Conceptual synthesis in community...
CONCEPTUAL SYNTHESIS IN COMMUNITY ECOLOGY Mark Vellend Departments of Botany and Zoology, and Biodiversity Research Centre, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4 e-mail: email@example.com keywords dispersal, drift, community ecology, population genetics, selection, speciation abstract Community ecology is often perceived as a ���mess,��� given the seemingly vast number of processes that can underlie the many patterns of interest, and the apparent uniqueness of each study system. However, at the most general level, patterns in the composition and diversity of species���the subject matter of community ecology���are influenced by only four classes of process: selection, drift, speciation, and dispersal. Selection represents deterministic fitness differences among species, drift represents stochastic changes in species abundance, speciation creates new species, and dispersal is the movement of organisms across space. All theoretical and conceptual models in community ecology can be understood with respect to their emphasis on these four processes. Empirical evidence exists for all of these processes and many of their interactions, with a predominance of studies on selection. Organizing the material of community ecology according to this framework can clarify the essential similarities and differences among the many conceptual and theoretical approaches to the discipline, and it can also allow for the articulation of a very general theory of community dynamics: species are added to communities via speciation and dispersal, and the relative abundances of these species are then shaped by drift and selection, as well as ongoing dispersal, to drive community dynamics. Introduction Cof OMMUNITY ECOLOGY is the study patterns in the diversity, abun- dance, and composition of species in com- munities, and of the processes underlying these patterns. It is a difficult subject to grasp in its entirety, with the patterns of interest seemingly contingent on every last detail of environment and species interac- tions, and an unsettling morass of theoret- ical models that take a wide variety of forms. Fifteen years ago, Palmer (1994) identified 120 different hypotheses to ex- plain the maintenance of species diversity, and the list would no doubt be even longer today. However, despite the overwhelmingly large number of mechanisms thought to un- derpin patterns in ecological communities, all such mechanisms involve only four dis- tinct kinds of processes: selection, drift, spe- ciation, and dispersal. Many biologists will recognize these four processes as close analogues of the ���big four��� in population genetics: selection, drift, mu- tation, and gene flow. Many ecologists, how- ever, might be skeptical that such a simple The Quarterly Review of Biology, June 2010, Vol. 85, No. 2 Copyright �� 2010 by The University of Chicago Press. All rights reserved. 0033-5770/2010/8502-0004$15.00 Volume 85, No. 2 June 2010 THE QUARTERLY REVIEW OF BIOLOGY 183
organizational scheme is applicable to the more complex subject of community ecol- ogy. Population genetics, despite being faced with essentially the same problem as commu- nity ecology���that is, understanding the com- position and diversity of alleles in popula- tions���is an easier subject to grasp, and I submit that the reason for this is not because of any fundamental difference in the com- plexity of the subject matter, but because of the coherence and simplicity of its the- oretical foundation. Every detail of the complex interactions between species and their environments that are studied by ecologists can also be important agents of natural selection, but it is quite useful to begin by recognizing just that: such inter- actions mostly fall under the single concep- tual umbrella of selection. Add the relatively simpler processes of drift, gene flow, and mu- tation to the mix, and you have the Modern Evolutionary Synthesis, which remains a ro- bust, general, and widely accepted theoretical foundation for population genetics and micro- evolution, notwithstanding arguments about whether it fully encompasses all facets of mod- ern evolutionary biology (Pigliucci 2007). The perspective that synthesis in com- munity ecology can be achieved by orga- nizing processes into the four categories of selection, drift, speciation, and dispersal flows directly out of a sequence of concep- tual developments that occurred over the last half century. In the 1950s and 60s, G. Evelyn Hutchinson and Robert MacArthur ushered in an era of community ecology in which the discourse was dominated by the deterministic outcome of local interactions between functionally distinct species and their environments���i.e., selection. Initial developments of mathematical theory in community ecology had occurred decades earlier (e.g., Lotka 1925), but, by all ac- counts, the 1960s marked the period during which theoretical development in commu- nity ecology flourished (Kingsland 1995 Cooper 2003). The importance of selective processes in local communities ruled the day, and the vast body of theoretical and empirical research in this vein has been dubbed ���traditional community ecology��� (Lawton 1999 see also Brown 1995). In response to the emphasis on local- scale selective processes almost to the ex- clusion of other factors, Ricklefs (1987) and others (Ricklefs and Schluter 1993 Brown 1995) argued for and successfully sparked a shift in emphasis to a more inclusive ap- proach in community ecology, explicitly recognizing the importance of processes occurring at broader spatial and temporal scales for understanding local-scale pat- terns. One key contribution here was the recognition that the composition and di- versity of species, even at a local scale, de- pend fundamentally on the composition and diversity of the regional pool of spe- cies, which, in turn, depend on the process of speciation. Just as mutation is