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Community Ecology
Neutral community ecology & metacommunities
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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.