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Community Ecology Neutral community ecology & metacommunities Click here for supplemental materials for today (PDF) Outline: 1. The waxing and waning of the importance of competition 2. Neutral theory vs. niche theory 3. Metacommunities 4. Final thoughts, course recap Terms/people: neutral theory (cf. niche theory) ecological equivalence metacommunity Stephen Hubbell ecological drift community phylogenetics By definition, community ecology is concerned with interspecific interactions. Such biotic interactions have been considered to be crucial in explaining organism-environment relationships and patterns in the abundance and distribution of organisms. Adaptive trade-offs preclude the evolution of a “super-species” that is superior at everything, so there is co-occurrence of interacting species. Of the biotic interactions, competition and predation have been considered the most ubiquitous and influential (with parasitism and mutualisms less so, and other types of interactions only rarely important and only under certain, limited circumstances). Competition has been particularly invoked (and attacked) by community ecologists since it involves multiple species simultaneously (and thus is in the purview of CE) compared to predation, which involves only 2 species (predator and prey) at a time (and thus is more relevant to PE). -competition is implicit in MANY aspects of CE, including niche theory (limiting similarity, character displacement, fundamental vs. realized niche), succession, community assembly and priority effects, island biogeography, etc. -however, the ubiquity of competition has been questioned: for example, it has been said that competition may be important primarily during "ecological crunches" (Wiens); more recently, an even greater downplaying of the importance of competition has emerged in neutral theory Neutral theory - Hubbell 2001, Rosindell et al. 2011 -“unified theory of biodiversity and biogeography”: IB doesn’t predict species abundances, and species-abundance models don’t predict species richness; neutral theory attempts to combine these two -"neutral" because "trophically similar" species are considered to be equal in terms of competitive abilities and fitness ("ecological equivalence": equality of fitness of individuals over different environmental conditions at various spatiotemporal scales) (terminology comes from Kimura and Crow’s (1964) neutral theory of molecular evolution whereby switching out alleles at a locus did not affect an organism’s fitness, meaning that organisms are more affected by mutation and drift than natural selection) therefore, higher-order (i.e., community) patterns (incl. species-abundance, species-area, diversity, distribution-abundance, and other patterns that are strikingly similar regardless of location or taxa) are largely due to stochastic factors and not biotic interactions -instead, dispersal is the key mechanism, not competition or density-dependence -"ecological drift" analogous to genetic drift (stochasticity implicit), whereby species that can withstand certain abiotic conditions wind up in certain areas; these species are thus alike in certain ways -changes in species abundances are due to random processes akin to genetic drift -dispersal limitation leads to clumping, which forces intraspecific competition to be more influential than interspecific competition therefore, community structure is donorcontrolled from input of new species (via dispersal) and by speciation -dispersal is considered to be more important than species-specific differences in demography -competition is not absent (a common misconception); instead, species are well-matched and intra > inter -cf. "niche theory" (see Chase and Leibold 1993, Leibold and McPeek 2006): communities assembled thru local environmental filters and interspecific competitive exclusion (think of work by Tilman, Diamond) -neutral theory does NOT mean that community patterns are purely random (a common misconception) instead, the main mechanism is dispersal limitation (but what about competition/dispersal tradeoffs?), which influence the relative strengths of intra- and interspecific interactions -the dynamics of ecological communities are thus a zero-sum game -because so much of CE invokes interspecific competition (from niche theory, IGP, food web dynamics, community assembly, succession, etc.), neutral theory has been both lauded and ridiculed a paradigm shift in the making? -“One of the main goals in producing the neutral theory was to stir the scientific pot vigorously, which in my opinion has been overdue in community ecology for a long time.” (Hubbell 2003) -criticisms: Enquist et al. 2002, Whitfield 2002, Clark 2009, Purves and Turnbull 2010, many others -claim of a "unified theory of biodiversity and biogeography" is pretentious because it is unlikely that a single theory will explain all patterns and processes at all scales at all times -perhaps ecological equivalence is the result of past competition? (Chave 2004) -very difficult to study because of all the species involved, issues of inclusion/exclusion, interactions and dispersal difficult to observe, timescales -empirical tests: reviewed by Holyoak and Loreau 2006, McGill et al. 2006 -often used as a null model, although doing so promotes duality thinking, yet both neutral and nonneutral factors may be present within a community (Tilman 2004) -useful because one doesn’t need to supply species-specific details about all members of a community to capture overall patterns (that is, neutral theory is simpler) Neutral theory assumes that there are local communities and metacommunities. Within local communities, extinction/recolonization follow MacArthur and Wilson’s theory of island biogeography. A metacommunity (see Leibold et al. 2004) is a community of communities that are linked by dispersal of multiple, potentially interacting species. The development of the