Showing papers in "Ecological Monographs in 2001"

A method for scaling vegetation dynamics: the ecosystem demography model (ed)

Abstract: The problem of scale has been a critical impediment to incorporating im- portant fine-scale processes into global ecosystem models. Our knowledge of fine-scale physiological and ecological processes comes from a variety of measurements, ranging from forest plot inventories to remote sensing, made at spatial resolutions considerably smaller than the large scale at which global ecosystem models are defined. In this paper, we describe a new individual-based, terrestrial biosphere model, which we label the eco- system demography model (ED). We then introduce a general method for scaling stochastic individual-based models of ecosystem dynamics (gap models) such as ED to large scales. The method accounts for the fine-scale spatial heterogeneity within an ecosystem caused by stochastic disturbance events, operating at scales down to individual canopy-tree-sized gaps. By conditioning appropriately on the occurrence of these events, we derive a size- and age-structured (SAS) approximation for the first moment of the stochastic ecosystem model. With this approximation, it is possible to make predictions about the large scales of interest from a description of the fine-scale physiological and population-dynamic pro- cesses without simulating the fate of every plant individually. We use the SAS approxi- mation to implement our individual-based biosphere model over South America from 15 8 Nt o1 5 8S, showing that the SAS equations are accurate across a range of environmental conditions and resulting ecosystem types. We then compare the predictions of the biosphere model to regional data and to intensive data at specific sites. Analysis of the model at these sites illustrates the importance of fine-scale heterogeneity in governing large-scale eco- system function, showing how population and community-level processes influence eco- system composition and structure, patterns of aboveground carbon accumulation, and net ecosystem production.

Trophic cascades, nutrients, and lake productivity: whole‐lake experiments

Abstract: Responses of zooplankton, pelagic primary producers, planktonic bacteria, and CO2 exchange with the atmosphere were measured in four lakes with contrasting food webs under a range of nutrient enrichments during a seven-year period. Prior to enrichment, food webs were manipulated to create contrasts between piscivore dominance and planktivore dominance. Nutrient enrichments of inorganic nitrogen and phosphorus exhibited ratios of N:P > 17:1, by atoms, to maintain P limitation. An unmanipulated reference lake, Paul Lake, revealed baseline variability but showed no trends that could confound the interpretation of changes in the nearby manipulated lakes. Herbivorous zooplankton of West Long Lake (piscivorous fishes) were large-bodied Daphnia spp., in contrast to the small-bodied grazers that predominated in Peter Lake (planktivorous fishes). At comparable levels of nutrient enrichment, Peter Lake's areal chlorophyll and areal primary production rates exceeded those of West Long Lake by factors of approximatel...

Introduced browsing mammals in new zealand natural forests: aboveground and belowground consequences

Abstract: Forest dwelling browsing mammals, notably feral goats and deer, have been introduced to New Zealand over the past 220 years; prior to this such mammals were absent from New Zealand. The New Zealand forested landscape, therefore, presents an almost unique opportunity to determine the impacts of introduction of an entire functional group of alien animals to a habitat from which that group was previously absent. We sampled 30 long-term fenced exclosure plots in indigenous forests throughout New Zealand to evaluate community- and ecosystem-level impacts of introduced browsing mammals, emphasizing the decomposer subsystem. Browsing mammals often significantly altered plant community composition, reducing palatable broad-leaved species and promoting other less palatable types. Vegetation density in the browse layer was also usually reduced. Although there were some small but statis- tically significant effects of browsing on some measures of soil quality across the 30 locations, there were no consistent effects on components of the soil microfood web (com- prising microflora and nematodes, and spanning three consumer trophic levels); while there were clear multitrophic effects of browsing on this food web for several locations, com- parable numbers of locations showed stimulation and inhibition of biomasses or populations of food web components. In contrast, all microarthropod and macrofaunal groups were consistently adversely affected by browsing, irrespective of trophic position. Across the 30 locations, the magnitude of response of the dominant soil biotic groups to browsing mammals (and hence their resistance to browsers) was not correlated with the magnitude of vegetation response to browsing but was often strongly related to a range of other variables, including macroclimatic, soil nutrient, and tree stand properties. There were often strong significant effects of browsing mammals on species composition of the plant community, species composition of leaf litter in the litter layer, and composition of various litter-dwelling faunal groups. Across the 30 locations, the magnitude of browsing mammal effects on faunal community composition was often correlated with browser effects on litter layer leaf species composition but never with browser effects on plant community composition. Browsing mammals usually reduced browse layer plant diversity and often also altered habitat diversity in the litter layer and diversity of various soil faunal groups. Across the 30 locations, the magnitude of browser effects on diversity of only one faunal group, humus-dwelling nematodes, was correlated with browser effects on plant diversity. However, browser effects on diversity of diplopods and gastropods were correlated with browser effects on habitat diversity of the litter layer. Reasons for the lack of unidirectional relationships across locations between effects of browsers on vegetation community attri- butes and on soil invertebrate community attributes are discussed. Browsing mammals generally did not have strong effects on C mineralization but did significantly influence soil C and N storage on an areal basis for several locations. However the direction of these effects was idiosyncratic and presumably reflects different mechanisms by which browsers affect soil processes. While our study did not support hypotheses predicting consistent negative effects of browsing mammals on the decomposer subsystem through promotion of plant species with poorer litter quality, our results still show that the introduction of these mammals to New Zealand has caused far-ranging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant com- munities and populations of most groups of litter-dwelling mesofauna and macrofauna.

Resistance and resilience of alpine lake fauna to fish introductions

Abstract: This paper reports on the response by amphibians, benthic macroinverte- brates, and zooplankton in naturally fishless alpine lakes to fish introductions and subsequent fish disappearance. We assessed resistance (the degree to which a system is altered when the environment changes) by comparing faunal distribution and abundance in lakes that have never been stocked with fish vs. the distribution and abundance in lakes that have been stocked and still contain fish. We assessed resilience (the degree and rate of a system's return to its previous configuration once the perturbation is removed) by comparing faunal distribution and abundance in lakes that were stocked at one time but have since reverted to a fishless condition (stocked-now-fishless lakes) vs. the distribution and abundance in lakes that have never been stocked. We quantified recovery rates and trajectories by com- paring faunal assemblages of stocked-now-fishless lakes that had been fishless for 5-10, 11-20, and .20 yr. Faunal assemblages in the study lakes had low resistance to fish introductions, but in general showed high resilience. The mountain yellow-legged frog (Rana muscosa), con- spicuous benthic macroinvertebrates, and large crustacean zooplankton (.1 mm) were dramatically reduced in distribution and abundance by fish introductions but generally recovered to predisturbance levels after fish disappearance. Inconspicuous benthic inver- tebrate taxa, small crustacean zooplankton (,1 mm), and rotiferan zooplankton (,0.2 mm) were either unaffected by fish or increased in the presence of fish. For both the benthic macroinvertebrate community and the zooplankton community as a whole, fish disappear- ance was followed by a steady change away from the configuration characteristic of fish- containing lakes and toward that of lakes that had never been stocked. Both communities remained markedly different from those in never-stocked lakes 5-10 yr after fish disap- pearance and converged on the configuration of never-stocked lakes only 11-20 yr after fish disappearance. Recovery was likely facilitated by the winged adult stages of many benthic macroin- vertebrates, resting eggs of zooplankton, and nearby source populations of frogs. However, many frog populations have disappeared since the time that lakes in this study reverted to a fishless condition, and the viability of zooplankton egg banks should decline in fish- containing lakes over time. As a result, faunal resilience may be lower in lakes that revert to a fishless condition today than is suggested by the results of our study. These findings have important implications for the restoration of alpine lake ecosystems.

Temporal and spatial variation in environmental impact assessment

Abstract: We compare two approaches to designing and analyzing monitoring studies to assess chronic, local environmental impacts. Intervention Analysis (IA) compares Before and After time series at an Impact site; a special case is Before–After, Control–Impact (BACI), using comparison sites as covariates to reduce extraneous variance and serial correlation. IVRS (impact vs. reference sites) compares Impact and Control sites with respect to Before–After change, treating the sites as experimental units. The IVRS estimate of an “effect” is the same as that of the simplest BACI (though not of others), but IVRS estimates error variance by variation among sites, while IA and BACI estimate it by variation over time. These approaches differ in goals, design, and models of the role of chance in determining the data. In IA and BACI, the goal is to determine change at the specific Impact site, so no Controls are needed. IA does not have controls and BACI's are not experimental controls, but covariates, deliberately chosen to ...

Landscape‐scale changes in plant species abundance and biodiversity of a sagebrush steppe over 45 years

Abstract: Increasing demands on arid and semiarid ecosystems, which comprise one-third of Earth's terrestrial environment, create an urgent need to understand their biodiversity, function, and mechanisms of change. Sagebrush (Artemisia) steppe, the largest semiarid vegetation type in North America, is endangered because of losses to agriculture, excessive grazing, and invasive species. Establishment in 1950 of what is now designated as the Idaho National Engineering and Environmental Laboratory (southeastern Idaho, USA) created the largest existing reserve of this extensive vegetation type. We used cover, density, and frequency data for vascular plants sampled on 79 permanent plots nine times during 45 years to (1) assess long-term changes in abundance and distribution of major species and life forms, (2) assess changes in species richness and plot similarity, and (3) test the hypotheses that plant cover and stability of cover are positively associated with species richness and that invasibility is inversely relate...

Linear home ranges: effects of smoothing, sample size, and autocorrelation on kernel estimates

Abstract: Simulations are necessary to assess the performance of home-range estimators because the true distribution of empirical data is unknown, but we must question whether that performance applies to empirical data. Some studies have used empirically based simulations, randomly selecting subsets of data to evaluate estimator performance, but animals do not move randomly within a home range. We created an empirically based simulation using a behavioral model, generated a probability distribution from those data, and randomly selected locations from that distribution in a chronological sequence as the simulated individual moved through its home range. Thus, we examined the influence of temporal patterns of space use and determined the effects of smoothing, number of locations, and autocorrelation on kernel estimates. Additionally, home-range estimators were designed to evaluate species that use space with few restrictions, traveling almost anywhere on the landscape. Many species, however, confine their movements ...

