Showing papers in "Ecology in 1997"
TL;DR: It is argued that engineering has both negative and positive effects on species richness and abundances at small scales, but the net effects are probably positive at larger scales encompassing engineered and nonengineered environments in ecological and evolutionary space and time.
Abstract: Physical ecosystem engineers are organisms that directly or indirectly control the availability of resources to other organisms by causing physical state changes in biotic or abiotic materials. Physical ecosystem engineering by organisms is the physical modification, maintenance, or creation of habitats. Ecological effects of engineers on many other species occur in virtually all ecosystems because the physical state changes directly create nonfood resources such as living space, directly control abiotic resources, and indirectly modulate abiotic forces that, in turn, affect resource use by other organisms. Trophic interactions and resource competition do not constitute engineering. Engineering can have significant or trivial effects on other species, may involve the physical structure of an organism (like a tree) or structures made by an organism (like a beaver dam), and can, but does not invariably, have feedback effects on the engineer. We argue that engineering has both negative and positive effects on species richness and abundances at small scales, but the net effects are probably positive at larger scales encompassing engineered and nonengineered environments in ecological and evolutionary space and time. Models of the population dynamics of engineers suggest that the engineer/habitat equilibrium is often, but not always, locally stable and may show long-term cycles, with potential ramifications for community and ecosystem stability. As yet, data adequate to parameterize such a model do not exist for any engineer species. Because engineers control flows of energy and materials but do not have to participate in these flows, energy, mass, and stoichiometry do not appear to be useful in predicting which engineers will have big effects. Empirical observations suggest some potential generalizations about which species will be important engineers in which ecosystems. We point out some of the obvious, and not so obvious, ways in which engineering and trophic relations interact, and we call for greater research on physical ecosystem engineers, their impacts, and their interface with trophic relations.
2,163 citations
TL;DR: The roles of life stage, physiology, indirect interactions, and the physical environment on the balance of competition and facilitation in plant communities are discussed.
Abstract: Interactions among organisms take place within a complex milieu of abiotic and biotic processes, but we generally study them as solitary phenomena. Complex combinations of negative and positive interactions have been identified in a number of plant communities. The importance of these two processes in structuring plant communities can best be understood by comparing them along gradients of abiotic stress, consumer pressure, and among different life stages, sizes, and densities of the interacting species. Here, we discuss the roles of life stage, physiology, indirect interactions, and the physical environment on the balance of competition and facilitation in plant communities.
1,784 citations
1,720 citations
TL;DR: Local biotic interactions and recruitment dynamics jointly determined diversity, species composition, and species abundances in these native grassland communities, suggesting that recruitment limitation may be more important, even on a local scale, than often recognized.
Abstract: Plant species composition, species abundances, and species richness were strongly recruitment limited in a 4-yr experiment in which seeds of up to 54 species were added to patches of native grassland. Four field seasons after a one-time addition of seed, many added species were still present and reproducing, with plots seeded at the highest rate having species richness that was 83% greater and total plant cover that was 31% greater than controls. Total plant community cover increased significantly with the number of species added as seed, but total cover of pre-existing species was independent of the number of species added as seed, suggesting that the new species mainly filled previously ''empty'' sites. The proportion of added species that became established was negatively correlated with initial species richness of plots, suggesting that species-rich sites were more resistant to invasion. Plot invasibility also depended on the abundances and species richness of plant functional groups in the plots, but was independent of seed size and of total plant cover. The major functional groups of plants differed in their abilities to invade as seed, with perennial grasses being the poorest invaders and herbaceous legumes being the best. Thus, local biotic interactions and recruitment dynamics jointly determined diversity, species composition, and species abundances in these native grassland communities. This supports a metapopulation-like perspective over a purely interspecific-interaction perspective or a purely regional perspective, suggesting that recruitment limitation may be more important, even on a local scale, than often recognized.
1,429 citations
TL;DR: In this article, the authors reviewed the contributing ornithologists' current research in birds' acoustic communication with an ecological and evolutionary focus, and also identified the areas they feel will dominate future research efforts.
Abstract: Twenty-five invited papers reviewing the contributing ornithologists' current research in birds' acoustic communication with an ecological and evolutionary focus, and also identifying the areas they feel will dominate future research efforts. The discussions center around the areas of vocal developm
1,307 citations
Journal Article•
TL;DR: In this article, the importance of space in the dynamics of individual species and on the structure, dynamics, diversity, and stability of multispecies communities is discussed, and the authors demonstrate that the spatial structure of a habitat can fundamentally alter both qualitative and quantitative dynamics and outcomes of ecological processes.
Abstract: Addresses the fundamental effects of space in the dynamics of individual species and on the structure, dynamics, diversity and stability of multispecies communities. The book aims to demonstrate that the spatial structure of a habitat can fundamentally alter both the qualitative and quantitative dynamics and outcomes of ecological processes. It highlights the importance of space to five areas: stability, patterns of diversity, invasions, coexistence and pattern generation. It illustrates both the diversity of approaches used to study spatial ecology and the underlying similarities of these approaches.
1,189 citations
TL;DR: A graphical model is used that visualizes how facilitative patterns can be understood from the simultaneous effects of plant canopies on microsite light and moisture, and the growth responses of establishing seedlings to those factors.
Abstract: If plants cannot simultaneously acclimate to shade and drought because of physiological trade-offs, then plants are expected to be less tolerant to shading under drier conditions. One observation that, at first sight, seems incompatible with this idea is the fact that the establishment of new plants in dry areas is often restricted to shady sites under the canopy of other plants, called “nurse plants.” We use a graphical model to resolve this paradox. The model visualizes how facilitative patterns can be understood from the simultaneous effects of plant canopies on microsite light and moisture, and the growth responses of establishing seedlings to those factors. The approach emphasizes the fact that positive and negative effects of plant canopies always occur simultaneously. In the presented light–water model, facilitation only occurs when the improvement of plant water relations under the canopy exceeds the costs caused by lower light levels. This may be true under dry conditions, whereas in less dry sit...
1,019 citations
TL;DR: The purpose of this communication is to identify the assumptions on which these inferences are based, to characterize the conditions in which they are not met, and to suggest the laboratory experiments that are needed to validate them.
Abstract: For decades, plant ecologists have used naturally occurring stable isotope ratios to disentangle ecological and physiological processes The methodology can also become a very powerful tool in animal ecology However, the application of the technique relies on assumptions that are not widely recognized and that have been rarely tested The purpose of this communication is to identify these assumptions, to characterize the conditions in which they are not met, and to suggest the laboratory experiments that are needed to validate them The ease with which isotopic data can be gathered and the growing popularity of the method are generating a large amount of data on the isotopic ecology of animals The proper interpretation of these data demands that we identify the assumptions on which these inferences are based, and that we conduct comparative laboratory experiments to assess their validity
1,014 citations
TL;DR: This new edition of "Methods in Stream Ecology" is updated to reflect recent advances in the technology associated with ecological assessment of streams, including remote sensing.
Abstract: "Methods in Stream Ecology" provides a complete series of field and laboratory protocols in stream ecology that are ideal for teaching or conducting research. This new edition is updated to reflect recent advances in the technology associated with ecological assessment of streams, including remote sensing. In addition, the relationship between stream flow and alluviation has been added, and a new chapter on riparian zones is also included.With a student-friendly price, this Second Edition is a key for all students and researchers in stream and freshwater ecology, freshwater biology, marine ecology, and river ecology. This text is also supportive as a supplementary text for courses in watershed ecology/science, hydrology, fluvial geomorphology, and landscape ecology. It includes: exercises in each chapter; detailed instructions, illustrations, formulae, and data sheets for in-field research for students; taxanomic keys to common stream invertebrates and algae; website with tables; and, link from chapter 22: Fish Community Composition to an interactive program for assessing and modeling fish numbers.
897 citations
TL;DR: In this paper, the authors evaluated the impact of predator manipulations on trophic cascades in an old-field system composed of herbaceous plants, grasshoppers, her-bivores, and spider predators.
Abstract: Trophic cascades are regarded as important signals for top-down control of food web dynamics. Although there is clear evidence supporting the existence of trophic cascades, the mechanisms driving this important dynamic are less clear. Trophic cascades could arise through direct population-level effects, in which predators prey on herbivores, thereby decreasing the abundance of herbivores that impact plant trophic levels. Trophic cascades could also arise through indirect behavioral-level effects, in which herbivore prey shift their foraging behavior in response to predation risk. Such behavioral shifts can result in reduced feeding time and increased starvation risk, again lowering the impact of her- bivores on plants. We evaluated the relative importance of these two mechanisms, using field experiments in an old-field system composed of herbaceous plants, grasshopper her- bivores, and spider predators. We created two treatments, Risk spiders that had their che- licerae glued, and Predation spiders that remained unmanipulated. We then systematically evaluated the impacts of these predator manipulations at behavioral, population, and food web scales in experimental mesocosms. At the behavioral level, grasshoppers did not dis- tinguish between Risk spiders and Predation spiders. Grasshoppers exhibited significant shifts in feeding-time budget in the presence of spiders vs. when alone. At the grasshopper population level, Risk spider and Predation spider treatments caused the same level of grasshopper mortality, which was significantly higher than mortality in a control without spiders, indicating that the predation effects were compensatory to risk effects. At the food web level, Risk spider and Predation spider treatments decreased the impact grasshoppers had on grass biomass, supporting the existence of a trophic cascade. Moreover, Risk spider and Predation spider treatments produced statistically similar effects, again indicating that predation effects on trophic dynamics were compensatory to risk effects. We conclude that indirect effects resulting from antipredator behavior can produce trophic-level effects that are similar in form and strength to those generated by direct predation events.
