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. :)

Global warming and shifts in food web strucutre

Predicting the effects of global warming on biological systems is of critical importance for informing proactive policy decisions. Most research so far has been on trying to predict shifts in species distributions and changes in interactions within local habitats. But what many of these studies assume is that the basic biological processes and requirements of the individual species will not change -that is their biology is fixed and they simply need to find the place that best suits them. Not so, say Mary O'Connor and colleagues, in a just-released study in PLoS Biology.

O'Connor and colleagues experimentally warmed marine microcosms and tested two alternative hypotheses on food web structure: 1) that productivity increases with warming; and 2) warming increases metabolic rates, thus changing consumer-autotroph (i.e., primary producers) interactions. What they found was that warming indeed altered consumer-autotroph interactions. Warming increased base metabolic rates of consumers, as well as primary production, and the net effect was that food webs shifted towards increasing consumer control (i.e., top-down control).

What this research means is that global warming may alter food web interactions by increasing resource needs of organisms as their metabolic rates increase. This may increase the stress on communities and change diversity patterns as increased needs may shift competitive hierarchies or affect autotroph's ability to withstand consumer effects.

O'Connor, M., Piehler, M., Leech, D., Anton, A., & Bruno, J. (2009). Warming and Resource Availability Shift Food Web Structure and Metabolism PLoS Biology, 7 (8) DOI: 10.1371/journal.pbio.1000178

Researcher spotlight: Tadashi Fukami

Increasingly, ecological explanations for extant community patterns are relying on dynamics operating across multiple spatial and temporal scales, linking small and large scales, and the here and now with evolutionary history. The traditional boundaries of sub-disciplines are blurring. I think that few other young scientists straddle these boundaries as successfully as Tad Fukami, and new assistant professor in the Department of Biology at Stanford University. Tad uses a broad array of theoretical and experimental approaches to understand how ecological communities are put together. From laboratory microcosms to rat-infested islands, and from the computer to remote locations, he is able to pull together disparate pieces of information into a central narrative about the assembly of communities.

I asked him why the question of community assembly interested him so much, and he gives much credit to his advisors, Jim Drake (also my PhD advisor) and Dan Simberloff both in the Department of Ecology and Evolutionary Biology at the University of Tennessee. But more than this, he says that:

“you need to look into the historical background of species interactions to understand the apparently inexplicable variation in the way species interact and the way communities are structured by the interactions.”

and certain aspects of this research obviously excite him. He goes on to say:

“One particularly intriguing thing is the great effect that small chance events that cause variation in early immigration history can have on long-term community development.”

Most ecologists gain their expertise by coming to understand and appreciate the details and intricacy of particular organisms or ecosystems. But Tad is especially noted for his use of an amazingly broad assemblage of systems and methods. I asked him why he used so many different systems, and how he chose those to test his ideas. He said that his work has benefitted from many exciting collaborations and that he has:

“been very lucky to meet many great people who have expertise on specific organisms and systems that a person with diffuse interests like me doesn't have.”

But I think that there may be something deeper and more reassuring. That is, the fact that one could study a multitude of systems, testing the basic dynamics of community assembly, means that there are regularities in how communities are assembled. That you can study stochastic historical events in bacterial microcosms and inform your understanding of plant succession means that while we individually take on these, at times, daunting research projects, our collective understanding of ecological processes are threaded together in a great fabric. And no one is a microcosm of this more than Tad Fukami.

Key recent papers

Fukami, T., Beaumont, H. J. E., Zhang, X.-X. & Rainey, P. B. (2007) Immigration history controls diversification in experimental adaptive radiation. Nature 446: 436-439.

Fukami, T., Wardle, D. A., Bellingham, P. J., Mulder, C. P. H., Towns, D. R., Yeates, G. W., Bonner, K. I., Durrett, M. S., Grant-Hoffman, M. N. & Williamson, W. M. (2006) Above- and below-ground impacts of introduced predators in seabird-dominated island ecosystems. Ecology Letters 9: 1299-1307.

Fukami, T., Bezemer, T. M., Mortimer, S. R. & Van der Putten, W. H. (2005) Species divergence and trait convergence in experimental plant community assembly. Ecology Letters 8: 1283-1290.