Fossil evidence for a diverse biota from kaua‘i and its transformation since human arrival

Abstract: Coring and excavations in a large sinkhole and cave system formed in an eolianite deposit on the south coast of Kaua‘i in the Hawaiian Islands reveal a fossil site with remarkable preservation and diversity of plant and animal remains. Radiocarbon dating and investigations of the sediments and their fossil contents, including diatoms, invertebrate shells, vertebrate bones, pollen, and plant macrofossils, provide a more complete picture of prehuman ecological conditions in the Hawaiian lowlands than has been previously available. The evidence confirms that a highly diverse prehuman landscape has been completely transformed, with the decline or extirpation of most native species and their replacement with introduced species. The stratigraphy documents many late Holocene extinctions, including previously undescribed species, and suggests that the pattern of extirpation for snails occurred in three temporal stages, corresponding to initial settlement, late prehistoric, and historic impacts. The site also records land-use changes of recent centuries, including evidence for deforestation, overgrazing, and soil erosion during the historic period, and biological invasion during both the Polynesian and historic periods. Human artifacts and midden materials demonstrate a high-density human presence near the site for the last four centuries. Earlier evidence for humans includes a bone of the prehistorically introduced Pacific rat (Rattus exulans) dating to 822 yr BP (calendar year [cal yr] AD 1039–1241). Vegetation at the site before human arrival consisted of a herbaceous component with strand plants and graminoids, and a woody component that included trees and shrubs now mostly restricted to a few higher, wetter, and less disturbed parts of the island. Efforts to restore lowland areas in the Hawaiian Islands must take into account the evidence from this study that the prehuman lowlands of dry leeward Kaua‘i included plants and animals previously known only in wetter and cooler habitats. Many species may be restricted to high elevations today primarily because these remote locations have, by virtue of their difficult topography and climate, resisted most human-induced changes more effectively than the coastal lowlands.

Landscape and regional impacts of hurricanes in new england

Abstract: Hurricanes are a major factor controlling ecosystem structure, function, and dynamics in many coastal forests, but their ecological role can be understood only by assessing impacts in space and time over a period of centuries. We present a new method for reconstructing hurricane disturbance regimes using a combination of historical research and computer modeling. Historical evidence of wind damage for each hurricane in the selected region is quantified using the Fujita scale to produce regional maps of actual damage. A simple meteorological model (HURRECON), parameterized and tested for selected recent hurricanes, provides regional estimates of wind speed, direction, and damage for each storm. Individual reconstructions are compiled to analyze spatial and temporal patterns of hurricane impacts. Long-term effects of topography on a landscape scale are then simulated with a simple topographic exposure model (EXPOS). We applied this method to the region of New England, USA, examining hurricanes since European settlement in 1620. Results showed strong regional gradients in hurricane frequency and intensity from southeast to northwest: mean return intervals for F0 damage on the Fujita scale (loss of leaves and branches) ranged from 5 to 85 yr, mean return intervals for F1 damage (scattered blowdowns, small gaps) ranged from 10 to >200 yr, and mean return intervals for F2 damage (extensive blowdowns, large gaps) ranged from 85 to >380 yr. On a landscape scale, mean return intervals for F2 damage in the town of Petersham, Massachusetts, ranged from 125 yr across most sites to >380 yr on scattered lee slopes. Actual forest damage was strongly dependent on land use and natural disturbance history. Annual and decadal timing of hurricanes varied widely. There was no clear century-scale trend in the number of major hurricanes. The historical-modeling approach is applicable to any region with good historical records and will enable ecologists and land managers to incorporate insights on hurricane disturbance regimes into the interpretation and conservation of forests at landscape to regional scales.

Estimating chaos and complex dynamics in an insect population

Abstract: A defining hypothesis of theoretical ecology during the past century has been that population fluctuations might largely be explained by relatively low-dimensional, non- linear ecological interactions, provided such interactions could be correctly identified and modeled. The realization in recent decades that such nonlinear interactions might result in chaos and other exotic dynamic behaviors has been exciting but tantalizing, in that attri- buting the fluctuations of a particular real population to the complex dynamics of a particular mathematical model has proved to be an elusive goal. We experimentally tested a model- predicted sequence of transitions (bifurcations) in the dynamic behavior of a population from stable equilibria to quasiperiodic and periodic cycles to chaos to three-cycles using cultures of the flour beetle Tribolium. The predictions arose from a system of difference equations (the LPA model) describing the nonlinear life-stage interactions, predominantly cannibalism. We built a stochastic version of the model incorporating demographic vari- ability and obtained conditional least-squares estimates for the model parameters. We gen- erated 2000 ''bootstrapped data sets'' with a time-series bootstrap technique, and for each set we reestimated the model parameters. The resulting 2000 bootstrapped parameter vectors were used to obtain confidence intervals for the model parameters and estimated distri- butions of the Liapunov exponents for the deterministic portion (the skeleton) of the model as well as for the full stochastic model. Frequency distributions of estimated dynamic behaviors of the skeleton at each experimental treatment were produced. For one treatment, over 83% of the bootstrapped parameter estimates corresponded to chaotic attractors, and the remainder of the estimates yielded high-period cycles. The low-dimensional skeleton accounted for at least 90% of the variability in the population abundances and accurately described the responses of populations to experimental demographic manipulations, in- cluding treatments for which the predicted dynamic behavior was chaos. Demographic stochasticity described the remaining noise quite well. We conclude that the fluctuations of experimental flour beetle populations are explained largely by known nonlinear forces involving cannibalistic-stage interactions. Claims of dynamic behavior such as periodic cycles or chaos must be accompanied by a consideration of the reliability of the estimated parameters and a realization that the population fluctuations are a blend of deterministic forces and stochastic events.