TL;DR: This work focuses on food-web dynamics on some small subtropical islands and on the interactions of top and intermediate predators, as well as on underappreciated interactions in terrestrial food webs.
Abstract: Section I: Detritus and nutrients: Introduction - Michael J. Vanni and Peter C. DeRuiter Food webs and nutrient cycling in soils - Janne Bengtsson, Heikki Setala and D.W. Zheng Energetics of detritivory and microbivory in soil in theory and practice - David C. Coleman Integrating the microbial loop and the classical food chain into a realistic planktonic food web - Karen G. Porter Trophic structure and carbon flow dynamics in the pelagic community of a large lake -- Ursula Gaedke, Dietmar Straile, and Claudia Pahl- Wostl Biogeochemistry and trophic ecology: a new food-web diagram - Robert W. Sterner, James J. Elser, Thomas H. Chrzanowski, John H. Schampel, and Nicholas B. George Nutrient transport and recycling by consumers in lake food webs: implications for algal communities - Michael J. Vanni Food-web structure and littoral zone coupling to pelagic trophic cascades - Daniel E. Schindler, Stephen R. Carpenter, Kathryn L. Cottingham, Xi He, James R. Hodgson, James F. Kitchell, and Patricia A. Soranno Section II: Interaction of Productivity and Consumption: Introduction - Donald L. DeAngelis, Lennart Persson, and Amy Rosemond Dynamics and interactions in food webs with adaptive foragers - Peter Abrams Nonlinear food web models and their responses to increased basal productivity - Roger Arditi and Jurzy Michalski The relative importance of resource limitation and predator limitation in food chains - Craig W. Osenberg and Gary G. Mittelbach Indirect effects of herbivores modify predicted effects of resources and consumption on plant biomass-- Amy Rosemond Food-web dynamics on some small subtropical islands: effects of top and intermediate predators - David A. Spiller and Thomas W. Schoener Subterranean species, endogenous defenses, plant nutrients, and competition: an example of underappreciated interactions in terrestrial food webs - Donald R. Strong, John L. Maron, and Peter G. Connors Section III: Causes and Effects: Introduction - Janne Bengtsson, Neo Martinez and Donald Strong Assessing the relative importance of trophic links in food webs - David G. Raffaelli and Stephen J. Hall Food webs and perturbation experiments: theory and practice - Peter Yodzis Energetics and stability in belowground food webs - Peter C. DeRuiter, Anje-Margriet Neutel and John C. Moore What equilibrium analysis of Lotka-Volterra equations do not tell us about food-web dynamics - Alan Hastings Effects of food-chain length and omnivory on population dynamics in experimental food webs - Peter J. Morin and Sharon P.
TL;DR: A likelihood- based approach to dealing with temporary emigration is presented that permits estimation under different models of temporary em migration and yields tests for completely random and Markovian emigration.
Abstract: Statistical inference for capture-recapture studies of open animal populations typically relies on the assumption that all emigration from the studied population is per- manent. However, there are many instances in which this assumption is unlikely to be met. We define two general models for the process of temporary emigration: completely random and Markovian. We then consider effects of these two types of temporary emigration on Jolly-Seber estimators and on estimators arising from the full-likelihood approach to robust design data. Capture-recapture data arising from Pollock's robust design provide the basis for ob- taining unbiased estimates of demographic parameters in the presence of temporary emi- gration, and for estimating the probability of temporary emigration. We present a likelihood- based approach to dealing with temporary emigration that permits estimation under different models of temporary emigration and yields tests for completely random and Markovian emigration. In addition, we use the relationship between capture probability estimates based on closed and open models under completely random temporary emigration to derive three ad hoc estimators for the probability of temporary emigration. Two of these should be especially useful in situations where capture probabilities are heterogeneous among indi- vidual animals. Ad hoc and full-likelihood estimators are illustrated for small-mammal capture-recapture data sets. We believe that these models and estimators will be useful for testing hypotheses about the process of temporary emigration, for estimating demographic parameters in the presence of temporary emigration, and for estimating probabilities of temporary emigration. These latter estimates are frequently of ecological interest as indicators of animal movement and, in some sampling situations, as direct estimates of breeding probabilities and proportions.
TL;DR: This article reviewed the available information, often making contrasts with wetter forests, and showed that the world's dry forests heterogeneity of structure and function is shown regionally, as does the spectrum of plant life-forms in terms of structure, physiology, phenology and reproduction.
Abstract: Prolonged seasonal drought affects most of the tropics, including vast areas presently or recently dominated by 'dry forests'. These forests have received scant attention, despite the fact that humans have used and changed them more than rain forests. This volume reviews the available information, often making contrasts with wetter forests. The world's dry forest heterogeneity of structure and function is shown regionally. In the neotropics, biogeographic patterns differ from those of wet forests, as does the spectrum of plant life-forms in terms of structure, physiology, phenology and reproduction. Biomass distribution, nutrient cycling, below-ground dynamics and nitrogen gas emission are also reviewed. Exploitation schemes are surveyed, and examples are given of non-timber product economies. It is hoped that this review will stimulate research leading to more conservative and productive management of dry forests.
