On the heels of our community structure discussion, these papers seemed like the perfect place to start discussing coexistence theory. How do species that are competing for the same resource coexist, when the principle of competitive exclusion suggests that the dominant species should outcompete all of the others. This week we discussed two papers that looked at the balance between coexistence and competitive exclusion and how it can affect community assembly.
We started with the Paradox of the Plankton (Hutchinson, 1961) to get a historical perspective first on coexistence theory. Hutchinson explores the concepts of coexistence theory with phytoplankton as his model system. He wonders how in an un-structured environment there so many species could coexist when ultimately one should dominate the system. He poses several hypotheses that fall into two broad categories, either biotic interactions (via facilitation or predation) or environmental conditions (variability in conditions). Our discussions of Hutchinson’s work felt reminiscent of discussing parasite aggregation in hosts, wherein a few number of hosts support the bulk of the parasite community. Similarly Hutchinson’s plankton seemed to demonstrate that a few number of species made up most of the abundance of plankton. He attributes this distribution to habitat heterogeneity, or temporal fluctuations in the environment, and suggests both as a mechanism driving the observed patterns of plankton coexistence.
In a more recent conceptual review, HilleRisLambers et al used coexistence theory as a tool to view community assembly. They highlight consider the main mechanisms, either stabilizing niche differences or relative fitness differences, both of which apply to between species differences, as well as frequency dependent population growth for within species dynamics. Using this approach, they evaluate the strengths and limitations of different empirical approaches to understanding the role of niche or fitness differences in community assembly, and ultimately advocate for a combination of approaches. While combining approaches may not always be feasible, HilleRisLambers emphasizes the importance of recognizing and considering the limitations of your own approach and what information could make your conclusions stronger.
While both Hutchinson and HilleRisLambers approach coexistence from the perspective of environment vs biotic interactions, the latter expressly addresses multiple sources of stabilizing mechanisms including resource partitioning or storage effects, that were really formalized through the work of Chesson in 2000. Hutchinson helped lay the groundwork for some of these ideas by introducing resource variation, differences in predation, and opportunism of different species.