Density dependence in an annual plant community: variation among life history stages

Abstract: Most studies of density-dependent demography in plants consider the density only of the single focal species being studied. However, density-dependent regulation in plants may frequently occur at the level of the entire community, rather than only within particular species. In addition, because density dependence may differ considerably (even in direction) among demographic parameters, generalizing about patterns of density dependence and extrapolating to lifetime fitness and to population dynamics require comparisons among life history stages, as well as among types of species and physical environments. We constructed seminatural communities of desert annuals composed of all the constituent species in the same relative proportions as found in the natural habitat. These experimental communities were planted at a range of densities that extended far above and below mean natural field density. We compared among physical environments (irrigation treatments), among communities from different physical environments, and among growth forms (dicot and graminoid) to search for generalizations about the magnitude and direction of density dependence. Strong evidence of community-level density dependence was detected at all three life history stages studied in these desert annuals: emergence, survival, and final size. However, both the direction and degree of consistency of this density dependence varied considerably among the stages. The strongest and most consistent competitive effects were experienced at the emergence stage, where the mechanism is most likely a form of interference competition rather than exploitation competition. At the survival stage, the magnitude of effects was highly variable among physical environments and source communities, but negative effects were relatively rare, with either positive or no significant effects of increasing density. Thus, exploitation competition was also unimportant at the survival stage. In contrast, for growth, exploitation competition appeared to be the primary mechanism of interaction influencing growth. This variation in mechanism, direction, and magnitude of interactions among life history stages suggests that current models of plant community structure that are based largely on exploitation competition as it influences growth (with mortality a simple function of growth) are inadequate for even this simple annual plant community. We also compared growth forms and found that graminoids were superior competitors to dicots at the emergence and survival stages; they also had higher emergence and survival, regardless of density. Consistent with this result, grasses are always the numerical dominants in the source communities. In contrast, the two growth forms did not differ in competitive ability for growth, and dicots were consistently larger individuals, independent of density, even though grasses were also usually the biomass dominants in the source communities. These results suggest the importance of nontrophic mechanisms of interaction in controlling community structure and again emphasize the importance of constructing and testing models that incorporate multiple mechanisms of interactions.

Comparative diving patterns of pinnipeds and seabirds

Abstract: General ecological information resulting from modern dive studies has been limited because analyses and conclusions are study- and species-specific. In this work, a series of unrelated divers was studied and compared using the same analytical procedures. More than 230 000 dives from 12 species were analyzed, and ∼140 000 of these dives were classified according to dive shape. The species included one cormorant, three penguins, two eared seals, five true seals, and a walrus. Dive profiles could generally be characterized as one of four shapes: square, V, skewed right, or skewed left. In light of this, a universal shape classification protocol was developed that also offers potential solutions for “on board” memory limitations and transmission constraints for archival time–depth recorders and satellite-linked time–depth recorders. Comparisons of dive data recorded with different sample intervals indicated the need for a standardization relative to mean dive duration (i.e., an equal number of data points per...

Sonoran desert columnar cacti and the evolution of generalized pollination systems

Abstract: We studied variation in flowering phenology, fruit and seed set, and the abundance of the pol- linators of four species of night-blooming Sonoran Desert columnar cacti for up to eight years at one site in Mexico and one year at one site in Arizona. We determined how spatiotemporal variation in plant-pollinator interactions affects the evolution of generalized pollination systems. We conducted pollinator exclusion and hand pollination experiments to document annual variability in pollinator reliability and to determine whether pollination systems were redundant (different species are partially or totally substitutable) or complementary (different species have an additive effect on fruit set). The cacti we studied included three species with generalized pollination systems involving bats, birds, and bees (cardon, Pachycereus pringlei; saguaro, Carnegiea gigantea; and organ pipe, Stenocereus thurberi) and one specialized moth-pollinated species (senita, Lophocereus schottii). We predicted that the migratory lesser long-nosed bat, Leptonycteris curasoae, is a less reliable pollinator than birds and bees, and that cacti with generalized pollination systems have more variable flowering phenologies than the specialized species. Annual time of peak flowering and mean size of flower crops were relatively invariant in saguaro and organ pipe. Time of peak flowering in cardon varied by as much as six weeks, and mean flower crop size varied three- fold over six years. In senita, peak flowering varied by as much as 5-8 wk among years. Peak numbers of the nectar bat L. curasoae varied among years, and bat density (0.9/ha) was an order of magnitude lower than that of cactus-visiting birds at both study sites. The abundance of migratory hummingbirds was also highly variable among years. Pollinator exclusion experiments indicated that bats were major pollinators of cardon, whereas diurnal visitors accounted for most fruit set in saguaro (except in 1995 when bats were most important) and organ pipe at our Mexican site; honeybees accounted for 64-87% of diurnal fruit set in these species. Annual variation in the contribution to fruit set by bats was substantially higher than that of diurnal pollinators in saguaro and organ pipe, but not in cardon. There was little geographic variation in the relative importance of nocturnal vs. diurnal pollinators in saguaro and senita, but bats were much more important for fruit set in organ pipe in Arizona than in Mexico. We generally detected no effect of different pollinators on number of seeds per fruit in any species. Annual variation in fruit set was lowest in saguaro, the species with the most diurnal pollination system, and highest in organ pipe, the species with the most generalized pollination system. Fruit set was strongly pollen limited only in females of cardon (a trioecious species) and in organ pipe (at both sites). The ''missing'' pollinators in both species are likely Leptonycteris bats. The pollination systems of saguaro and cardon were partially redundant, whereas that of organ pipe was complementary. The four species of cactus that we studied occur at the northern geographic limits of Mexican columnar cacti where many vertebrate pollinators are seasonal migrants. In the Sonoran Desert, variation in rainfall and spring temperatures affects timing of flowering and the extent of competition between cacti for pollinator visits and causes the relative importance of particular pollinators, especially Leptonycterisbats, for fruit set to vary annually. Under such conditions, selection has favored generalized pollination systems (as seen in organ pipe) or shifts from reliance primarily on nocturnal pollinators (as seen in cardon) to reliance primarily on diurnal pollinators (as seen in saguaro). Nonetheless, as exemplified by the senita-senita moth system, highly specialized pollination mutualisms can also evolve in this habitat in plants that rely on sedentary insects rather than migratory bats