the ulti- mate source of genetic variation, so too is speciation the ultimate source of the spe- cies that make up ecological communities. The next key addition to the mix was ecological drift. Ecologists have long rec- ognized that changes in the composition and diversity of species can have an impor- tant stochastic element (e.g., Chesson and Warner 1981). However, it was not until Hubbell (2001) imported the neutral the- ory of population genetics into ecology that drift was incorporated into theory as something much more than ���noise��� in an otherwise deterministic world. Pure ecological drift happens when individuals of different spe- cies are demographically identical, which is ex- ceedingly unlikely. But drift need not be the only active process in order to be an important process, and, in many groups of species that show only modest functional differentia- tion, drift may indeed be quite important (McPeek and Gomulkiewicz 2005). The fact that neutral theory was imported into ecology essentially unchanged from popu- lation genetics suggests the possibility of a broader synthesis of processes in both popula- tion genetics and community ecology, neutral and otherwise (Vellend and Geber 2005 Hu et al. 2006 Vellend and Orrock 2009). The final key process is dispersal���the eco- logical equivalent of gene flow in population genetics. Dispersal has been incorporated into ecological theories of all kinds over the past fifty years, but, in recent years, it has been brought to the forefront in the form of 184 Volume 85 THE QUARTERLY REVIEW OF BIOLOGY
the metacommunity concept (Holyoak et al. 2005), which is explicitly concerned with the role of dispersal among local communities in influencing community patterns at multiple scales. The movement of organisms across space can have a variety of important conse- quences in communities. For each of the latter three processes���spe- ciation, drift, and dispersal���conceptual devel- opments were motivated by a perceived lack of emphasis in the literature on the impor- tance of the process in question. Selection, in the form of deterministic interactions among species and between species and their environments, was always recognized as important. With the additions of specia- tion, drift, and dispersal, we now have a logically complete set of process categories within which all other more specific pro- cesses can be placed. I believe that organiz- ing the overwhelming number of specific ecological theories for communities under this scheme can help achieve at least two important goals. First, the essential similar- ities and differences between different eco- logical models can be clarified in fairly straightforward terms, thereby making the full set of models easier to understand, apply, and teach to students. Second, we can articulate a very general theory of com- munity dynamics, which may on the sur- face sound obvious and too generalized to make any specific predictions, but may, nonetheless, serve the same critical func- tion as foundational theory in population genetics. Before proceeding, I should emphasize that I am not arguing that the parallels be- tween processes or models in population ge- netics and community ecology are perfect. For example, selection among individuals across species can be manifested in ways that are rare or absent within species (e.g., tro- phic or parasitic interactions), and specia- tion is a far more complicated process than mutation. The list could go on. Rather, my argument is that we can define a similar set of four logically distinct processes in commu- nity ecology in order to provide a coherent conceptual framework for the discipline. The rest of this paper is structured as follows. I first specify more precisely the motivation for conceptually organizing the material in community ecology, and pro- vide operational definitions of important terms. I then illustrate, with separate sec- tions on theory and data, how the subject matter of community ecology can be pre- sented using the proposed organizational framework, describing the ways in which selection, drift, speciation, and dispersal influence communities. I then touch on some of the general patterns that commu- nity ecologists have traditionally been in- terested in, and I discuss how pattern is connected with process. Finally, I compare the framework presented here with other conceptual frameworks in community ecol- ogy. Community Ecology Is a Mess Based largely on empirical results, Lawton (1999) famously called community ecology ���a mess,��� and ascribed this mess to the inher- ent contingency of ecological patterns on the details of how the underlying processes or rules act. ���The rules are contingent in so many ways . . . as to make the search for patterns unworkable��� (Lawton 1999:181). One source of motivation for the present paper is that even theoretical community ecology can be considered a mess for much the same reason: each and every twist added to theoretical models seems to matter, mak- ing an overarching treatment of the subject very difficult. Consider the number of differ- ent models that can be constructed from the simple Lotka-Volterra formulation of inter- actions between two species by layering on realistic complexities, one by one. First, there are at least three qualitatively distinct kinds of interactions (competition, predation, mu- tualism). For each of these, we can have ei- ther an implicit accounting of basal re- sources (as in the Lotka-Volterra model), or we can add an explicit accounting in one particular way. That gives six different mod- els so far. We can then add spatial heteroge- neity or not ( 2), temporal heterogeneity or not ( 2), stochasticity or not ( 2), immigra- tion or not ( 2), at least three kinds of func- tional relationships between species (e.g., predator functional responses 3), age/size structure or not ( 2), a third species or not June 2010 185 SYNTHESIS IN COMMUNITY ECOLOGY