grasslands in the southwestern United States have been virtually replaced by shrubs during the past century. Understanding the causes and consequences of such vegetation dynamics requires that we elucidate the interplay between external forces of change (e.g., climate, human impacts) and the internal forces within these ecosystems that foster resilience and/or stability. Several conceptual models of arid ecosystems address this interplay by including the potential role of autogenic shrub effects on ecosystem processes, which lead to the formation of “resource islands” and tend to promote shrub persistence. Specifically, during the process of shrub establishment and maturation, the cycling of nutrients is progressively confined to the zones of litter accumulation beneath shrubs, while bare intershrub spaces become increasingly nutrient poor. As shrub resource islands develop, there is increased interception and stemflow by shrub canopies, confining infiltration of nutrient-enriched ...

Climate‐related change in an intertidal community over short and long time scales

Abstract: Changes in the abundance of macroinvertebrate species documented in a rocky intertidal community between surveys in 1931-1933 and 1993-1996 are consistent with the predicted effects of recent climate warming. We resampled 57 0.84-M2 plots of an intertidal transect first surveyed by W. G. Hewatt at Hopkins Marine Station (HMS), Pacific Grove, California, between 1931 and 1933. Replicating precisely the location of the plots and methodology used by Hewatt, we documented changes in the abundances of 46 invertebrate species, indicating that this intertidal community changed significantly during the 60 yr between surveys. Changes in abundance were related to geographic ranges of species. Most southern species (10 of 11) increased in abundance, whereas most northern species (5 of 7) decreased. Cosmopolitan species showed no clear trend, with 12 increasing and 16 decreasing. Although Hewatt did not record algal species as thoroughly as inver- tebrates, we were able to document a massive decline in cover of Pelvetia compressa, a cosmopolitan fucoid alga that is typically more common in the southern part of its range. Shoreline ocean temperature, taken daily at HMS, warmed by 0.790C during this 60-yr period, with average summer temperatures up to 1.940C warmer in the 13 yr preceding our study than in the 13 yr preceding Hewatt's. The hypothesis that-climatic warming drove the observed range-related community shifts is supported further by historical records and data from other investigators. Several alternative hypotheses to explain changes in the invertebrate community at HMS, including habitat changes, anthropogenic effects, indirect biological interactions, El Nifio-Southern Oscillation (ENSO) events, and upwelling are considered to be less important than climate change. Changes in species' abundances over a short period (3 yr) were relatively small compared to large species shifts over 60 yr and were unrelated to geographic range of the species, indicating that short-term population fluctuations play a relatively minor role in the long-term community changes that we observed.

Physical-biological coupling on oyster reefs: how habitat structure influences individual performance

Abstract: A large-scale field experiment was conducted to test whether the physical structure of biogenic reef habitat controls physical conditions (hydrodynamics and hydro- graphics) with subsequent influence on the performance (recruitment, growth, and survival) of a benthic suspension feeder. The experimental system consisted of restored subtidal oyster reefs inhabited by the eastern oyster Crassostrea virginica. To determine whether the size of reefs influences the flow environment and oyster performance, reefs of four heights-tall (2 m), short (1 m), dredged (0.6 m), and low (0.1 m)-were constructed at 3-m water depth in the Neuse River estuary, North Carolina, USA. To test whether oyster performance varies with water depth and hydrographic conditions, tall and short reefs were also constructed at 6-m water depth. Flow speed, sedimentation, temperature, salinity, dissolved oxygen, and the performance of oysters were measured as a function of reef height, position on reef, and water depth over a 10-mo period. Flow speed was found to increase on reefs with reef height and elevation on reefs. Rates of sediment deposition were seasonally highest where flow speed was lowest, at the bases of reefs, and seasonally decreased with increasing water depth. More than 90% of the surface area of low reefs was buried after only 16 mo of exposure in the estuary, indicating that reef height controls habitat quality (and quantity) indirectly through its effect on flow. Short reefs and the bases of tall reefs at 6-m depth were exposed to a total of 26 d of hypoxia/anoxia. No other reef treatment was exposed to >5 d of hypoxia. Physical conditions on experimental reefs had a profound influence on the performance of oysters as the flow environment alone explained 81 % of variability in oyster growth and mortality. Recruitment of oysters over a 2-mo period was slightly higher on the front bases than the crests of reefs, but did not vary with reef height or water depth. After 10 mo, the shell growth and condition index of genetically similar, hatchery-raised oysters were greatest on the crests of tall and short reefs, where flow speed and quality of suspended food material were highest, and sediment deposition was lowest. Growth was greatest overall at the crests of tall reefs located at 6-m water depth where flow speed was high, and the numbers of days exposed to hypoxia/anoxia and variation in salinity were lowest. Total percentage mortality of oysters after 10 mo was greater on low reefs located at 3-m depth than on all other reef types and was greater on the bases than crests of tall, short, and dredged reefs. Predation by crabs and fishes accounted for 4-20% of total oyster mortality and showed no pattern across reef treatments. Results of this experiment indicate (1) that the physical structure and location of biogenic habitat controls local physical variables and (2) that, in turn, physical variables, especially flow speed, have a profound influence on the performance of a resident species. Realization that an ecological function of habitat is to indirectly control local population production through physical-biological coupling should improve our ability to conserve, restore, and manage habitat and associated species diversity. Better ecological engineering of restored oyster reef habitat is likely to improve fishery production and help maintain estuarine biodiversity.