The influence of habitat quality on dispersal, demography, and population dynamics of voles

Abstract: We compared the effects of habitat quality on dispersal, demography, dynamics, and fitness of prairie vole (Microtus ochrogaster) and meadow vole (M. pennsylvanicus) populations by manipulating habitat patches in experimental landscapes. Four habitat patches in each of four replicate landscapes differed in availability of high-quality food and amount of vegetative cover in a 2 × 2 factorial design. High cover had a strong positive effect on basic habitat quality, as reflected by the performance of founders early in the season, but supplemental food had only a small effect. Population growth ceased for prairie voles after week 18 (mid-October) when densities had reached much higher levels in habitats with high cover (260 ± 27 voles/ha in high cover with either high or low food; mean ± 1 se) than in habitats with low cover (115 ± 38 voles/ha in high food, low cover and 60 ± 15 voles/ha in low food, low cover). Population growth had ceased in three habitat types for meadow voles by week 20 at much higher den...

Retrospective analysis of demographic responses to environmental change: a lesser snow goose example

Abstract: In general, analysis of population dynamics can proceed either prospectively or retrospectively. In the case of the former, asymptotic expectations are generally derived (analytically or numerically) from analysis of the potential effects of perturbation of the elements of the life table. However, the vital rates that are indicated by prospective analysis to contribute the most to projected growth rate are not necessarily those that have contributed to observed variation in growth rate over time. We used a retrospective analysis to analyze the life table responses of a population of long-lived herbivorous geese to a systematic reduction in food abundance within the traditional breeding colony. Typical of long-lived species, adult survival rate has been shown previously in a prospective perturbation analysis to have the largest potential impact on projected growth of the population. However, despite a significant long-term increase in adult survival over the course of the study, there has been a long-term decline in growth rates of the population inhabiting the traditional sampling areas, although absolute numbers of individuals in both populations increased over time. Retrospective assessment of the relative contributions of variation in underlying vital rates (adult and juvenile survival, in situ recruitment, emigration and immigration rates into the population) to projected growth showed that the long-term dynamics of the nesting population primarily reflected the combined effects of changes in postfledging juvenile survival, while changes in the postnesting population were most influenced by variation in juvenile survival and adult fidelity rate to the traditional brood-rearing areas. Decreases in both juvenile survival and fidelity reflect systematic reductions in food abundance over the course of the study. Our results confirm previous suggestions that philopatry to the brood-rearing areas may be a significantly more plastic trait than fidelity to nesting areas.

A long-term field study on biodiversity × elevated co2 interactions in grassland

Abstract: Interactive effects of increases in atmospheric CO2 and reductions in plant species diversity were investigated in planted calcareous grassland communities in northwestern Switzerland. The experimental communities were composed of 5, 12, and 31 species assembled from the native species pool. The study aimed at testing whether the CO2 responses of ecosystems change when specific sets of species are lost from plant communities. Species were selected so that the proportion of grasses, legumes, and non-legume forb individuals remained constant across levels of diversity. The most diverse plant community had approximately the same diversity as the surrounding grassland, and species occurring in less diverse communities were subsets of the species in the more diverse communities. The factorial atmospheric-CO2 treatment was applied using 50-cm-tall, open-bottom, open-top wind screens. Plant community-level responses and the responses of the individual species were assessed over a period of five years. A signific...

Bottom-up and top-down forces in tide pools: test of a food chain model in an intertidal community

Abstract: A simple food chain model of community structure was used to evaluate the roles of bottom-up and top-down factors in a rocky intertidal community. Predictions of the model were modified to incorporate known variation in the strength of species inter- actions and nutrient delivery rates along a wave-exposure gradient. To test the predictions of the model, I manipulated nutrients and consumers in tide pools chiseled into mudstone benches at two sites that varied in degree of wave exposure. The pools were located in the mid-intertidal zone between ;1 and 1.5 m above mean lower low water (MLLW), at Boiler Bay, Oregon, USA. The focal organisms were the benthic macroalgae and mobile inver- tebrate herbivores that dominate naturally occurring tide pools at this site. I manipulated nutrient levels and the abundance of herbivores in these tide pools in a fully factorial randomized block design replicated six times at a wave-exposed and a wave- protected site. The experiment was maintained for two years (1994-1996). The abundances of herbivores and macroalgae were monitored in the spring, summer, and fall of each year. I measured primary productivity in the tide pools during the summer. Herbivores had a negative impact on algal abundance. The total effect of herbivory, and the efficiency of herbivores per se, on algal abundance was lower at the wave-exposed site. Nutrient additions had a positive effect on algal abundance, but this effect was reduced at the wave-exposed site. Nutrients also appeared to increase algal productivity, but only where herbivore abundance was low. Algal abundance patterns were generally consistent with model predictions for bottom-up, top-down, and hydrodynamic effects. In contrast to model predictions, herbivore abundance did not respond to the nutrient treatment. The decoupling of consumers from resource dynamics is interpreted to be the result of an herbivore preference for noncalcified seaweeds with higher potential growth rates. In wave-protected pools, where nutrients were most limiting and consumers were most effi- cient, seaweeds with the potential to translate elevated nutrient levels into growth had no effective refuge from consumers. The difference in scale between resource patches (tide pools) and the foraging range of the dominant herbivore, Tegula funebralis, may have augmented the ability of this herbivore to virtually exclude fleshy seaweeds from wave- protected pools. Expanding the domain of applicability of food chain models requires the incorporation of consumer preferences, variation in plant growth rates, environmental gra- dients, and differences in the relative scales of resource patches and foraging ranges of consumers.