TL;DR: The first approximation to predicting the impact of agricultural or permanently managed edges on forest songbird reproductive success is to assess habitat characteristics at the landscape scale.
Abstract: Ecological processes neat" habitat edges often differ from processes away fromedges. Yet, the generality of "edge effects" has been hotly debated because results vary tremendously. To understand the factors responsible for this variation, we described nest predation and cowbird distribution patterns in forest edge and forest core habitats on 36 randomly selected plots in three states in the midwestern United States. We tested four hypotheses that may explain the variation and mechanisms responsible for edge effects - among the 36 plots: (1) the landscape context, (2) the local predator community, (3) the local bird (host-prey) community, and (4) the nest site microhabitat structure. We used artificial nests baited with quail and clay eggs to determine nest predation patterns and " predators and used point count surveys to determine cowbird and host abundance in forest edge and forest core habitats. Raccoons, Opossums, canids, and birds accounted for most predation of artificial nests. Neither local host abundance nor mean nest concealment of artificial nests significantly •influenced nest predation rates in habitat edge or in habitat core. Nest predation was sig- nificantly greater in highly fragmented landscapes than in unfragmented landscapes and was significantly higher in edge habitats than in core habitats. However, detection of edge effects varied, depending upon landscape type. Higher predation rates in edge habitats were detected in highly and moderately fragmented landscapes, but not in unfragmented land- , , s.capes. Both mammalian and avian predator groups contributed to higher predation rates along edges in highly and moderately fragmented landscapes. ,Cowbird abundance was significantly related to host abundance, but the effect of hosts varied depending upon habitat type. In edge habitats, cowbird abundance was negatively associated with host abundance in all three landscapes studied. By contrast, cowbird abun- dance was positively associated with host abundance in core habitats. Once the effects of host abundance were removed, cowbird abundance in core habitat was greater in highly fragmented landscapes than in moderately and unfragmented landscapes, but did not differ between the latter two. In edge habitat, cowbird abundance did not differ between land- scapes, but abundance in edges tended to be highest in the highly fragmented landscape and lowest in the unfragmented landscape. Cowbird abundance did not vary between edge and core habitat in any of the landscapes studied. We suggest that the first approximation to predicting the impact of agricultural or permanently managed edges on forest songbird reproductive success is to assess habitat characteristics at the landscape scale. Given geographic location, local factors such as host abundance and predator composition should be assessed.
TL;DR: Re- sampling methods should be incorporated in meta-analysis studies, to ensure proper evaluation of main effects in ecological studies, and confidence limits based on bootstrapping methods were found to be wider than standard confidence limits, implying that resampling estimates are more conservative.
Abstract: Meta-analysis is a statistical technique that allows one to combine the results from multiple studies to glean inferences on the overall importance of various phenomena. This method can prove to be more informative than common ''vote counting,'' in which the number of significant results is compared to the number with nonsignificant results to determine whether the phenomenon of interest is globally important. While the use of meta- analysis is widespread in medicine and the social sciences, only recently has it been applied to ecological questions. We compared the results of parametric confidence limits and ho- mogeneity statistics commonly obtained through meta-analysis to those obtained from re- sampling methods to ascertain the robustness of standard meta-analytic techniques. We found that confidence limits based on bootstrapping methods were wider than standard confidence limits, implying that resampling estimates are more conservative. In addition, we found that significance tests based on homogeneity statistics differed occasionally from results of randomization tests, implying that inferences based solely on chi-square signif- icance tests may lead to erroneous conclusions. We conclude that resampling methods should be incorporated in meta-analysis studies, to ensure proper evaluation of main effects in ecological studies.
TL;DR: The relationship between soil net nitrogen (N) mineralization, aboveground N cycling, and aboveground net primary production (ANPP) for temperate forest ecosystems is unclear as discussed by the authors.
