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Disentangling the Importance of Ecological Niches from Stochastic Processes Across Scales Jonathan M

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Disentangling the Importance of Ecological Niches from Stochastic Processes Across Scales Jonathan M Phil. Trans. R. Soc. B (2011) 366, 2351–2363 doi:10.1098/rstb.2011.0063 Research Disentangling the importance of ecological niches from stochastic processes across scales Jonathan M. Chase* and Jonathan A. Myers Department of Biology and Tyson Research Center, Washington University in St Louis, St Louis, MO 63130, USA Deterministic theories in community ecology suggest that local, niche-based processes, such as environmental filtering, biotic interactions and interspecific trade-offs largely determine patterns of species diversity and composition. In contrast, more stochastic theories emphasize the import- ance of chance colonization, random extinction and ecological drift. The schisms between deterministic and stochastic perspectives, which date back to the earliest days of ecology, continue to fuel contemporary debates (e.g. niches versus neutrality). As illustrated by the pioneering studies of Robert H. MacArthur and co-workers, resolution to these debates requires consideration of how the importance of local processes changes across scales. Here, we develop a framework for disentan- gling the relative importance of deterministic and stochastic processes in generating site-to-site variation in species composition (b-diversity) along ecological gradients (disturbance, productivity and biotic interactions) and among biogeographic regions that differ in the size of the regional species pool. We illustrate how to discern the importance of deterministic processes using null- model approaches that explicitly account for local and regional factors that inherently create stochastic turnover. By embracing processes across scales, we can build a more synthetic framework for understanding how niches structure patterns of biodiversity in the face of stochastic processes that emerge from local and biogeographic factors. Keywords: b-diversity; biogeography; community assembly; ecological drift; niche selection; regional species pool 1. INTRODUCTION of evolutionary processes that shape the structure of A species’ niche is determined by the traits that allow it ecological assemblages (e.g. [10,11]). Armed with to gather resources, evade enemies and any other these distinct but related components of the niche, factor that influences its relative birth and death rates we can develop a wide variety of expectations for [1,2]. Conditions within the N-dimensional hyper- how niches influence species distributions and range volume of a species’ needs [2], in which birth rates limits, the distribution and co-occurrence of dis- are equal to or greater than death rates, represent a parately or closely related species, and patterns of species’ ‘requirement’ (sensu [3,4]) or ‘Grinnellian’ biodiversity along environmental and biogeographic niche (sensu [5]). A niche can also be described as gradients (e.g. [4,5,9,12–14]). the interactions a species has with its biotic and abi- Although the niche is often heralded as one of the otic environment [6]. The corresponding influence most important concepts in community ecology (e.g. that a species has on its environment is known as its [4,15–19]), studies of the niche are often considered ‘impact’ niche (sensu [3,4]) or ‘Eltonian’ niche (sensu tautological, owing to the seemingly insurmountable [5]). Niche concepts were instrumental in the devel- task of defining the dimensions of the niche, especially opment of the ‘local perspective’ of ecological those that help maintain species diversity [16,20,21]. communities, which views communities as limited- Furthermore, it has recently been argued that evidence membership assemblages in which interspecific biotic for niche differences that promote stable coexistence of interactions and abiotic conditions are the primary fac- species is, in fact, quite sparse [22]. Empirical evi- tors influencing community composition and diversity dence for niche-based coexistence has been limited, (e.g. [7,8]). Today, niche concepts are used widely in in part, by the challenges associated with testing stabi- ecology (e.g. [4]), biogeography (e.g. ecological or lizing and equalizing mechanisms that promote species environmental niche models; [9]) and in the study coexistence in many communities [15,19]. Nonetheless, criticisms of niche theory, many quite valid, have stimulated numerous theoretical and empirical investi- *Author for correspondence ([email protected]). gations that call into question the centrality of the One contribution of 10 to a Theme Issue ‘Biogeography and niche concept and the role of local niche-based pro- ecology: two views of one world’. cesses in generating patterns of biodiversity (e.g. 2351 This journal is q 2011 The Royal