Showing posts with label metacommunity. Show all posts
Showing posts with label metacommunity. Show all posts

Monday, December 26, 2011

Rumors of community ecology’s death were greatly exaggerated: reflections on Lawton 1999

In 1999, John Lawton, eminent British ecologist, published a lament for the state of community ecology entitled “Are there general laws in ecology?” Cited more than 600 times, Lawton’s paper forced a re-evaluation of community ecology’s value, success, and even future existence. Other scientists at the time seemed to agree, with papers starting with phrases like “Although community ecology is a struggling science…” and “Given the lack of general laws in ecology…”. Lawton appeared to be suggesting that community ecology be abandoned for the generality of macroecology or the structure of population ecology.

An important point to be made is that Lawton was simply making a particularly public expression of ecology’s growing pains. In 1999, ecology was at a crossroads between the traditional approach of in-depth system-based study, and a fairly single-minded focus on competition as an explanation for patterns (e.g., Cooper 1993 ‘The Competition Controversy in Community Ecology’ Biology and Philosophy 8: 359-384), while at the same time there were emergent approaches and explanations like neutrality, macroecology, spatial ecology, ecophylogenetics, and improved computer and molecular methods. There was also growing dissent about ecology’s philosophical approach to ecology (e.g., Peters 1991 ‘A Critique for Ecology’; Haila and Heininen 1995 ‘Ecology: A New Discipline for Disciplining’ Social Text 42: 153-171): ecologists tended to ignore the Popperian approach, which required falsification of existing hypothesis, instead tending to look for support for an existing hypothesis, or at least advocated looking for patterns without considering alternative mechanisms. Not only this, but the applications for ecology were more clear than ever – the Intergovernmental Panel for Climate Change was meeting , and the ecological consequences of human actions were perhaps more obvious they had ever been. But ecologists were failing at providing solutions –Lawton argued-correctly-that in 1999 ecologists could provide little insight into how a community might change in structure and function in response to changing climate.

Although everyone should read Lawton’s paper, a simple synthesis of his concerns would be this – that community ecology is too contingent, communities are too complex, and therefore community ecology cannot formulate any laws, cannot make predictions, cannot be generalized from one system to another. This makes community ecology suspect as a science (physics being the most common example of an “ideal” science), and certainly not very useful. Lawton suggests that population ecology, where only a few models of growth could explain the majority of species’ dynamics, or macroecology, which focuses on the most general, large-scale patterns, were a better example of how ecology should be practiced.

Community ecology, rather than dying, has experienced an incredible surge in popularity, with a large contingent represented at meetings and in journal publications. Ecology itself is also thriving, as one of the fastest growing departments in universities. So what, if anything, has changed? Has ecology addressed Lawton’s criticisms?

Two major things happened in the late 1990’s and early 2000’s, which helped ecologists see beyond this general malaise. The first was that a number of well-thought out alternative ecological mechanisms explaining community membership were published. Before the late 90’s community ecologists looked for evidence of competition in patterns of community composition, either among locales or through time following disturbance. When local competition was insufficient to explain patterns, researchers likely cited, but did not test other mechanisms. Or if they did test other mechanisms, say predation, it was as an alternative, mutually exclusive mechanism. The new publications, drawing on previous ideas and concepts formalized assembly mechanisms like neutral processes or metacommunity dynamics where uneven fitnesses in a heterogeneous landscape can affect local coexistence. More than these as solely alternative mechanisms, these allowed for a synthesis where multiple mechanisms operate simultaneously to affect coexistence. Probably the most emblematic paper of this renewed excitement is Peter Chesson’s 2000 ‘Mechanisms of maintenance of species diversity’ published in Annual Reviews of Ecology and Systematics. This paper, cited over a thousand times, offers a way forward with a framework that includes competitive and niche differences but can also account for neutral dynamics.

A second major development that rejuvenated ecology was the formation of technological and statistical tools engendering broad-scale synthetic research. Suddenly the search for general explanations – Lawton’s most piercing criticism - became more common and more successful. With the advent of on-line databases, meta-analytic procedures and centers (e.g., the National Center for Ecological Analysis and Synthesis) that foster synthetic research, ecologists routinely test hypotheses that transcend local idiosyncrasies. Often, the capstone publication on a particular hypothesis is no longer a seminal experiment, but rather a meta-analysis that is combines all the available information to assess how strongly and how often a particular mechanism affects patterns.

