Ecology needs more evolution (and vice versa)

One historical weakness in community ecology is its singular focus on ecology in the absence of any consideration of the role of evolution. Ecological theory may attempt to explain and expand on mechanisms of coexistence, but this is done in ignorance of whether such a mechanism could have reasonably evolved in the first place. Evolutionary biology has equally ignored the role of ecology (for example, just-so stories invented in the absence of ecological support). Fortunately, it is becoming more common to see papers that incorporate, empirically or theoretically, evolution and community ecology.

A recent paper by Robin Snyder and Peter Adler attempts to incorporate both ecology and evolution in reference to the storage effect, a mechanism in which species coexist as a result of environmental variability and corresponding differential variation in species fecundity in response to the environment. As a simplistic example, consider a system of two annual plants for which each species has their highest recruitment at different temperatures, and temperature varies randomly between years. Each species is expected to have high recruitment in different years/environmental conditions, and this high recruitment in good years can then buffer that species’ fitnesses in years of poor conditions, provided the species have some way of “storing” fitness (such as long-lived seedbanks). The storage effect therefore predicts that environmental variability can mediate the coexistence of otherwise unequal competitors. Because the requirements of the storage effect appear so ubiquitous (environmental variation, differential species responses to the environment, some sort of buffer), it seems that the storage effect could be very common. However, there is also theory suggesting that variation in demographic rates should come at a fitness cost, since the long term mean growth rate will be lower if demographic rates vary than if they are fixed (as the result of geometric averaging). This predicts that there should be selection against flexible—rather than fixed—demographic rates, including rates that vary in response to environmental or other cues. Is it possible then for variable demographic rates, which are necessary for the storage effect, to evolve?

Snyder and Adler discuss this disconnect between community ecology and evolution, questioning whether the storage effect can be supported by both evolutionary and ecological theory. To this end, the authors explore whether, and under what conditions, the storage effect could evolve. Snyder and Adler use a simple model of competition between two annual plants, in which fecundity fluctuates due to environmental variation, and germination rate can be temporally fixed, or variable. Germination rates should be constant, despite environmental variation, due to the cost of variability. Germination rates would be expected to vary year to year only if this conferred a fitness benefit to the species. Hypothesized benefits of variable germination rates include if germination rates are positively correlated with fecundity (that is, in good years germination is higher as well), or if it allows a species to avoid competition (by having high germination when their competitor has low germination). To test this hypothesis, the authors varied the correlation between fecundity and germination, and the correlation between the two species’ germination rates. They then examined the conditions under which variable germination rates were an evolutionarily stable strategy (ESS).

Snyder and Adler’s results suggest that the storage effect is expected to evolve only under anarrow set of conditions. A variable germination rate was most likely to evolve if there was a strong correlation between fecundity and germination rate. They note that such a correlation might occur if seed production and germination depended on similar environmental cues or similar resource requirements. A variable germination rate was also a stable strategy if one species was limited in its ability to evolve, in which case the other species evolved variable germination rates. If these specific conditions didn’t hold, the storage effect was not evolutionarily stable.

These results are meaningful because they highlight how different the conclusions of community ecology, which has proposed that the storage effect could be a widespread contributor to coexistence, and evolutionary theory, which suggests that the storage effect may only occur under particular conditions, can be. This kind of reconciliation of community ecology and evolution tells us more about natural systems than either approach can on its own. It also hints that theory and conclusions we’ve drawn in community ecology in the absence of evolution may be limited and incomplete.