Society 2352 J. M. Chase & J. A. Myers Ecological niches across scales [8,23,24]). Consequently, there has been a major the face of stochastic ecological processes that emerge shift towards more regional perspectives on ecologi- from local and biogeographic factors. cal communities, as well as integration of local and regional perspectives, to better understand how processes at broader scales influence biodiversity (a) The niche across scales: revisiting (e.g. [8,25–28]). MacArthur’s paradox Here, we develop a framework for testing the Robert H. MacArthur’s pioneering work helped to lay importance of ecological niches in generating com- the foundations for several modern subfields of ecol- munity structure at local and biogeographic scales. ogy, including evolutionary and foraging ecology We begin by illustrating how the role of niches varies [34], coexistence and biodiversity theory (e.g. [31]), with scale using examples from pioneering studies by island biogeography theory [29,30] and large-scale Robert H. MacArthur et al. (e.g. [29–31]). We then studies across biogeographic gradients (i.e. temperate describe two complementary approaches that combine versus tropical communities; [35]). Importantly, each patterns of site-to-site variation in species composition of these fields focuses on different spatial scales, (b-diversity) with null-model analyses to disentangle ranging from variation among individuals and popula- the importance of niche-based process from stochastic tions within localities to variation among communities ecological processes at local and biogeographic scales. and regions across biogeographic provinces. The con- We contrast completely deterministic, niche-based cept of the ecological niche, as developed by Grinnell processes with more stochastic (e.g. neutral) [1] and Elton [6], and codified by Hutchinson (e.g. processes, such as chance colonization, random extinc- [2,36,37]), was central to MacArthur’s work. How- tion and ecological drift (random changes in species ever, the prominence of the niche concept—the relative abundance; [8]). Note, however, that we do interaction between a species’ traits and its environ- not generally consider the issue of speciation (neutral ment—varied depending on the primary spatial scale or otherwise) in our conceptual framework; instead, of inquiry. At one extreme, MacArthur’s work on we leave those discussions to our evolutionary biology limiting similarity, competition and coexistence (e.g. colleagues (e.g. [14]). Throughout the paper, we use [31]) viewed species trait–environment relations and the term ‘deterministic process’ to refer to any ecologi- interspecific trade-offs as paramount for species coex- cal process that involves non-random, niche-based istence and diversity at local spatial scales. At the other mechanisms, and we use the term ‘stochastic process’ extreme, MacArthur’s work on the theory of island to refer to any ecological process that gives rise to biogeography [29,30] ignored any differences among patterns of species diversity, relative abundance and species in their traits (i.e. species were neutral) when composition that are indistinguishable from random predicting species diversity on islands that vary in chance alone. Although debate exists as to how some their size and isolation from the mainland as a function processes influence both stochastic and determinis- of colonization and extinction rates. tic community assembly (e.g. dispersal limitation; ‘MacArthur’s Paradox’ [38,39] highlights the fact [15,32]), it is not the goal of our paper to resolve such that his different theories placed very distinct emphasis debates. on the relative importance of niche-based proces- Our framework provides two general insights into ses. His use of niche concepts in theoretical studies how ecological niches influence patterns of biodiver- depended on the phenomena under investigation sity. First, it explicitly considers how the relative and the spatial scale of the study: limiting similarity importance of stochastic processes changes with scale and coexistence were used to understand patterns of both within (e.g. between local habitats that vary in coexisting species at local scales whereas island bio- size) and among (e.g. between regions with large geography theory was used to understand patterns of versus small species pools) biogeographic regions. compositional turnover and diversity at broader spatial Second, it offers an approach for disentangling the scales. However, he was well aware of the connections relative importance of deterministic and stochastic between the two scales, and incorporated larger spatial processes in generating b-diversity along ecological processes into local community interactions using nas- gradients (disturbance, productivity and biotic inter- cent metacommunity models (e.g. [40,41]), as well as actions) and among biogeographic regions that differ
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