While these theoretical and technological developments have been essential ingredients in this ecological rejuvenation, there has also been a subtle shift the philosophical approach to what it is ecological theory can and should do. Criticism in the 1990’s (e.g., Peters 1991 ‘A Critique for Ecology’) centered on the inability of ecological theory to make accurate predictions. The concept of science common in ecology in the 1990’s was that a rigorous, precise science (i.e., with laws) results in the ability to accurately predict species composition and species abundances given a set of mechanisms. This view of ecological science has been criticized as simplistic ‘physics-envy’ (e.g., see Massimo Pigliucci’s PhD dissertation ‘Dangerous habits: examining the philosophical baggage of biological research’published by the University of Tennessee in 2003). The subtle philosophical change has been a move from law=prediction to law=understanding. This is as true for physics as it is for ecology. We don’t expect a physicist to predict precisely where a falling feather will land, but we do expect to totally understand why it landed where it did based on fundamental processes. (for more on the contrast of prediction and understanding, see Wilhelm Windelband’s nomothetic and idiographic knowledge)

While the feather example above is simplistic, it is telling. In reality a physicist can produce probability contours of where the feather is likely to land, which could be very focused on a calm day or broad on a windy one. This is exactly what ecologists do. Once they understand how differing mechanisms come together to shape diversity, they make probabilistic predictions about the outcome of a set of known mechanisms.

Ecology today is as vibrant as ever. This is not a result of finding new laws that proved Lawton incorrect. Rather, ecologists now have a more sophisticated understanding of how various mechanisms operate in concert to shape diversity. Moreover, conceptual, technological and philosophical revolutions have fundamentally changed what ecologists do and what they are trying to explain. It is a great time to be an ecologist.

Lawton, J. H. (1999). Are there general laws in ecology? Oikos, 84(2), 177-192.

By Marc Cadotte and Caroline Tucker

Wednesday, June 15, 2011

Metacommunity data and theory: the tortoise and the hare

Empirical approaches to metacommunities: a review and comparison with theory
Logue et al. 2011

The recognition that community composition is a function of both local and regional-scale processes, meaning that a community cannot be understood in isolation from the network of communities with which it interacts, is the fundamental idea behind metacommunity ecology. In a relatively short period of time, metacommunity ecology has integrated concepts from spatial ecology, metapopulation ecology, and community ecology with novel ideas, and developed a strong body of theory. However, metacommunity theory has advanced much more rapidly than empirical tests of that theory. In an interesting review in TREE, Logue et al. examine whether empirical data needs to catch up with the pace of theory development, or whether theory is moving too fast to incorporate the information available from empirical data.

The types of systems used in the 34 experimental and 74 observational studies that Logue et al. found were very limited – the most common experimental approach involved setting up aquatic microcosms of unicellular organisms.* Observational studies similarly tested microorganisms, usually in aquatic systems. The organisms so beloved in the rest of community ecology (plants? vertebrates?) barely feature. Most studies focus on aquatic systems composed of multiple patches (such as microcosms, ponds, pitcher plant communities) because systems with discrete boundaries are more amenable to testing current theory. However, natural systems are rarely configured into a clear “patch” versus “matrix” dichotomy. Instead they are complex and heterogeneous, and may lack clear boundaries.

Dynamics in metacommunities are generally described using four dominant paradigms: mass-effects, species sorting, neutral perspective, or patch-dynamics. These paradigms reflect the most important processes structuring communities, that is, either dispersal between communities, environmental differences between communities, dynamics driven by the tenets of neutral theory, or extinction and colonization, respectively. Strikingly, experimental studies mostly tested for mass-effects or patch dynamics, and observational studies mostly tested for species-sorting and mass effects paradigms. The neutral paradigm was rarely tested in any type of study. Logue et al. found that many studies had difficulty designing experiments that tested for evidence of specific paradigms, because natural communities are much more complex than the simple paradigms suggest. Most studies that did test for evidence of particular paradigms found evidence for multiple paradigms or had difficulty disentangling different mechanisms.

The metacommunity theory that has developed in the last five years is among the most exciting and interesting work in ecology. However, the slower pace of experimental work means that theory has developed with little feedback. For example, Logue et al. make a strong argument that the results from these studies suggest that it is time to integrate the four-paradigm system into a single, comprehensive framework (see figure). Theory is only valuable if it’s useful - this paper is an important reminder that there is an important feedback loop between theory and data, and successful science requires input from both.

*Important disclaimer: at this very moment I'm running aquatic microcosms of microscopic protists in the lab. We all have room for improvment. :)

Tuesday, April 27, 2010

Niche or Neutral? Why size matters.

Metacommunity dynamics (i.e. the effects of dispersal among connected communities) have become an increasingly common lens through which to explain community structure. For example, competition-colonization models explain the coexistence of superior and inferior competitors as the result of a trade-off in colonization and competitive ability. Species are either superior competitors, with high probabilities of establishing in patches, but low ability to move between patches, or superior colonizers, which have tend to lose in competitive interactions but can travel easily between patches. Under this framework, the ability of superior colonizers to reach and maintain populations in patches where their superior competitors are absent allows them to avoid extinction.

One problem with these types of models is that they rarely acknowledge the importance of ecological drift – that is, that chance events also affect species interactions. This despite the fact that we know that in “real life”, chance events likely play a major role in producing assemblages different than those we might predict based on theory. One of the strengths of the Hubbell’s neutral model is that it recognizes and embraces the importance of randomness.

A recent paper by Orrock and Watling (2010) examines how chance events can alter the predictions of the classic competition-colonization model. Orrock and Watling show that the size of communities in a metacommunity (which is assumed to correlate with the strength of ecological drift) determines whether community dynamics are niche-structured or neutral in nature. In large communities, predictions agree closely with those of the classic competition-colonization model, and niche-based interactions (i.e. competitive hierarchies) dominate. It’s in small communities that things get interesting: ecological drift becomes more important, so that differences in competitive ability between species are effectively neutralized. As a result, small communities begin to resemble neutral assemblages in which species abundances don’t relate to differences in competitive ability. An interesting consequence of this outcome is that species who are poor competitors but good colonizers have an additional refuge – simply by escaping to small communities, even if these communities contain superior competitors, they can persist in a metacommunity.

Beyond the theoretical implications of this model, the applied implications are what really matter. Habitat destruction and fragmentation are an growing problem due to human activities. Habitat patches are often smaller, and of lower quality, decreasing the size of the community each patch can support. Even if these patches are still connected and functioning as a metacommunity, species which rely on their strong competitive ability for persistence will lose this advantage as assemblages become increasingly neutral. Under this model, community diversity declines even more as habitat is lost than in the traditional competition-colonization model, and superior competitors face even greater extinction risk than previously predicted.

Since in reality, metacommunities are likely to consist of patches of different sizes, rather than all large or all small patches, the predictions here remain to be extended to more realistic metacommunities. However, Orrock and Watling have produced a useful model for understanding how ecological drift can affect diversity in a metacommunity and alter the expectations of traditional competition-colonization models.

Orrock, J.L. and Watling, J.I. (2010) Local community size mediates ecological drift and competition in metacommunities. Proc. R. Soc. B.

Tuesday, February 3, 2009

Local extinctions reveal metacommunity dynamics.

ResearchBlogging.orgMetacommunity dynamics (i.e., that dispersal limitation among locales creates spatially-contingent community processes) have been in vogue over the past half-decade. Many of the advances in this field have come from theoretical models, computer simulations, artificial laboratory assemblages of micro-organisms (with yours truly being a major offender) and field experiments using small-bodied, short-lived organisms. An oft-repeated criticism has been that the necessary conditions for metacommunity processes are what are manipulated in simulations or lab tests and that simple extinction-colonization dynamics are rarely observed for larger, longer-lived organisms. In a recent paper by Kevin Burns and Christopher Neufeld, high levels of extinction and colonization are shown in patchy communities of woody plants. They sampled 18 islands off the west coast of Canada in 1997 then again in 2007 and found that substantial numbers of local extinctions were observed. These results reveal that what we often think of as relatively stable communities (woody plant species) are actually quite dynamic, creating the conditions were metacommunity processes are an important mechanisms driving patterns of diversity. They further show that communities with greater exposure to ocean storms had higher extinction risk and species with hardier leaves were less prone to local extinctions.

Kevin C. Burns, Christopher J. Neufeld (2009). Plant extinction dynamics in an insular metacommunity Oikos, 118 (2), 191-198 DOI: 10.1111/j.1600-0706.2008.16816.x