Thursday, March 26, 2015

Ecology in evolutionary times

Ecological and evolutionary perspectives on community assembly. 2015. Gary G. Mittelbach, Douglas W. Schemske. Trends in Ecology and Evolution.

Phylogenetic patterns are not proxies of community assembly mechanisms (they are far better). 2015. Pille Gerhold, James F. Cahill Jr, Marten Winter, Igor V. Bartish and Andreas Prinzing. Functional Ecology

Community assembly has always provided some of the most challenging puzzles for ecologists. Communities are complex, vaguely delimited, involve multi-species interactions, and assemble with seemingly immense variation. Thousands of papers have been dedicated to understanding community assembly, and many have proposed different approaches understanding communities. These range from the ever popular abiotic/biotic filtering concept, functional traits, coexistence theory, island biogeography, metacommunity theory, neutral theory, and phylogenetic patterns. It is probably fair to say that no one existing approach is adequate to completely describe or predict community assembly.

One response to this problem is the growing demand to expand the lens of “community” to cover greater spatial and temporal scales. This owes a lot, directly and indirectly, to Robert Ricklefs’ influential Sewall Wright Award lecture on the Disintegration of the Ecological Community. There is also a strong trend towards re-integrating evolutionary history into studies of community ecology. Coincidentally, or perhaps not, this is occurring as so-called ‘eco-phylogenetic’ approaches have been increasingly criticised. If nothing else, eco-phylogenetics provided a path for, and popularized, the idea of reintegrating evolution into community ecology.

I’ll highlight two particular papers that address this re-integration in surprisingly convergent ways. Both have macroevolution slants (that is, they focus on the impacts and drivers of speciation and extinction, sympatry, allopatry, etc), and an interest in the feedbacks between community interactions and these processes. The first, from Pille Gerhold, James F. Cahill Jr, Marten Winter, Igor V. Bartish and Andreas Prinzing, positions itself as the phoenix from the ashes of eco-phylogenetics (as seen in their particularly enthusiastic title :) ). Evolutionary history, captured by phylogenies, was originally of interest to ecologists not for what it was, but because it could (sometimes, maybe) act as a proxy for species traits and niches. This paper does an excellent job of laying out the various hypotheses that went behind this type of approach and showing why they are not reliably true. If for no other reason, it is worth reading the paper for its clear critique of the foundation of eco-phylogenetics. Using patterns in phylogenies as proxies for the outcomes of particular ecological processes being clearly suspect, the authors argue that explicitly thinking of phylogenetic patterns as the result of both ecological and evolutionary processes is far more informative. [I’ll return to this in a bit with their examples below].

The second paper is written by two big names in their respective fields: Gary Mittlebach (ecology) and Doug Schemske (evolution). The title is a bit vague (“Ecological and evolutionary perspectives on community assembly”), but it turns out that they too have converged on the importance of considering evolutionary history in order to understand community assembly. In particular they focus on the problematic nature of the species pool: species pools are nearly always treated as a static object changing little through time or space and are notoriously difficult to define. However, the species pool underlies null model approaches used to test communities for differences from a random expectation. So defining it correctly is important.

From the early days, Elton and others defined the species pool as the group of species that can disperse to and colonize a community. However, the species pool may be dynamic, and they note “To date, relatively little attention has been focused on the feedback that occurs between local community species composition, biotic interactions, and the diversification processes that generate regional species pools.”

This paper does an excellent job of explaining how macroevolutionary processes can alter a regional species pool. The most obvious example is the process of adaptive radiation in island-like systems, where competition for resources drives ecological divergence and speciation. Darwin’s finches, Anolis lizards, and cichlid fishes provide well-known examples of this rapid expansion of the species pool through inter-specific interactions. On mainland systems, speciation may be more likely to occur in allopatry, and the rate limiting step for range expansion (leading to secondary sympatry and only then increasing a species pool) is often interspecific interactions. One study found that secondary sympatry took 7my on average, though speciation alone took only 3my. So the species pool is the outcome of constant feedbacks between species interactions and evolutionary processes.
From Mittlebach & Schemske. Figure illustrating the feedbacks between evolution and ecological interactions, in producing the species pool.
Both papers provide useful examples of how such incorporating evolution into community ecology may prove useful. As a simple example, Mittlebach and Schemske point out that evolution can greatly alter the utility of Island Biogeography Theory: given enough time, speciation events including adaptive radiations, greatly increase the (non-mainland) species pool and would strongly alter predictions of diversity, especially for distant islands.

The Gerhold et al. paper provides the below illustrations as additional possibilities for how evolution and community interactions may feedback.
From Gerhold et al. Two examples of how evolution and communities might interact.

It is certainly interesting to see this shift towards how we envision and study communities. The historical focus on local space and time no doubt reflects ecologists' attempt to limit the problem to a manageable frame. But there is some logic behind expanding our definition of communities to larger spatial scales and greater time periods, especially since there are usually no true boundaries defining communities in space and time. Answering which specific time scales and spatial scales most useful to understanding communities is difficult: if we increase the time or space we consider, how and when does the additional information provided decline? The next step is to consider evolution in this fashion for real organisms, and evaluate the true utility of this approach.  

No comments: