Robert D. Holt

Predation, apparent competition, and the structure of prey communities

Abstract It is argued that alternate prey species in the diet of a food-limited generalist predator should reduce each others equilibrial abundances, whether or not they directly compete. Such indirect, interspecific interactions are labeled apparent competition . Two examples are discussed in which an observed pattern of habitat segregation was at first interpreted as evidence for direct competition, but later interpreted as apparent competition resulting from shared predation. In order to study the consequences of predator-mediated apparent competition in isolation from other complicating factors, a model community is analyzed in which there is no direct interspecific competition among the prey. An explicit necessary condition for prey species coexistence is derived for the case of one predator feeding on many prey species. This model community has several interesting properties: (1) Prey species with high relative values for a parameter r a are “keystone” species in the community; (2) prey species can be excluded from the community by “diffuse” apparent competition; (3) large changes in the niche breadth of the predator need not correspond to large changes in predator density; (4) the prey trophic level as a whole is regulated by the predator, yet each of its constituent species is regulated by both the predator and available resources; (5) increased productivity may either increase, decrease, or leave unchanged the number of species in the community; (6) a decrease in density-independent mortality may decrease species diversity. These conclusions seem to be robust to changes in the prey growth equations and to the incorporation of predator satiation. By contrast, adding prey refugia or predator switching to the model weakens these conclusions. If the predator can be satiated or switched, the elements a ij comprising the community matrix may have signs opposite the long-term effect of j upon i . The effect of natural selection upon prey species coexistence is discussed. Unless r i , K i , and a i are tightly coupled, natural selection within prey species i will tend to decrease the equilibrial abundance of species j .

Trophic Downgrading of Planet Earth

Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind’s most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.

Niche conservatism as an emerging principle in ecology and conservation biology.

The diversity of life is ultimately generated by evolution, and much attention has focused on the rapid evolution of ecological traits. Yet, the tendency for many ecological traits to instead remain similar over time [niche conservatism (NC)] has many consequences for the fundamental patterns and processes studied in ecology and conservation biology. Here, we describe the mounting evidence for the importance of NC to major topics in ecology (e.g. species richness, ecosystem function) and conservation (e.g. climate change, invasive species). We also review other areas where it may be important but has generally been overlooked, in both ecology (e.g. food webs, disease ecology, mutualistic interactions) and conservation (e.g. habitat modification). We summarize methods for testing for NC, and suggest that a commonly used and advocated method (involving a test for phylogenetic signal) is potentially problematic, and describe alternative approaches. We suggest that considering NC: (1) focuses attention on the within-species processes that cause traits to be conserved over time, (2) emphasizes connections between questions and research areas that are not obviously related (e.g. invasives, global warming, tropical richness), and (3) suggests new areas for research (e.g. why are some clades largely nocturnal? why do related species share diseases?).

Impacts of biodiversity on the emergence and transmission of infectious diseases

Current unprecedented declines in biodiversity reduce the ability of ecological communities to provide many fundamental ecosystem services. Here we evaluate evidence that reduced biodiversity affects the transmission of infectious diseases of humans, other animals and plants. In principle, loss of biodiversity could either increase or decrease disease transmission. However, mounting evidence indicates that biodiversity loss frequently increases disease transmission. In contrast, areas of naturally high biodiversity may serve as a source pool for new pathogens. Overall, despite many remaining questions, current evidence indicates that preserving intact ecosystems and their endemic biodiversity should generally reduce the prevalence of infectious diseases.

Intraguild predation: The dynamics of complex trophic interactions

There is a long-standing debate in ecology concerning the relative importance of competition and predation in determining community structure. Recently, a novel twist has been added with the growing recognition that potentially competing species are often engaged in predator-prey interactions. This blend of competition and predation is called intraguild predation (IGP). The study of IGP will lead to a reconsideration of many classical topics, such as niche shifts, species exclusion and cascading interactions in food webs. Theoretical models suggest that a variety of alternative stable states are likely in IGP systems, and that intermediate predators should tend to be superior in exploitative competition. Many field studies support these expectations. IGP is also important in applied ecological problems, such as the conservation of endangered species and fisheries management.

A framework for community interactions under climate change.

Predicting the impacts of climate change on species is one of the biggest challenges that ecologists face. Predictions routinely focus on the direct effects of climate change on individual species, yet interactions between species can strongly influence how climate change affects organisms at every scale by altering their individual fitness, geographic ranges and the structure and dynamics of their community. Failure to incorporate these interactions limits the ability to predict responses of species to climate change. We propose a framework based on ideas from global-change biology, community ecology, and invasion biology that uses community modules to assess how species interactions shape responses to climate change.

