Metacommunities

Just as the dispersal of individuals may link the dynamics of populations in space, the dispersal of species among communities may link local communities into a metacommunity. Four different perspectives characterize how dispersal rates, environmental heterogeneity, and species traits interact to influence diversity in metacommunities. These perspectives are: patch dynamics, species sorting, mass effects, and the neutral perspective. The neutral perspective stands in stark contrast to the other three perspectives in that it assumes that niche differences between species are unimportant and that species are demographically identical in terms of their birth, death, and dispersal rates. Under the neutral perspective, species diversity is maintained by a balance between speciation, extinction, and dispersal. Although neutral theory is incompatible with realistic modes and rates of speciation, it has been enormously influential in focusing our attention on the linkages between species interactions on local scales, and evolutionary and biogeographic processes occurring on large scales.

Community assembly and species traits

There is perhaps no more fundamental question in ecology than what determines the number and kinds of species found in a community and their relative abundances. This chapter lays out a powerful approach to answering this question, based on the concepts of a regional species pool and environmental filters. The species pool is the set of species that could potentially colonize a local site or community. Of these potential colonists, some species are limited in their ability to disperse to site, some are limited by their ability to survive the abiotic environment, and some are limited by their interactions with other species. These “filters” act individually or in concert, and the functional traits of species determine their success in passing through these filters to colonize a local site. There is growing empirical evidence that both abiotic and biotic processes select for specific functional traits. Focusing on the functional traits of species may lead to rules of community assembly that are general and help unify a variety of more specific theories.

Beneficial interactions in communities: mutualism and facilitation

The consequences of beneficial interactions for the diversity and functioning of communities remain poorly understood, but this is changing. This chapter examines how mutualism may evolve in the face of cheating, using the concept of biological markets where members of each species exchange resources and services, with associated costs and benefits. Understanding the evolution and maintenance of positive interactions in communities requires that we consider the broader web of interactions and abiotic conditions in which mutualisms are embedded—their context dependency. Ant-plant mutualisms, plant-Rhizobium mutualisms, and plant-mycorrhizal fungi mutualisms are discussed as examples of shifting costs and benefits based on context dependency. Recent advances at incorporating positive interactions into community theory allow species to have both positive and negative effects on each other’s population growth rate. For example, the presence of a neighboring plant may enhance survival in a harsh environment, but may reduce plant growth due to competition for resources.

Population growth and density dependence

This chapter reviews the basic mathematics of population growth as described by the exponential growth model and the logistic growth model. These simple models of population growth provide a foundation for the development of more complex models of species interactions covered in later chapters on predation, competition, and mutualism. The second half of the chapter examines the important topic of density-dependence and its role in population regulation. The preponderance of evidence for negative density-dependence in nature is reviewed, along with examples of positive density dependence (Allee effects). The study of density dependence in single-species populations leads naturally to the concept of community-level regulation, the idea that species richness or the total abundance of individuals in a community may be regulated just like abundance in a single-species population. The chapter concludes with a look at the evidence for community regulation in nature and a discussion of its importance.

The fundamentals of predator–prey interactions

This chapter introduces the concept of the consumer-resource link, the idea that each species in a community consumes resources and is itself consumed by other species. The consumer–resource link is the fundamental building block from which more-complex food chains and food webs are constructed. The chapter continues by exploring what is arguably the simplest consumer–resource interaction—one predator species feeding on one species of prey. Important topics discussed in the context of predator–prey interactions are the predator’s functional response, the Lotka–Volterra predator–prey model, the Rosenzweig–MacArthur predator–prey model, and the suppression-stability trade-off. Isocline analysis is introduced as a method for visualizing the outcome of species interactions at steady-state or equilibrium. Herbivory and parasitism are briefly discussed within the context of general predator–prey models.

Species in variable environments

Species and communities may exist in a dynamic state of change in response to environmental variation and disturbance. This chapter explores the consequences of variable environments and disturbance to species interactions and community structure. In particular, it examines how disturbance can result in the succession of ecological communities, how disturbance may promote (or hinder) species coexistence, how a varying environment can promote species coexistence through a mechanism called the “storage effect”, and how communities may shift between alternative states in response to environmental change. The latter topic is particularly relevant to the management of biotic resources and the restoration of degraded ecosystems, as systems may respond to environmental change abruptly at a “tipping point”, leading to alternative community states that can be difficult to reverse.

Patchy environments, metapopulations, and fugitive species

Populations and species are distributed heterogeneously across the landscape and this has important consequences for their abundance, persistence, and interactions with other species. This chapter introduces the concept of a metapopulation, a “population of populations”, where populations occur in patches of suitable habitat surrounded by areas of unsuitable habitat (“matrix”), and where dispersal serves to connect patch dynamics. Metapopulation theory provides an important conceptual underpinning to the field of conservation biology, fostering the study of corridors and assisted migration as important conservation tools. There also are important parallels between metapopulation theory and epidemiology. The study of patchily distributed populations leads naturally to considerations of species interactions, where it is shown that an inferior competitor may coexist with a superior competitor if the inferior competitor is better a colonizing open patches—a “fugitive species”. This competition-colonization trade-off can be a strong stabilizing mechanism for maintaining biodiversity in a patchy environment.

Species coexistence and niche theory

Ecologists have long puzzled over the question of how multiple species may coexist in a community in the face of strong interspecific competition. This chapter explores the answers that modern coexistence theory provides to this question. Spatial and temporal heterogeneity in the environment, in conjunction with species differences in resource use and dispersal (niche differences and trade-offs) provide important general coexistence mechanisms. More specifically, theory shows that stable species coexistence is enhanced when intraspecific competition is stronger than interspecific competition (α‎jj>α‎ji). Recent theoretical developments show how this criteria for coexistence (α‎jj>α‎ji) is related to two different aspects of the niche. Less niche overlap between competing species provides a positive stabilizing effect on species coexistence, whereas greater equality in the invasion fitness of competing species provides a positive equalizing effect on coexistence. Analysis of models of apparent competition due to shared predation yield similar conclusions.

Biodiversity and ecosystem functioning

This chapter examines the relationship between biodiversity (most often measured as species richness) and the functioning of ecosystems. Examined in detail are the effects of biodiversity on: ecosystem productivity, nutrient use and nutrient retention, community and ecosystem stability, and invasibility by exotic species. A careful look, over two decades, at experimental results and meta-analyses confirms the positive impact of species richness on productivity, ecosystem stability, and nutrient retention. Thus, we can confidently conclude that biodiversity matters to the healthy functioning of ecosystems, although we do not yet know how many species are needed to ensure the successful functioning of any given ecosystem. The chapter concludes with a discussion of six important, but unanswered question in the study of biodiversity and ecosystem functioning.

Some concluding remarks and a look ahead

This chapter reflects on the successes achieved and challenges that remain in the study of ecological communities. It concludes with a discussion of research topics expected to occupy the attention of community ecologists for the next decade or so and that may yield big dividends in terms of understanding the processes that structure communities and govern their functioning. These include metacommunities and the integration of local and regional processes; the drivers of regional biodiversity; community assembly and functional traits; pathogens, parasites and natural enemies; biodiversity and ecosystem functioning; changing technology will change how we collect data; eco-evolutionary feedbacks and regional pool processes; climate change, and its effects on species distributions and species interactions; and the role of time.