Temporal and spatial scales of kelp demography: the role of oceanographic climate

Abstract: This paper integrates long-term descriptive and experimental studies of the effects of ocean climate on inter- and intraspecific competition, as expressed by recruitment, density, survivorship, growth, and reproduction of the most conspicuous kelp species in the Point Loma kelp forest community off San Diego, California, USA. The species included Macrocystis pyrifera,with a floating canopy; Pterygophora californicaand Eisenia arborea, which rely on stipes to support their canopy; Laminaria farlowii, with a prostrate canopy; and a speciose red algal turf. To evaluate the roles of large-scale oceanographic processes on biological processes across important depth gradients, the study was carried out over nine years during a cold-water, nutrient-rich La Nina event (1988-1989) and a warm-water, nutrient-stressed El Nino period (1992-1994), over a depth range of 8-23 m. This depth range encompassed strong physical gradients involving factors that are critical for kelp growth, including bottom temperatures (correlated with nutrients) and light levels. To examine interactions among these kelps, we established clearings across the depth gradient and then manipulated Macrocystis recruit densities. The demographic responses offer an understanding of the ''fundamental'' vs. ''realized'' niches of these species. Eval- uating these patterns, as they are influenced by inter- and intraspecific competition, offers insights into the ''realized niches'' of the kelps. With the exception of some understory effects on Macrocystis recruitment and some evidence of intraspecific competition during the nutrient-rich La Nina conditions, we found little influence of competitive effects on Macrocystis. The response of Pterygophora to manipulations and disturbances suggests light-limited recruitment, and competition with Macrocystis was exhibited via reduced growth and reproduction, but not survivorship. No nutrient stress was observed in Ptery- gophora reproduction. Eisenia recruitment is rare, but once established, juveniles had very good survivorship, with growth and reproduction reduced by depth; the Macrocystis treat- ment was more important than depth, suggesting the importance of light to Eisenia re- cruitment and growth. In general, Macrocystis had massive effects on Laminaria growth and reproduction, the strength varying with depth. In particular, there were very strong effects of competition with Macrocystis during the nutrient-rich La Nina period when Macrocystis had a dense surface canopy. In addition to the Macrocystis effects, there were some significant Pterygophora effects on Laminaria growth during El Nino. The strongest biological definition of realized niches occurred during the nutrient-rich La Nina period, especially in shallow depths. One of the most important conclusions of this paper is the appreciation of the importance of scaling in time to include oceanographic climate. There are many seasonal patterns, but the interannual scales that encompass El Ninos and La Ninas, and ultimately the interdecadal-scale oceanographic regime shifts that affect the intensity of canopy competition with Macrocystis, are critical for this system because surface-water nutrients have pervasive long-term effects on the other kelps. Small- scale patterns are driven by local processes (competition, disturbance, dispersal, etc.) that potentially are important at larger scales; however, our most lasting effects result from very large-scale, low-frequency episodic changes in nutrients, with cascading competitive con- sequences to the other algal populations in the community.