Abstract: The generality of relationships between soil net nitrogen (N) mineralization, aboveground N cycling, and aboveground net primary production (ANPP) for temperate forest ecosystems is unclear. It is also not known whether these variables and their rela- tionships differ between evergreen and deciduous forests, or across soil types. To address these questions we compiled data on annual rates of in situ net N mineralization and ANPP for 16 conifer and 34 hardwood forests, including plantations and natural stands on a range of soils at six locations in Wisconsin and Minnesota, USA. For 31 natural stands, 48 stands with native species (including plantations), and all data, ANPP increased linearly with annual net N mineralization rates. Native evergreen conifer and two deciduous hardwood types (oaks and mesic hardwoods) followed similar patterns in this regression, indicating common functional relationships at the ecosystem level. The relationship of N mineraliza- tion and ANPP differed between finer textured Alfisol soils and sandier Entisols, with higher ANPP at any given N mineralization level in Alfisols. A multiple regression of N mineralization on soil texture (percentage silt plus clay), litterfall N, and mean annual temperature explained 81% of the variance in annual N mineralization for natural stands, and a multiple regression of ANPP on soil texture and annual N mineralization rate explained 83% of the variance in ANPP. Naturally regenerated forest types differed in mean annual net N mineralization, litterfall N, and ANPP, and all were greater in oaks than in mesic hardwoods or conifers, respectively. However, differences among the 50 stands and six locations were largely a result of dif- ferences in soils and stand origin. For all natural hardwood stands, ANPP and N miner- alization were greater on fine-textured Alfisols than on sandy Entisols. For evergreen co- nifers, ANPP and N mineralization were greater in plantations on Alfisols than in natural stands on Histosols, Entisols, or Spodosols. Hardwood and evergreen conifer stands did not differ significantly in ANPP or N mineralization on comparable soils and stand origin: they differed neither as plantations on Alfisols nor as natural stands on Entisols. This suggests that observed average differences among natural forest types in ANPP and N mineralization resulted largely from variation in their distribution on differing soils, and not from feedback effects on N mineralization or differing productivity per available N. These data suggest that, at a regional scale, at least half of the variation in ANPP can be attributed to variation in annual N mineralization. Both ANPP and N mineralization differ more strongly with soil type/parent material than with forest type; ANPP at any given level of N mineralization is higher on silty/loamy Alfisols than on sandy Entisols, Histosols, or Spodosols, but not different for coniferous and broad-leaved deciduous species. There is no indication of N saturation of ANPP within the range of natural N availability in these
TL;DR: The slow-growth–high-mortality hypothesis predicts that prolonged development in herbivorous insects results in greater exposure to natural enemies and a subsequent increase in mortality, and this hypothesis was tested using the cabbage butterfly Pieris rapae and its larval parasitoid Cotesia glomerata.
Abstract: The slow-growth–high-mortality hypothesis predicts that prolonged development in herbivorous insects results in greater exposure to natural enemies and a subsequent increase in mortality. We tested this hypothesis using the cabbage butterfly Pieris rapae and its larval parasitoid Cotesia glomerata. We conducted a series of field and laboratory experiments to determine how variation in the larval development of P. rapae within and among four species of host plants (Brassica oleracea, Tropaeolum majus, Lunaria annua, and Cleome spinosa) influenced parasitism rates by C. glomerata. On the same host plant species, fast-developing larvae incurred less parasitism than slow-developing larvae of the same age, because once larvae reached the third instar they became much less vulnerable to attack due to the onset of encapsulation. Thus, the “window of vulnerability” was extended for slow-developing larvae. Similarly, the window of vulnerability was prolonged and larvae were susceptible to parasitism for a longer p...
TL;DR: This paper analyzes patterns of cyanobacterial dominance observed in the field and shows that these patterns imply that the algal community is a hysteretic system with two alternative equilibria, and constructs a simple competition model to show that hysteresis should in fact be expected from differences in physiology between cyanobacteria and algae.
Abstract: The phytoplankton community of eutrophic shallow lakes is often dominated by filamentous cyanobacteria of the family Oscillatoriaceae. In this paper we follow two independent approaches to show that this situation is likely to be one of two alternative stable states of the algal community. First we analyze patterns of cyanobacterial dominance observed in the field, and show that these patterns imply that the algal community is a hysteretic system with two alternative equilibria. Then, we construct a simple competition model to show that hysteresis should in fact be expected from differences in physiology between cyanobacteria and algae. The basic mechanism is that cyanobacteria are the superior competitors under conditions of low light, but also promote such conditions, as they can cause a higher turbidity per unit of phosphorus than other algae. This mechanism of hys- teresis offers an explanation for the resistance of cyanobacteria dominance in shallow lakes to restoration efforts by means of nutrient reduction.
TL;DR: In this paper, a 4-yr average of the ingegral of the NDVI (NDVI-J) using spatially aggregated values of ANPP was calibrated for temperate perennial grasslands, and a positive and statistically significant relationship between NDVI-I and ANPP for grassland areas with mean annual precipitation between 280 and 1150 mm, and mean annual temperature between 40 and 20'C.
Abstract: Several studies have suggested the existence of a positive relationship be- tween the Normalized Difference Vegetation Index (NDVI) derived from AVHRR/NOAA satellite data and either biomass or annual aboveground net primary production (ANPP) for different geographic areas and ecosystems. We calibrated a 4-yr average of the ingegral of the NDVI (NDVI-J) using spatially aggregated values of ANPP. We also provided an estimate of the energy conversion efficiency coefficient (?) of Monteith's equation. This is the first attempt to calibrate a standard NDVI product for temperate perennial grasslands. We found a positive and statistically significant relationship between NDVI-I and ANPP for grassland areas with mean annual precipitation between 280 and 1150 mm, and mean annual temperature between 40 and 20'C. Depending on the method used to estimate the fraction of photosynthetic active radiation, the energy conversion officency coefficient was constant (0.24 g C/MJ), or varied across the precipitation gradient, from 0.10 g C/MJ for the least productive to 0.20 g C/MJ for the most productive sites.