THE EVOLUTION OF DISPERSAL IN SPATIALLY AND TEMPORALLY VARYING ENVIRONMENTS

Using a simple two-patch model, we examine how patterns of spatial and temporal variation in carrying capacities affect natural selection on dispersal. The size of the population in each patch is regulated separately, according to a discrete-generation logistic equation, and individuals disperse from each patch at propensities determined by their genotype. We consider genotypes that express the same dispersal propensities in both patches and genotypes that express patch-specific disperal propensities. Contrary to previous analyses, our results show that some level of dispersal is favored by selection under almost all regimes of habitat variability, including a spatially varying and temporally constant environment. Furthermore, two very different polymorphisms are favored under different conditions. When carrying capacities vary spatially but not temporally, any number of genotypes with patch-specific dispersal propensities in ratios inversely proportional to the ratio of the carrying capacities can coexist. This result extends previous analyses to show that dispersal is favored in such an environment if individuals can alter dispersal propensities in response to environmental cues. In contrast, when carrying capacities vary both spatially and temporally but differ in mean or variance, a polymorphism of only two genotypes (a high-dispersal and a no-dispersal genotype) is favored when the only genotypes possible are ones expressing the same dispersal propensity in both patches. However, this dimorphism can be invaded and replaced by one genotype with a particular combination of patch-specific dispersal propensities in a ratio also inversely proportional to the ratio of the average population sizes. We discuss a number of testable predictions this model suggests about the amounts of phenotypic and genetic variation in dispersal characters that are expected both within and between populations, and the degree to which the expression of phenotypic characters affecting dispersal propensity should be sensitive to environmental conditions. The model also suggests novel mechanisms for coexistence between competing species in varying environments.

Population dynamics in two-patch environments: Some anomalous consequences of an optimal habitat distribution

Abstract The effect of dispersal on population size and stability is explored for a population that disperses passively between two discrete habitat patches. Two basic models are considered. In the first model, a single population experiences density-dependent growth in both patches. A graphical construction is presented which allows one to determine the spatial pattern of abundance at equilibrium for most reasonable growth models and rates of dispersal. It is shown under rather general conditions that this equilibrium is unique and globally stable. In the second model, the dispersing population is a food-limited predator that occurs in both a source habitat (which contains a prey population) and a sink habitat (which does not). Passive dispersal between source and sink habitats can stabilize an otherwise unstable predator-prey interaction. The conditions allowing this are explored in some detail. The theory of optimal habitat selection predicts the evolutionarily stable distribution of a population, given that individuals can freely move among habitats so as to maximize individual fitness. This theory is used to develop a heuristic argument for why passive dispersal should always be selectively disadvantageous (ignoring kin effects) in a spatially heterogeneous but temporally constant environment. For both the models considered here, passive dispersal may lead to a greater number of individuals in both habitats combined than if there were no dispersal. This implies that the evolution of an optimal habitat distribution may lead to a reduction in population size; in the case of the predator-prey model, it may have the additional effect of destabilizing the interaction. The paper concludes with a discussion of the disparate effects habitat selection might have on the geographical range occupied by a species.

Habitat fragmentation and its lasting impact on Earth's ecosystems

Urgent need for conservation and restoration measures to improve landscape connectivity. We conducted an analysis of global forest cover to reveal that 70% of remaining forest is within 1 km of the forest’s edge, subject to the degrading effects of fragmentation. A synthesis of fragmentation experiments spanning multiple biomes and scales, five continents, and 35 years demonstrates that habitat fragmentation reduces biodiversity by 13 to 75% and impairs key ecosystem functions by decreasing biomass and altering nutrient cycles. Effects are greatest in the smallest and most isolated fragments, and they magnify with the passage of time. These findings indicate an urgent need for conservation and restoration measures to improve landscape connectivity, which will reduce extinction rates and help maintain ecosystem services.

Analysis of adaptation in heterogeneous landscapes: Implications for the evolution of fundamental niches

SummaryThe fundamental niche is a description of the range of environmental conditions in which the mean fitness of a population exceeds or equals unity, and outside of which its mean fitness is less than one. The fundamental niche is a mean phenotype of a population, a trait that can evolve by natural selection. In the analysis of the evolution of adaptations by natural selection one must specify the range of environments within which the relative fitnesses of alternative phenotypes are compared. Population dynamics automatically biases the environments experienced by an evolutionary lineage, simply because more individuals tend to be found within the fundamental niche than outside it (unless the population as a whole is going extinct). We argue that this basic asymmetry biases adaptive evolution toward further improvement to conditions inside the fundamental niche, even at the expense of fitness outside it. This suggests that natural selection may act principally as a conservative force on fundamental niches. We place the particular problem of the evolution of fundamental niches into the general framework of specifying the spatiotemporal scale for the analysis of adaptation in heterogeneous environments and introduce the notion of a ‘phylogenetic envelope’, a heuristic representation of this scaling. Because all of microevolution necessarily occurs within the constraint of the evolutionary dynamics of the fundamental niche, we conclude that understanding such dynamics should be of central concern to evolutionary ecologists.