Partially specified ecological models

Abstract: Models are useful when they are compared with data. Whether this comparison should be qualitative or quantitative depends on circumstances, but in many cases some statistical comparison of model and data is useful and enhances objectivity. Unfortunately, ecological dynamic models tend to contain assumptions and simplifications that enhance tractability, promote insight, but spoil model fit, and this can cause difficulties when adopting a statistical approach. Furthermore, the arcane numerical analysis required to fit dynamic models reliably presents an impediment to objective model testing by fitting. This paper presents methods for formulating and fitting partially specified models, which aim to achieve a measure of generality by avoiding some of the irrelevant incidental assumptions that are inevitable in more traditional approaches. This is done by allowing delay differential equation models, difference equation models, and differential equation models to be constructed with part of their structure represented by unknown functions, while part of the structure may contain conventional model elements that contain only unknown parameters. An integrated practical methodology for using such models is presented along with several examples, which include use of models formulated using delay differential equations, discrete difference equations/matrix models, ordinary differential equations, and partial differential equations. The methods also allow better estimation from ecological data by model fitting, since models can be formulated to include fewer unjustified assumptions than would usually be the case if more traditional models were used, while still including as much structure as the modeler believes can be justified by biological knowledge: model structure improves precision, while fewer extraneous assumptions reduce unquantifiable bias.

Elevated co2 differentiates ecosystem carbon processes: deconvolution analysis of duke forest face data

Abstract: Quantification of the flux of carbon (C) through different pathways is critical to predict the impact of global change on terrestrial ecosystems. Past research has en- countered considerable difficulty in separating root exudation, root turnover rate, and other belowground C fluxes as affected by elevated CO2. In this study we adopted a deconvolution analysis to differentiate C flux pathways in forest soils and to quantify the flux through those pathways. We first conducted forward analysis using a terrestrial-C sequestration (TCS) model to generate four alternative patterns of convolved responses of soil surface respiration to a step increase in atmospheric CC)2. The model was then validated against measured soil respiration at ambient CO2 before it was used to deconvolve the CO2 stim- ulation of soil respiration. Deconvolved data from the Duke Forest free-air CO2 enrichment (FACE) experiment suggest that fast C transfer processes, e.g., root exudation, are of minor importance in the ecosystem C cycling in the Duke Forest and were not affected by elevated CO2. The analysis indicates that the fine-root turnover is a major process adding C to the rhizosphere. This C has a residence time of several months to -2 yr and increases signif- icantly with increased CO2. In addition, the observed phase shift in soil respiration caused by elevated CO2 can be only reproduced by incorporation of a partial time delay function in C fluxes into the model. This paper also provides a detailed explanation of deconvolution analysis, since it is a relatively new research technique in ecology.

Community structure of pleistocene coral reefs of curacao, netherlands antilles

Abstract: The Quaternary fossil record of living coral reefs is fundamental for understanding modern ecological patterns. Living reefs generally accumulate in place, so fossil reefs record a history of their former biological inhabitants and physical environments. Reef corals record their ecological history especially well because they form large, resistant skeletons, which can be identified to species. Thus, presence–absence and relative abundance data can be obtained with a high degree of confidence. Moreover, potential effects of humans on reef ecology were absent or insignificant on most reefs until the last few hundred years, so that it is possible to analyze “natural” distribution patterns before intense human disturbance began. We characterized Pleistocene reef coral assemblages from Curacao, Netherlands Antilles, Caribbean Sea, focusing on predictability in species abundance patterns from different reef environments over broad spatial scales. Our data set is composed of >2 km of surveyed Quaternary reef. Taxonomic composition showed consistent differences between environments and along secondary environmental gradients within environments. Within environments, taxonomic composition of communities was markedly similar, indicating nonrandom species associations and communities composed of species occurring in characteristic abundances. This community similarity was maintained with little change over a 40-km distance. The nonrandom patterns in species abundances were similar to those found in the Caribbean before the effects of extensive anthropogenic degradation of reefs in the late 1970s and early 1980s. The high degree of order observed in species abundance patterns of fossil reef coral communities on a scale of tens of kilometers contrasts markedly with patterns observed in previous small-scale studies of modern reefs. Dominance of Acropora palmata in the reef crest zone and patterns of overlap and nonoverlap of species in the Montastraea “annularis” sibling species complex highlight the tendency for distribution and abundance patterns of Pleistocene corals to reflect environmental preferences at multiple spatial scales. Wave energy is probably the most important physical environmental variable structuring these coral communities. The strong similarity between ancient and pre-1980s Caribbean reefs and the nonrandom distribution of coral species in space and time indicate that recent variability noted at much smaller time scales may be due to either unprecedented anthropogenic influences on reefs or fundamentally different patterns at varying spatio-temporal scales.