TL;DR: The results of an experimental manipulation on New England rocky shores are presented that suggest that group benefits are as important in maintaining the upper intertidal limits of dominant spaceholders on rocky shores as the negative forces of competition and predation are in maintaining lower dunes.
Abstract: Positive interactions that result from neighbors buffering one another from stressful conditions are predictably important community forces in physically stressful habitats. Here, we examine the generality of this hypothesis in marine intertidal communities. Intertidal communities have historically played a large role in the development of community ecology since they occur across pronounced physical gradients and are easily manipulated. Positive interactions, however, have not been emphasized in studies of intertidal communities. We first review studies of intertidal marsh plant communities that suggest that positive interactions play a dominant role in the structure and dynamics of these common assemblages. We then present the results of an experimental manipulation on New England rocky shores that suggests that group benefits are as important in maintaining the upper intertidal limits of dominant spaceholders on rocky shores as the negative forces of competition and predation are in maintaining lower d...
TL;DR: In this article, the authors investigate the relationship between local and regional species richness in a broad array of taxa from around the world to address five questions: 1) is the relationship among local and local species richness linear, or does local richness accumulate more slowly at pro- gressively higher regional diversities, suggesting local saturation of species diversity? Sec- ond, do these relationships vary with locality size? Third, do taxa and continents differ in the form of relationships between local on regional diversity? Fourth, do relationships between global diversity depart from that expected from a null
Abstract: The extent to which species richness in local communities is determined by regional and historical processes is not well understood. An increasingly popular way to investigate these large-scale processes is through regressions of local on regional species richness. We sampled local and regional species richness in a broad array of taxa from around the world to address five questions. First, is the relationship between local and regional species richness linear, or does local richness accumulate more slowly at pro- gressively higher regional diversities, suggesting local saturation of species diversity? Sec- ond, do these relationships vary with locality size? Third, do taxa and continents differ in the form of relationships between local and regional diversity? Fourth, do relationships between local and regional diversity depart from that expected from a null model in which all individuals of a locality are randomly sampled from a regional pool of species whose abundances have a canonical log-normal distribution? Fifth, using this same null model, how does the expected relationship between local and regional species richness depend on the sampling intensity within localities? We used distribution maps to ensure that diversity was sampled in a consistent manner across diverse taxa. Each region was 500 3 500 km, and localities were 1% and 10% of the region size. There was no evidence of local species saturation, as local species richness was strongly and linearly related to regional richness at both spatial scales. Between scales, local diversity accumulated faster as a function of regional diversity at the larger spatial scale. The slope of this relationship between local and regional diversity was the same among taxa across continents, and between Australia and North America across taxa. In other words, at each spatial scale one relationship between local and regional diversity describes most cases very well. The null model showed that approximately linear relationships between local and regional diversity are expected when regional species abundances are log-normal and when the number of individuals sampled within localities is large (roughly 200 times the number of species in the most species-rich region examined). However, empirical slopes were less than expected from the null model, which we interpret as an effect of spatial turnover of species (beta diversity). Since these slopes were nevertheless similar among taxa and between regions, rates of spatial turnover must be approximately the same among these taxa and regions. The log-normal model also showed that nonlinear (concave down) relationships between local and regional diversity are expected under random sampling when sample size is small relative to regional diversity. Therefore, nonlinear relationships are not necessarily indicative of saturation. Our results suggest that at the scales investigated here local communities are unsaturated and that their diversities are strongly limited by species richness of the surrounding regions. Similarity between taxa and continents in the form of the local-regional diversity relationship implies that ''rules'' governing the assembly of local communities may be widely consistent. If so, understanding species diversity in local assemblages will require knowledge of processes acting at larger spatial scales, in- cluding determinants of regional species richness and spatial turnover of species.
TL;DR: The fourth-corner method offers a way of analyzing the relationships between the supplementary variables associated with the rows and columns of a binary data table, and is shown to be applicable to a wide class of ecological problems.