Fire history in northern patagonia: the roles of humans and climatic variation

Abstract: The effects of humans and climatic variation on fire history in northern Patagonia, Argentina, were examined by dating fire scars on 458 trees at 21 sites in rain forests of Fitzroya cupressoides and xeric woodlands of Austrocedrus chilensis from 39° to 43° S latitude. Climatic variation associated with fires was analyzed on the basis of 20th-century observational records and tree ring proxy records of climatic variation since approximately AD 1500. In the Austrocedrus woodlands, fire frequency increases after about 1850, coincident with greater use of the area by Native American hunters. Increased burning, particularly in the zone of more mesic forests, is also strongly associated with forest clearing by European settlers from about 1880 to the early 1900s. The marked decline in fire frequency during the 20th century coincides with both the demise of Native American hunters in the 1890s and increasingly effective fire exclusion. Strong synchroneity in the years of widespread fire at sample sites disperse...

Density‐dependent processes in structured fish populations: interaction strengths in whole‐lake experiments

Abstract: The goal of this study is to identify the mechanisms and measure the strengths of interactions within and among size classes in experimental populations of rainbow trout, Onchoranchus mivkiss. The metric that we used to assess the density-dependent effects was based on consumptive allometry and predator-prey theory. We demonstrate that the inter- actions among size classes were asymmetrical, favoring larger-bodied individuals. Descrip- tions of diet and spatial resource use, measures of prey availability, and risk to intra-specific interactions allowed assessment of the relative contributions of exploitative and interference competitive interactions among size classes. Growth of the larger classes was strongly density-dependent and driven primarily by exploitative competition. Growth of the smallest size class was controlled by a combination of exploitative competition within and among size classes and interference competition with larger-bodied conspecifics. This combination of interactions among size classes within populations resulted in a body-size-based asymmetry favoring the larger size classes. Sur- vival of all size classes was positively related to both body size and growth rate. We speculate that the net result of these processes within size-structured populations is compensatory, leading to stable population dynamics.

Top‐down and bottom‐up regulation of new zealand rocky intertidal communities

Abstract: Studies on the west coast of North America suggest that nearshore oceanographic conditions can have important effects on rocky intertidal community structure and dynamics. Specifically, upwelling-dependent processes in coastal waters can affect both top-down and bottom-up processes on adjacent rocky shores. As a first step in testing the prediction that similar linkages occur elsewhere, we investigated the effects and rates of predation, grazing, and recruitment on rocky intertidal community dynamics at upwelling and non-upwelling sites on the South Island of New Zealand. Comparative-experimental studies were done at each of two sites on both the east and west coasts of the South Island. We quantified benthic community structure, maximal wave force, nearshore sea-surface temperature, air temperature at low tide, nutrient concentrations, survival of mussels, rates and effects of predation, rates and effects of limpet grazing, recruitment of mussels and barnacles, and RNA:DNA ratios (a growth index) of muss...

Theory of marine communities: competition, predation, and recruitment-dependent interaction strength

Abstract: Of the marine animals that spend their adult lives inhabiting benthic communities, most have a planktonic larval phase. In this paper, we derive the relationship between the physical oceanographic processes that transport these larvae and the strength of species interactions in the benthic habitat. We review a model of hierarchical competition for space between two species with planktonic larvae and develop a model for predator–prey dynamics in which prey are space-limited. Lotka-Volterra approximations to these models are developed. The approximations provide per capita interaction strength (the effect of an individual of one species on the per capita growth rate of another) and population interaction strength (the effect of a population of one species on the per capita growth rate of another) as functions of parameters in the original model. Per capita and population interaction strengths of dominant competitors on subordinates decrease in magnitude as offshore advection of larvae increases. The per cap...