Source–sink dynamics for a generalist insect predator in habitats with strong higher-order predation

Abstract: The functional importance of higher-order predators in terrestrial ecosystems is currently an area of active inquiry. In particular, an understanding of the relative influences of prey availability and higher-order predation on predator populations is of immediate relevance to the theory of biological control of herbivorous arthropods. Biological control workers have repeatedly speculated that one cause of failure to establish predators and parasitoids in novel environments is the strong mortality imposed on released agents by higher-order predators. Nevertheless, the ability of higher-order predators to create a habitat where mortality exceeds natality (a “sink” habitat) has never been tested experimentally with a biological control agent in nature. Although in isolation the predatory lacewing Chrysoperla carnea can consistently produce strong suppression of populations of the aphid Aphis gossypii, the full community of predators when tested together exerts minimal aphid control. The age structure of Chrysoperla spp. populations in cotton fields harboring low to intermediate densities of aphid prey is characterized by a sharp drop in densities from the egg to the first larval instar; this observation is consistent with heavy mortality during either the egg or first larval stage. Egg cohorts followed under unmanipulated field conditions showed relatively high rates of successful hatch, suggesting that the vulnerable developmental stage is the young larva. Larval survival is relatively high in the absence of hemipteran predators, suggesting that prey availability is not the primary limiting factor. Depressed survival is observed in the presence of Geocoris spp., Nabis spp., and Zelus renardii, all common hemipteran predators in cotton. Predation on lacewing larvae appears to disrupt the strong top-down control of aphid populations in cotton. Chrysoperla spp. densities declined in fields harboring intermediate aphid densities when lacewing subpopulations were experimentally caged to block immigration and emigration. In one year (1993) Chrysoperla spp. densities fell to very low levels, suggesting that the field was either a true sink habitat or a pseudosink with a very low equilibrium density. In a second year (1994), densities declined to what appeared to be a lower but stable density, suggesting that the habitat was a pseudosink. Thus, in both years, declines in Chrysoperla spp. densities were observed following caging, suggesting that Chrysoperla spp. populations are spatially subsidized. Aphid prey availability and higher-order predation interacted strongly in their influence on C. carnea survival: larval survival in the presence of higher-order predators was 5.6% when prey availability was intermediate and 40.5% when prey were superabundant. Spatial heterogeneity in aphid prey densities modulates the intensity of higher-order predation and thereby appears to produce source–sink dynamics of Chrysoperla spp. in cotton fields.

Host-plant change in marine specialist herbivores: ascoglossan sea slugs on introduced macroalgae

Abstract: In the British Isles, oligophagous marine herbivores, particularly the ascoglossan (= sacoglossan) sea slug Elysia viridis, associate with the introduced green macroalga Codium fragile ssp tomentosoides Slugs prefer to associate with and consume the introduced C fragile to the native C tomentosum Our investigation of adult, larval, and juvenile E viridis focused on whether this association is attributable (1) to a host switch or (2) to an expansion from native hosts to the introduced C fragile ssp tomentosoides Growth rates and maximum body sizes of E viridis on introduced hosts were greater than on natives Although the native Cladophora rupestris induced a high rate of slug metamorphosis, recently metamorphosed juvenile E viridis (from Codium fragile-feeding parents) were generally not able to feed or grow on the native alga In contrast, juveniles from Cladophora-feeding parents could eat Cladophora, although their performance was highly variable Small, postlarval slugs (<800 μm long) could

The effects of neighbors on the demography of a dominant desert shrub (ambrosia dumosa)

Abstract: We used patch-specific matrix models to test the influence of neighboring plants on the demography of Ambrosia dumosa, a dominant perennial shrub of the Colorado Desert in southern California. In the desert literature, the presence or absence of neighboring plants is reported to influence plant growth and survival and has long been associated with plant–plant interactions that range from extreme competition to facilitation. Here we consider the less addressed question of how neighbor-defined population subsets contribute differently to overall population dynamics. Demographic data collected from >6000 individual plants from a permanently mapped hectare over 10 yr were used to divide this A. dumosa population into subsets that were defined by the presence or absence of neighbors. By partitioning the population in this way, we documented differences in population growth, elasticity structure, and stable size structure among the distinct patch types, and evaluated the contributions of each patch type to over...

Global, regional, and local patterns in species richness and abundance of butterflyfishes

Abstract: Butterflyfish communities vary in richness longitudinally from highs of >30 species to lows of 2–3 species. We investigated this gradient because it provides an opportunity to address some basic questions relating to determinants of community structure, e.g., are richness and abundance functions of geography or of local conditions? How are richness and density of species populations related? Do community-level processes, such as competition, determine community structure? In a broader sense, to what extent are community properties internally deterministic as opposed to being functions of higher-level processes? From 1981 through 1997 we counted kinds and number of butterflyfishes on over 2000 10-min transects distributed from the eastern Caribbean to the western Indian Ocean and Red Sea. We grouped transects at the same site into stations, stations in the same local area into islands, and islands into 18 geographic regions, thus organizing the data into four spatial levels. For each station, extent of live coral cover and degree of open-ocean influence were estimated. Richness is highest in the Philippine–Bornean–New Guinean region and declines radially from that center. Total number of individual butterflyfish also declines over that gradient. On more local scales, increased open-ocean exposure and live-coral cover enhance both richness and numerical abundance. Because species richness declines more rapidly with distance from the richness center than does abundance, across the same gradient average abundance per benthivore species rises. In peripheral, species-poor reaches of the Indopacific, the Eastern Tropical Pacific, and the Caribbean the communities are dominated by one or a few very common species. Butterflyfishes respond to changing richness of their communities by adjusting the abundance of individual species. Use of space by individual species decreases with increasing richness. Global richness is determined chiefly by geography; within local areas local factors are more important. Community structure is predominantly organized by richness, hence first by geography and then by habitat characteristics.