Abstract: This paper addresses the following question: how does one relate the biological and behavioral characteristics of animals to habitat characteristics of the locations at which they are found? Ecologists often assemble data on species composition at different localities, habitat descriptions of these localities, and biological or behavioral traits of the species. These data tables are usually analyzed two by two: species composition against habitat characteristics, or against behavioral data, using such methods as canonical analysis. We propose a solution to the problem of estimating the parameters describing the relationship between habitat characteristics and biology or behavior, and of testing the statistical significance of these parameters; this problem is referred to as the fourth-corner problem, from its matrix formulation. In other words, the fourth-corner method offers a way of analyzing the relationships between the supplementary variables associated with the rows and columns of a binary (presence or absence) data table. The test case that motivated this study concerns a coral reef fish assemblage (280 species). Biological and behavioral characteristics of the species were used as supplementary variables for the rows, and characteristics of the environment for the columns. Parameters of the association between habitat characteristics (distance from beach, water depth, and substrate variables) and biological and behavioral traits of the species (feeding habits, ecological niche categories, size classes, egg types, activity rhythms) were estimated and tested for significance using permutations. Permutations can be performed in different ways, corresponding to different ecological hypotheses. Results were compared to predictions made independently by reef fish ecologists, in order to assess the method as well as the pertinence of the variables subjected to the analysis. The new method is shown to be applicable to a wide class of ecological problems.
TL;DR: A qualitative theoretical model is presented that predicts that direct positive interactions increase species diversity by facilitating species thatmight not normally survive under very high physical disturbance, stress, or predation, and suggests that facilitator species that might normally be competitively excluded are released from competition.
Abstract: Direct positive interactions (mutualisms and commensalisms) are generally accepted as important processes in communities. They appear to be most common in environments with relatively high physical disturbance, stress, or predation, where associated species can increase the growth and survival of other species unable to survive in isolation. Although ecologists have documented direct positive interactions among species for decades, there is less known about how these interactions affect community species diversity patterns.
In this paper, we present a qualitative theoretical model that considers how direct positive interactions affect community species diversity. The model uses, as its basis, familiar unimodel species diversity models (i.e., “compensatory mortality” and “intermediate disturbance” hypothesis) to understand where direct positive interactions are likely to be important. Initially, it predicts that direct positive interactions increase species diversity by facilitating species that might not normally survive under very high physical disturbance, stress, or predation. In addition, it suggests that, under intermediate physical disturbance, stress, or predation, facilitator species that might normally be competitively excluded are released from competition. We suggest that facilitator species may then create new interaction webs that would not be possible in their absence.
To illustrate these ideas, we describe a case study taken from a New England salt marsh community where a gradient in physical conditions occurs. In this community, direct positive interactions, and their indirect effects, are predicted to increase the species diversity by at least 35%. This empirical case study and model show that by incorporating direct positive interactions into ecological experiments and theory, it is possible to expand our understanding of the mechanisms responsible for community species diversity patterns.
TL;DR: This work used a combination of observations of natural ponds, “experimental natural history” of artificial ponds, and controlled experiments in an array of 144 replicate ponds to develop, then test, hypotheses about how the structures of food webs are regulated.
Abstract: A food web graphically represents the paths of nutrients and energy through the living components of an ecosystem and the context in which individuals exploit their prey and avoid their enemies. Temporary ponds are excellent arenas for the study of food webs because they are discrete communities that can be mimicked in containers that approach the realism of natural habitats. Artificial ponds permit repeatable initial conditions and sufficient replication of independent experimental units in complex experiments to test hypotheses about the control of structure and function in natural communities. I used a combination of observations of natural ponds, “experimental natural history” of artificial ponds in my study area, and controlled experiments in an array of 144 replicate ponds to develop, then test, hypotheses about how the structures of food webs are regulated. Understanding food webs begins with population biology. Amphibians use the aquatic larval stage of their biphasic life cycle to exploit ephemer...
TL;DR: No significant change in species richness was detected as a result of the harvesting, except in the 1-ha connected fragments, where the number of species increased two years after isolation, suggesting that the adjacent buffer strips were being used as movement corridors.
Abstract: We studied the effect of habitat fragmentation on the richness, diversity, turnover, and abundance of breeding bird communities in old, boreal mixed-wood forest by creating isolated and connected forest fragments of 1, 10, 40, and 100 ha. Connected fragments were linked by 100 m wide riparian buffer strips. Each size class within treatments and controls was replicated three times. We sampled the passerine community using point counts before, and in each of two years after, forest harvesting, accumulating 21340 records representing 59 species. We detected no significant change in species richness as a result of the harvesting, except in the 1-ha connected fragments, where the number of species increased two years after isolation. This increase was accounted for by transient species, suggesting that the adjacent buffer strips were being used as movement corridors. Diversity (log series alpha index) was dependent on area in the isolated fragments only after cutting, having decreased in the smaller areas. Tur...
TL;DR: In this paper, the authors present several new measures of transient response that complement resilience as a description of the response to perturbation and demonstrate the calculation of these indices using previously published linear compartment models (two models for phosphorus cycling through a lake ecosystem and one for the flow of elements through a tropical rain forest).