Gradients in the composition, structure, and diversity of remnant oak savannas in southern wisconsin

Abstract: Savannas with scattered oak canopies, once the most widespread communities in southern Wisconsin, now are among the most endangered. Surprisingly little is known about the composition, structure, and horizontal patterning of their species-rich ground layers. This study relates the distribution and ecological characteristics of 417 ground-layer species to local and regional gradients in soil composition and light regime, based on an analysis of 722 1-m2 quadrats in 12 remnant savannas. Our findings have important implications for efforts to restore/conserve midwestern oak savannas. Ground-layer composition was strongly related to among-site differences in soil texture and within-site differences in light availability, with variation in sand vs. silt content accounting for twice as much turnover in species composition as that accounted for by direct photon flux density (PFD) estimated from hemispherical photographs. Most species reached peak coverage under sunny or partly shaded conditions. Flowering/fruiting was often skewed toward sunnier microsites. Absolute forb cover increased with silt content and declined with PFD. Graminoid cover showed a curvilinear relationship to soil texture and light, being highest in well-lit, moderately sandy microsites. Total ground-layer cover increased with silt content at a given irradiance; it increased with PFD on silty sites and decreased with PFD on sandy sites. Forb cover increased regularly with PFD and sand content. When quadrats were stratified by sand content and PFD, species richness of forbs and graminoids increased linearly with coverage by each group, with far more forb species present at a given coverage. Among graminoids, C4 grasses were common only in bright, sandy microsites; C3 grasses and sedges had broader ecological distributions. Among forbs, leaf width increased and leaf inclination became more horizontal toward shadier and siltier microsites; tall herbs were generally found in silty areas with dense ground-layer cover. Plants with N-fixing symbioses were found mostly on sandy, well-lit microsites, although climbing species occured on shadier and/or siltier microsites, where N-fixation might be expected to be less advantageous. Most ground-layer species were perennial (88%), with few annuals (6%) or biennials (6%). For the 85 most common species, breadth of distribution across savanna microhabitats (4 soil × 4 light) was significantly correlated with presence across 34 Wisconsin community types, suggesting that similar factors help to constrain distributions at local and regional scales. Species in the two largest genera (Aster and Solidago) differed significantly in distribution according to the Syrjala test, supporting habitat partitioning as a mechanism of coexistence. Oak savannas are unusually diverse. At small spatial scales, there were 16.1 ± 1.3 species/m2 (mean ± 1 sd), compared with 11.4 ± 2.7 for prairies and 8.2 ± 2.5 for forests. At slightly larger scales, savannas showed high spatial turnover in ground-layer composition: 89.3 ± 12.0 species/20 m2, compared with 41.5 ± 8.3 for prairies and 42.8 ± 8.0 for forests. At large spatial scales, a survey of 22 savanna remnants (42 ha) showed extensive floristic differentiation: 507 native plant species, ∼27% of Wisconsin's indigenous vascular flora (∼14 × 106 ha). Contrary to previous reports, Midwest oak savannas are forb dominated, except on the sandiest or sunniest microsites. Release of forbs from competition with C4 grasses under partly shaded conditions may help to account for the high diversity of savanna ground layers relative to prairies. Divergent distributions of plants with different adaptations for energy capture, together with large variation within sites in ground-layer light regime and among sites in soil texture, suggest that partitioning of light and soil gradients is important for maintaining the high plant diversity of oak savannas. Mass effects (involving dispersal subsidies from favorable microsites) may also play a role in maintaining diversity within these mosaic communities.

Preference–performance relationships and effects of host plant choice in an herbivorous marine amphipod