metacommunity concept illustrates the growing complexity in our thinking about biodiversity, and why certain species can and do co-occur. But some (e.g. Cavender-Bares et al. 2009) have argued that these processes are not relevant on evolutionary time scales, and argue for a more phylogenetic approach to examining how communities are structured and maintained: On the one hand, environmental filtering will select for species with similar traits in the same environment, but then fierce competition among similar species will limit the similarity that is possible. Community phylogenetics examines the relative importance of competitive exclusion and character displacement in community assembly. The premise is that IF environmental filtering dictates community structure, THEN species in a community should be more similar in traits than expected (trait clustering). But IF competition is the driver, THEN trait overdispersion would be expected. But even this isn’t straightforward and is scale-dependent: at a regional scale, environmental filtering appears to lead to trait clustering, but at a local scale, competition/limiting similarity leads to trait overdispersion (e.g. Cavender-Bares et al. 2004 on FL oaks, Graham et al. 2009 on tropical hummingbirds). Course recap: - a community is a multispecies assemblage living within a circumscribed area - members of a community interact with each other and with their environment, both directly and indirectly - communities have a structure based on diversity properties - communities are dynamic over time - community structure is often repeated over space (due to similar environmental conditions and/or convergent evolution), resulting in consistent community patterns - these patterns inspire hypotheses about processes, which must be tested against null models about community assemblage - these patterns must be recognized as being scale-dependent - anthropogenic changes to the environment impact community structure - communities are subject to both local (e.g. competition, predation) and regional effects (e.g. dispersal) Perhaps most importantly, you have been exercising your critical thinking skills throughout this course. Critical thinking is a vital skill in evaluating scientific claims in community ecology and claims in life in general. There is a risk in exercising this skill, however: you soon realize just how little is known with certainty! Or as the German writer/philosopher Goethe put it, “Doubt grows with knowledge.” There are numerous “reflections” on community ecology, ranging from historical (Roughgarden 2009) to personal and pointedly critical (Lawton 2000) to unneeded borrowing from population genetics (Vellend 2010). I urge you to read these (and others) to gain greater perspective about the field, for there is more than I have had time to cover in this course. The world has changed, from glaciers, meteors, fire and fire suppression, invasive species, Pleistocene overkill, habitat loss and fragmentation, etc., resulting in an impoverished world: there are fewer songbirds, large predators, legendary fish, and exquisite wildflowers. Global biodiversity is now less than it was at the start of this semester. What are we going to do about that? Watch it diminish, or put our knowledge and skills to use? References: NOTE: Ecology vol. 87 iss. 6 from 2006 has a special feature on neutral community ecology. Cavender-Bares, J., D.D. Ackerly, D.A. Baum, and F.A. Bazzaz. 2004. Phylogenetic overdispersion in Floridian oak communities. Am. Nat. 163:823-843. Cavender-Bares, J., K.H. Kozak, P.V.A. Fine, and S.W. Kembel. 2009. The merging of community ecology and phylogenetic biology. Ecol. Lett. 12:693-715. Chase, J.M., and M.A. Leibold. 1993. Ecological Niches: Linking Classical and Contemporary Approaches. University of Chicago Press, Chicago, IL. Chave, J. 2004. Neutral theory and community ecology. Ecol. Lett. 7:241-253. Clark, J.S. 2009. Beyond neutral science. TREE 24:8-15. Enquist, B.J., J. Sanderson, and M.D. Weiser. 2002. Modeling macroscopic patterns in ecology. Science 295:1835-1836. Graham, C.H., J.L. Parra, C. Rahbek, and J.A. McGuire. 2009. Phylogenetic structure in tropical hummingbird communities. PNAS 106:19673-19678. Holyoak, M., and M. Loreau. 2006. Reconciling empirical ecology with neutral community models. Ecology 87:1370-1377. Hubbell, S.P. 2001. The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press, Princeton, NJ. Hubbell, S.P. 2003. Modes of speciation and the lifespans of species under neutrality: a response to the comment of Robert E. Ricklefs. Oikos 100:193-199. Kimura, M., and J.R. Crow. 1964. The number of alleles that can be maintained in a finite population. Genetics 49:725-738. Lawton, J.H. 2000. Community Ecology in a Changing World. Ecology Institute, Oldendorf, Germany. Leibold, M.A., M. Holyoak, N. Mouquet, P. Amarasekare, J.M. Chase, M.F. Hoopes, R.D. Holt, J.B. Shurin, R. Law, D. Tilman, M. Loreau, and A. Gonzalez. 2004. The metacommunity concept: a framework for multi-scale community ecology. Ecol. Letters 7:601-613. Leibold, M.A., and M.A. McPeek. 2006. Coexistence of the niche and neutral perspectives in community ecology. Ecology 87:1399-1410. McGill, B.J., B.A. Maurer, and M.D. Weiser. 2006. Empirical evaluation of neutral theory. Ecology 87:1411-1423. Purves, D., and L. Turnbull. 2010. Different but equal: the implausible assumption at the neart of neutral theory. J. Anim. Ecol. 79:1215-1225. Rosindell, J., S.P. Hubbell, and R.S. Etienne. 2011. The Unified Neutral Theory of Biodiversity and Biogeography at age ten. TREE 26:340-348. Roughgarden, J. 2009. Is there a general theory of community ecology? Biology and Philosophy 24:521-529. Tilman, D. 2004. Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly. PNAS 101:10854-10861. Vellend, M. 2010. Conceptual synthesis in community ecology. Quarterly Review of Biology 85:183-206. Whitfield, J. 2002. Ecology: neutrality versus the niche. Nature 417:480-481.