Abstract: Resilience is a component of ecological stability; it is assessed as the rate at which perturbations to a stable ecological system decay. The most frequently used estimate of resilience is based on the eigenvalues of the system at its equilibrium. In most cases, this estimate describes the rate of recovery only asymptotically, as time goes to infinity. However, in the short term, perturbations can grow significantly before they decay, and eigenvalues provide no information about this transient behavior. We present several new measures of transient response that complement resilience as a description of the response to perturbation. These indices measure the extent and duration of transient growth in models with asymptotically stable equilibria. They are the reactivity (the maximum possible growth rate immediately following the perturbation), the maximum amplification (the largest proportional deviation that can be produced by any perturbation), and the time at which this amplification occurs. We demonstrate the calculation of these indices using previously published linear compartment models (two models for phosphorus cycling through a lake ecosystem and one for the flow of elements through a tropical rain forest) and a standard nonlinear predator-prey model. Each of these models exhibits transient growth of perturbations, despite asymptotic stability. Measures of relative stability that ignore transient growth will often give a misleading picture of the response to a perturbation.
TL;DR: A new synthesis of theoretical and empirical evidence is provided that elucidates and extends a mechanism of population regulation for species whose individuals preemptively use sites that differ in suitability.
Abstract: The nature and extent of population regulation remains a principal unanswered question for many types of organisms, despite extensive research. In this paper, we provide a new synthesis of theoretical and empirical evidence that elucidates and extends a mechanism of population regulation for species whose individuals preemptively use sites that differ in suitability. The sites may be territories, refuges from predation, oviposition sites, etc. The mechanism, which we call site dependence, is not an alternative to density dependence; rather, site dependence is one of several mechanisms that potentially generate the negative feedback required for regulation. Site dependence has two major features: (1) environmentally caused heterogeneity among sites in suitability for reproduction and/or survival; and (2) preemptive site occupancy, with the tendency for individuals to move to sites of higher quality as they become available. Simulation modeling shows that these two features, acting in concert, generate nega...
TL;DR: It is found that the joint pattern of selection acting on tolerance and two resistance traits, trichome density and total glucosinolate concentration, indicated that there were not alternate peaks in the fitness landscape favoring either resistance or tolerance, and selection favored the retention of both tolerance and resistance.
Abstract: Plants can employ two general strategies to defend themselves against herbivory: they can either reduce the amount of damage they experience (resistance), or they can tolerate herbivore damage Theoretical considerations suggest that, in many cases, tolerance and resistance are redundant strategies, and may therefore be mutually exclusive adaptations In this investigation of natural populations of the annual plant Arabidopsis thaliana we examine whether the pattern of selection acting on resistance and tolerance favors the evolution of one defense strategy, or the other, but not both We found that the joint pattern of selection acting on tolerance and two resistance traits, trichome density and total glucosinolate concentration, indicated that there were not alternate peaks in the fitness landscape favoring either resistance or tolerance Rather, selection favored the retention of both tolerance and resistance One reason for the absence of mutually exclusive alternative resistance/tolerance strategies is the absence of a negative genetic correlation between resistance and tolerance An unexpected result is the detection of disruptive selection acting on tolerance, which seems to result from a nonlinear relationship between tolerance and its costs
TL;DR: A mechanistic model is developed that reproduces the spatial and temporal dynamics of phytoplankton prior to the invasion of the zebra mussel and suggests that the fate of light- scattering inorganic particles (turbidity) is a key feature determining the impact of benthic grazers in aquatic systems.
Abstract: Changes in the biomass of benthic bivalves can cause dramatic changes in total grazing pressure in aquatic systems, but few studies document ecosystem-level impacts of these changes. This study documents a massive decline in phytoplankton biomass con- current with the invasion of an exotic benthic bivalve, the zebra mussel ( Dreissena poly- morpha), and demonstrates that the zebra mussel actually caused this decline. In the fall of 1992 the zebra mussel became established at high biomass in the Hudson River Estuary, and biomass of mussels remained high during 1993 and 1994. During these 2 yr, grazing pressure on phytoplankton was over 10-fold greater than it had been prior to the zebra mussel invasion. This increased grazing was associated with an 85% decline in phyto- plankton biomass. Between 1987 and 1991 (pre-invasion), summertime chlorophyll aver- aged 30 mg/m 3 ; during 1993 and 1994 summertime concentrations were ,5 mg/m 3 . Over this same period, light availability increased, phosphate concentrations doubled, some planktonic grazers declined, and average flow was not different from the pre-invasion period. Thus, changes in these other factors were not responsible for phytoplankton declines. We developed a mechanistic model that reproduces the spatial and temporal dynamics of phytoplankton prior to the invasion of the zebra mussel (1987-1991). The model ac- curately predicts extreme declines in phytoplankton biomass after the invasion (1993-1994). The model demonstrates that zebra mussel grazing was sufficient to cause the observed phytoplankton decline. The model also allows us to test which features make the Hudson River sensitive to the impact of benthic grazers. The model suggests that the fate of light- scattering inorganic particles (turbidity) is a key feature determining the impact of benthic grazers in aquatic systems.