Abstract: The fitness of herbivores that have limited mobility as juveniles or larvae may often depend on the host choice behavior of adults. For such herbivores, selection should favor adults that choose plants that maximize the performance of their offspring, resulting in positive correlations across host plants among adult preferences, offspring performance (growth, survival, etc.), and for herbivores that are restricted to living on host plants, population-level parameters such as abundance on different hosts. We tested this hypothesis for the marine, nest-building amphipod Peramphithoe parmerong, using a series of behavioral and performance assays and relevant field data. Adults displayed strong preferences among eight species of brown algae in habitat choice assays, with Sargassum linearifolium and S. vestitum highly preferred; Colpomenia peregrina and Padina crassa of lower preference; and Dictyopteris acrostichoides, Dictyota dichotoma, Dilophus marginata, and Zonaria diesingiana consistently avoided. Juvenile amphipods were relatively immobile and, thus, mostly constrained to the host alga selected by their mother. Differences in the growth and survival of juvenile amphipods raised on single-species diets were consistent with adult nest-building preferences among algae, with the best performance on the two high-preference species of Sargassum. Thus, adult preferences for host plants largely determined juvenile performance due to (1) restricted movement by juveniles and (2) differences among algal species in their effects on growth, survival, and onset of reproduction. In contrast to host plant preferences by adults, feeding rates on different algae were not as clearly correlated with juvenile performance. In particular, the low-preference C. peregrina was consumed at a high rate, but survivorship on this alga was relatively poor. Differences in abundances of P. parmerong on different host algae in the field were consistent with laboratory preferences and not related to relative algal abundance or epiphyte loads, supporting the hypothesis that there were population-level consequences of host plant choice in this species. With regard to mechanisms underlying host use, amphipod preferences were closely (negatively) correlated with the presence of nonpolar secondary metabolites. Nonpolar crude extracts from nonhost species, all of which contain secondary metabolites, affected amphipod host acceptance behavior, whereas extracts from host species, all of which lack such metabolites, did not. Overall, host plant use was unrelated to the nutritional value of the algal species, but one low-preference host, C. peregrina, was notably nutritionally poor. These observations, and the correlations among adult preferences, offspring performance, and field densities for P. parmerong, are consistent with intrinsic host plant qualities determining host plant range for this species. This contrasts with previous studies that emphasized the importance of extrinsic factors, particularly predation, in determining amphipod host use.

Facilitation of conspecific seedling recruitment and shifts in temperate savanna ecotones

Abstract: In contrast to documented increases in woody plant dominance of savannas and grasslands of North America, oak (Quercus L.) savannas that form lower tree lines in the southwestern United States and northwestern Mexico have been stable over the last several centuries. We sought to identify potential biotic and abiotic constraints on seedling recruitment of Quercus emoryi within the context of potential shifts in lower tree line. We used field surveys to describe seedling distribution at and below lower tree line, and to determine the potential for acorn dispersal from lower tree line into adjacent grassland. Field and greenhouse experiments were used to test explanatory hypotheses generated by descriptive surveys. Q. emoryi seedlings were located almost exclusively beneath mature, conspecific tree canopies within the woodland and savanna and were absent from adjacent semidesert grassland in 1993 and 1995. Seed bank surveys indicated that acorns were concentrated beneath tree canopies and were dispersed into adjacent grassland in low numbers. Although soil N, C, and P were about two times greater beneath trees than in adjacent grassland, experimental nutrient amendments to subcanopy and grassland soils indicated that soil nutrients did not limit Q. emoryi growth. Reciprocal transfers of subcanopy and grassland soil to subcanopy and grassland microsites indicated that microsite was more important than soil source for seedling growth. Overstory shade was important at all stages of seedling development investigated: the provision of artificial or natural shade increased rates of seedling emergence and subsequent survival as much as 19-fold and increased recruitment rates between 30- and 60-fold. We conclude that rates of Q. emoryi recruitment within grasslands below tree line are relatively low and are constrained by low rates of seed dispersal coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where they have a higher probability of emergence than in adjacent grassland. Survival rates of emerged seedlings were low, regardless of landscape position. Thus, observed patterns of seedling distribution on the landscape resulted from interactions between seed dispersal and habitat-specific response of seedlings to environmental variation. Results of this and complementary research suggest that the lower tree line in southern Arizona is stabilized by self-enhancing feedback mechanisms of overstory shade, seed dispersal, and seedling establishment, coupled with strong abiotic constraints beyond the current ecotone. These processes stabilize the woodland–grassland ecotone both spatially and temporally, consistent with Wilson and Agnew's one-sided positive feedback switch. Although this switch would not produce an indefinitely stable vegetation mosaic, upslope or downslope shifts in lower tree line are apparently resistant to decadal or even century-scale climatic perturbation. The observed shift in tree line in the last millennium was less likely the result of slow, spatial progression of autogenic safe sites than the result of episodic and infrequent allogenic processes that simulated or negated the importance of conspecific, biogenic safe sites.

Individual‐based model of yellow perch and walleye populations in oneida lake

Abstract: Predator–prey dynamics and density dependence are fundamental issues in ecology. We use a detailed, individual-based model of walleye and yellow perch to investigate the effects of alternative prey and compensatory responses on predator and prey population dynamics. Our analyses focus on the numerical and developmental responses of the predator, rather than the traditional emphasis on functional responses. The extensive database for Oneida Lake, New York, USA was used to configure the model and ensure its realism. The model follows the daily growth, mortality, and spawning of individuals of each species through their lifetime. Three ecologically distinct periods in the history of Oneida Lake were simulated: baseline, high mayfly densities, and high forage fish densities. Mayflies and forage fish act as alternative prey for walleye. For model corroboration, the three periods were simulated sequentially as they occurred in Oneida Lake. Model predictions of abundances, size at age, and growth and survival ra...

Settlement vs. environmental dynamics in a pelagic-spawning reef fish at caribbean panama

Abstract: How are the dynamics of the settlement of pelagic larvae of marine shore organisms into benthic habitats affected by larval production and mortality dynamics, and by environmental (wind and tidal) control of settler delivery? We examined the dynamics of reproduction of the pelagic-spawning fish Thalassoma bifasciatum from that perspective at San Blas, Caribbean Panama. During April–June 1993, ∼75% of females spawned each day, with mean output per female per day at about one-third the potential maximum. Output peaked several days before the new and full moon, when tides were high during the afternoon spawning period. Daily fluctuations in output were weakly correlated with light (+), swell (−), and onshore wind (−) on preceding days. Settler production (determined from otolith back calculations of settlers' fertilization dates) peaked during the wet season (May–November), when winds were light and variable, spawning-period tidal heights were moderate to low, and tidal flows were off-reef at night. These conditions correspond to when most settlement occurs. Settler production was minimal during the dry season (January–March), when onshore trade winds were strong and consistent, and tidal conditions were the reverse of those in the wet season. Settlement usually occurred in one variably timed pulse per lunar cycle, around the new moon. Variation in monthly settlement strength over 11 yr (1986–1996) was unrelated to onshore wind stress during either settler arrival or settler production. The daily dynamics of settlement were unrelated to the concurrent dynamics of wind or tides, or wind dynamics up to a week before settlement. Settler production was much less continuous than spawning. Wet season settler-production dynamics were unrelated to tidal dynamics, but (weakly) negatively related to onshore wind stress. Although the dynamics of settler production and settlement were similar, bouts of settler production were about twice as long as bouts of settlement. The average lunar patterns of settler production and observed spawning were similar (weakly bimodal). Settlers produced during different lunar phases differed in the length and variability of their pelagic larval durations (PLDs). Fish with different PLDs all tended to settle around the new moon. Weakly semilunar spawning may reflect effects of weak tidal influences on egg survivorship, or of environmental constraints on adults' spawning capacity that also limit spawning to well below the potential maximum. Dry season trade winds may produce the seasonal low of settlement by depressing spawning or forcing the onshore transport of eggs and larvae. If settlement is transport-dependent, weak variable winds may have little effect on currents during the wet season, or local geography may limit such effects; tidal dynamics may not affect wet-season settlement dynamics because small (<0.6 m) tides produce weak flows. Alternatively, settlement may not be dependent on transport mechanisms. Short-term settlement dynamics result from both the dynamics of larval survival (as most larvae have near-mean PLDs) and larvae delaying or advancing settlement to settle around the new moon. Larvae spawned at different lunar phases have different patterns of variation in their PLDs but achieve similar relative settlement success. Larvae with extended PLDs may not suffer low settlement success because the inevitable age-dependent decline in survival has smaller effects on relative settlement success than does large monthly variation in the survivorship of fish spawned during different lunar phases.

