Stochastic and Deterministic Mechanisms of Assembly

Recently in seminar we discussed two influential papers:

  • Vellend, B. M. 2010. Conceptual Synthesis in Community Ecology. The Quarterly Review of Biology 85:183–206. (pdf)
  • Chase, J. M. 2007. Drought mediates the importance of stochastic community assembly. Proceedings of the National Academy of Sciences of the United States of America 104:17430–17434. (pdf)

I chose these papers because I feel that they do a good job of both laying the conceptual and practical groundwork necessary to advance  the debate between the relative importance of stochastic and deterministic processes in community ecology.

Before our discussion began there was a flurry of interesting tweets inspired by the readings:

tweet2 tweetsbeth_ross_tweetThese tweets helped set the tone for our discussion and led to some interesting discussion.

The key questions that we discussed during the seminar were:

1) What are stochastic and deterministic processes?

2) Why or why is it not useful to characterize biological processes that influence communities in this way.?

3) What aspects of ecological assembly are ignored or misrepresented using the analogy of population genetics in which species are thought of as alleles?

4) How do we operationalize these concepts, quantify their importance, and predict how their importance will vary?

Vellend proposes that 4 processes: ecological drift, speciation, dispersal, and selection can be used to unify community ecology under a general theory of community dynamics in which

…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…

Visually the process of community assembly can be thought of like this (Vellend 2010, Fig 4):

vellend_model

This is an important and exciting idea for a couple of reasons:

1) These processes appear to provide a minimal set of mechanisms required to unify many ecological theories of community assembly (see Vellend 2010 Table 2). This is important because it provides a way of synthesizing across seemingly disparate theories.

2) The typical filter model of community assembly (figure below) ignores ecological drift and speciation and focuses instead on biotic and environmental filters (selective forces) and their interaction with chance dispersal

filter_model(Figure 1 from HilleRisLambers et al. 2012).

The inclusion of speciation as a key process in ecological assembly places emphasis on the fact that species pool (in the filter model above) is not static. Vellend’s recognition of ecological drift as a key processes is important because it suggests that some patterns of abundance are simply stochastic which results from the demographic equivalence or similarity between species.  In the classic filter model, stochasticity resulted only from variation around the relative stable environmental filter or from chance dispersal. The recognition of ecological drift suggests that we will not be able to explain all variation in community composition even with perfect knowledge of a species niche and dispersal capacity.

To answer @bethross ‘s question above, we felt in general that the analogy between population genetics and community ecology was actually fairly accurate if you are willing to ignore intraspecific demographic variation which most ecological theories do for the sake of simplicity.  Vellend does bring up a few caveats though in this regard. More importantly though the fact that population genetics does not have to be changed to accommodate ecological assembly suggests the intriguing idea that these two frameworks could be unified into a single cohesive theory that links variation in alleles and species.

I especially liked pairing Vellend’s conceptual paper with Jon Chase’s empirical study because Chase lays out a simple way to operationalize the concepts that Vellend discussed and he suggests some intriguing hypotheses for how the importance of Vellend’s processes should tradeoff.

Chase reduces Vellend’s processes even further into either deterministic or stochastic components of assembly.  He considers stochastic processes to include the influence of dispersal and ecological drift while deterministic processes include the influence of “niche-selection” by which he means filtering by the environment and by biotic interactions for particular traits.  The key insight that Chase recognized is that if communities are primarily driven by stochastic processes then their specific composition should be  unpredictable relative to the species pool, alternatively if communities are strongly deterministically controlled then composition in a particular environment should be fairly predictable. Lastly, Chase hypothesized that the influence of deterministic processes would become stronger when the environmental filter was stronger, in other words when the environment was less benign (i.e., drought years for his aquatic systems).

Chase’s results can be summed up with a relatively common analysis and a simple visual diagram:treatment_effectThe figure above summarizes the differences between sites in terms of their species composition. The closer two sites (the dots) are in one the plot the more similar their composition is. Because the drought ponds are clustered together and the permanent ponds are over-dispersed it suggests that the drought sites had much more similar species composition which fits with Chase’s hypothesis that community composition will become more predictable under harsher environmental conditions.

Today, I think that the ideas that Vellend and Chase articulated not too many years ago have begun to permeate the ecological community. At this year’s ESA meeting in Minneapolis the controversy over the existence of neutral or stochastic dynamics had really died down. It was far more common for a speaker to recognize upfront that both niche and neutral processes were operating in their community. The important challenge now is to understand what the relative importance of these processes changes across scales, taxa, and ecosystems and how we can use this information to formulate predictions for the future of communities in light of rapid global change.

This post was written by Dan McGlinn you can learn about his research here

References:

Vellend, B. M. 2010. Conceptual Synthesis in Community Ecology. The Quarterly Review of Biology 85:183–206

HilleRisLambers, J., P. B. Adler, W. S. Harpole, J. M. Levine, and M. M. Mayfield. 2012. Rethinking Community Assembly through the Lens of Coexistence Theory. Annual Review of Ecology, Evolution, and Systematics 43:227–248.

Chase, J. M. 2007. Drought mediates the importance of stochastic community assembly. Proceedings of the National Academy of Sciences of the United States of America 104:17430–17434.

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