Consequences of diet choice by a small generalist herbivore

Abstract: We examined the energy, nutrient, time, and habitat consequences of diet choice by a generalist herbivore, the hispid cotton rat (Sigmodon hispidus) on the Texas coastal prairie. Benefit-cost and linear programming models were developed and used to test several specific hypotheses about diet choice, foraging strategy, and habitat use by cotton rats. Within cotton rat habitat, there were significant differences among both seasons and habitat patches in the abundance and biomass of foods available. Monocot patches typically were higher in standing crops of energy and nutrients than were either dicot or mixed patches. Relative to available biomass and nutrients in dicot foods, this cotton rat habitat was both spatially and temporally heterogeneous and dynamic. Although the abun- dance of dicot foods in a habitat patch did influence costs of foraging time, time costs of cotton rats foraging for suitable foods did not decrease when standing crops of energy and nutrients in these habitats increased. Higher requirements for energy and nutrients needed for lactation by reproductive cotton rats resulted in higher foraging costs than for non- reproductive female rats. A mixed diet incorporating both monocot and dicot foods was nutritionally superior to either a monocot-only diet or a dicot-only diet. A diet of only monocots would meet minimum daily energy requirements and would not exceed either maximum daily digestive capacity or maximum daily foraging time, but it would not meet minimum daily requirements for protein and phosphorus during reproduction. Conversely, a diet of only dicots would meet nutrient requirements for reproduction but would exceed maximum daily foraging time. Mixing of monocot and dicot plant parts in the diet met both net energy and nutritional requirements, and increased foraging time, as compared to a diet of only monocots. Diets of cotton rats foraging to maximize net energy intake each season differed from the observed diets. Likewise, diets of cotton rats foraging to minimize total daily foraging time differed from the observed diets. Thus, we conclude that repro- ductive females of this generalist herbivore neither maximized their energy nor minimized their time, but rather, consumed a mixed diet to balance intake of requisite nutrients. Accordingly, a diet choice that included a mixture of monocot and dicot foods was most efficient when foraging occurred in a habitat having a mixture of monocot and dicot plants. Mixed patches provided higher energy and nutrient benefits to foraging cotton rats than did either monocot or dicot patches. Finally, we observed that reproductive female cotton rats concentrated their activities in mixed-patch habitats.

Habitat complexity, spatial interference, and “minimum risk distribution”: a framework for population stability

Abstract: In the past century, the debate over whether or not density-dependent factors regulate populations has generally focused on changes in mean population density, ignoring the spatial variance around the mean as unimportant noise. In an attempt to provide a different framework for understanding population dynamics based on individual fitness, this paper discusses the crucial role of spatial variability itself on the stability of insect populations. The advantages of this method are the following: (1) it is founded on evolutionary principles rather than post hoc assumptions; (2) it erects hypotheses that can be tested; and (3) it links disparate ecological schools, including spatial dynamics, behavioral ecology, preference-performance, and plant apparency into an overall framework. At the core of this framework, habitat complexity governs insect spatial variance. which in turn determines population stability. First, the minimum risk distribution (MRD) is defined as the spatial distribution of individuals that results in the minimum number of premature deaths in a population given the distribution of mortality risk in the habitat (and, therefore, leading to maximized population growth). The greater the divergence of actual spatial patterns of individuals from the MRD, the greater the reduction of population growth and size from high, unstable levels. Then, based on extensive data from 29 populations of the processionary caterpillar, Ochrogaster lunifer, four steps are used to test the effect of habitat interference on population growth rates. (1) The costs (increasing the risk of scramble competition) and benefits (decreasing the risk of inverse density-dependent predation) of egg and larval aggregation are quantified. (2) These costs and benefits, along with the distribution of resources, are used to construct the MRD for each habitat. (3) The MRD is used as a benchmark against which the actual spatial pattern of individuals is compared. The degree of divergence of the actual spatial pattern from the MRD is quantified for each of the 29 habitats. (4) Finally, indices of habitat complexity are used to provide highly accurate predictions of spatial divergence from the MRD, showing that habitat interference reduces population growth rates from high, unstable levels. The reason for the divergence appears to be that high levels of background vegetation (vegetation other than host plants) interfere with female host-searching behavior. This leads to a spatial distribution of egg batches with high mortality risk, and therefore lower population growth. Knowledge of the MRD in other species should be a highly effective means of predicting trends in population dynamics. Species with high divergence between their actual spatial distribution and their MRD may display relatively stable dynamics at low population levels. In contrast, species with low divergence should experience high levels of intragenerational population growth leading to frequent habitat-wide outbreaks and unstable dynamics in the long term. Six hypotheses, erected under the framework of spatial interference, are discussed, and future tests are suggested.