Early post-settlement mortality in a coral reef fish and its effect on local population size

Abstract: The relative importance of recruitment and post-recruitment factors in controlling population fluctuations and determining population sizes in open marine systems is still poorly understood. To address this issue, I conducted an experiment investigating whether very early post-recruitment mortality of a coral reef fish, Thalassoma bifasciatum, is dependent on recruitment density. To quantify the extent to which local populations are likely to be controlled primarily by recruitment or by post-settlement density-dependent mortality, I also measured natural mortality rates and recruitment variation at sites around the island of St. Croix, U.S. Virgin Islands. I manipulated densities of 1–3-d post-settlement T. bifasciatum on large natural patch reefs in Tague Bay, St. Croix, U.S. Virgin Islands. Manipulated recruits were followed for ∼30 d. The loss rate over the initial 24-h period following placement of recruits on the reef was positively related to the density at the beginning of that period. After this period, initial recruit density had no effects on mortality. Natural monthly mortality rates approximating the monthly mortality monitored during the experiment were measured for naturally recruited fish at sites around the island. Mortality was positively correlated with recruitment density. The effect was strong enough that across sites the relationship between recruit density and juvenile density the following month was not proportional; that is, juvenile density tended to reach an asymptote with increasing recruitment. The relationship between total recruitment per season at a site and adult abundance at the end of the season was similarly asymptotic, indicating the persistence of a density-dependent effect at the scale of the island. Mortality rates around the island were extremely variable and density independent at densities below ∼1 recruit/m2. Above that level, mortality rates were higher and tended to increase with further increases in recruitment density. Around St. Croix, three out of eight sites consistently received recruitment above this “threshold,” while five sites never experienced recruitment above that level during the study. Because spatial patterns of recruitment around St. Croix were consistent over time, some local populations may be controlled mainly by recruitment and density-independent mortality at all times, while others may be chronically affected by post-settlement density dependence. This study gives support to the idea that recruitment limitation and density dependence are not mutually exclusive. Rather, local populations must be characterized by the degree of recruitment, the resultant population density, and the subsequent degree of density-dependent regulation.

Coexistence of similar species in a space-limited intertidal zone

Abstract: The lower intertidal zone (0.0 to +1.0 m mean low water [MLW]) of rocky shores in New England is a space-limited community occupied by two similar rhodophyte seaweeds, Chondrus crispus and Mastocarpus stellatus, that overlap broadly in their use of three essential resources: space, light, and nutrients. C. crispus coexists primarily with the prostrate-crust generation of M. stellatus lower on the shore (less than +0.35 m MLW) and with the alternate upright-frond generation higher on the shore (greater than +0.35 m MLW). Our objectives were to determine (1) whether these two species compete and (2) if so, what process(es) enable their coexistence. Upright fronds of M. stellatus transplanted to the lowest intertidal zone (less than +0.25 m MLW) where C. crispus predominates grew faster and showed higher survivorship after 2 yr than those transplanted to areas where M. stellatus predominates. However, the failure of upright fronds of M. stellatus to consistently recruit limits their abundance in the lowest z...

Population dynamics of fraxinus nigra in response to flood-level variations, in northwestern quebec

Abstract: Population dynamics of black ash (Fraxinus nigra) were investigated in five stands with different exposures to spring flooding at the southwestern limit of Quebec's boreal forest. Fifty-four 100-M2 plots were sampled along 25 transects laid out perpendicular to Lake Duparquet. In each plot, all black ash >5 cm in diameter at breast height (dbh) were aged using dendrochronological methods. An age-height regression was used to cal- culate the age of those individuals <5 cm in dbh. Stump sprouts had a significantly faster height and diameter growth rate than individuals established from sexual origin. Two stands in this study showed deficits in sexual recruitment. One stand, located on a deep organic layer, also showed recent colonization by F. nigra that may have started during the 1930s. This stand had the highest stem density, and sprouting was restricted to the younger age classes. Ordination analysis of dispersion indices characterizing the age distribution of F. nigra indicated a definite pattern toward higher sexual recruitment in plots and stands that were less exposed to flooding. In contrast, poor sexual recruitment was observed on more exposed sites. However, this situation was compensated for by abundant vegetative re- cruitment. Analysis of the periods of growth release and suppression of 153 trees revealed that F. nigra population dynamics are not regulated by major disturbance events. A general upward trend in radial growth and F. nigra expansion toward higher elevation sites observed since the end of the 19th century may be due to changes in Lake Duparquet's hydrological regime. Increases in the severity of flooding events may result in the decrease in seedling abundance observed in the more exposed stands. F. nigra populations were found to be resistant to long-term natural water fluctuations due to their ability to regenerate both by seeds and by sprouts.