Evolutionary and plastic rescue in multitrophic model communities

Background/Question/Methods

Under changing environmental conditions, intraspecific variation can potentially rescue populations from extinction. Two fundamental sources can provide the variation that may ultimately lead to population rescue: genetic diversity and phenotypic plasticity. In the present study, we compare the potential for evolutionary rescue (through genetic diversity) and plasticity rescue (through phenotypic plasticity) by analyzing their differential ability to produce dynamical stability and persistence in mathematical tri-trophic food webs.  We also evaluate how the trophic location of variation affects rescue.  We systematically test the following hypotheses: (i) Plastic communities are more likely to exhibit stability and persistence than communities where genetic diversity provides the same range of traits. (ii) Variation at the lowest trophic level promotes stability and persistence more than variation at higher levels. (iii) Communities with variation (either of plastic or genetic origin) at two levels have greater likelihoods of stability and persistence than communities with variation at only one level.  The way we approach the phenomenon of “rescue” in our study differs from previous ones in two important ways: we use increased natural mortality and the bottom-up effects of nutrient enrichment to generalise environmental stress and evaluate how defensive traits can mitigate the effects of these stressors.  Secondly, we consider “rescue” in the framework of community dynamics; that is, rather than focussing on the persistence of single, isolated populations we evaluate the potential for rescue of ensembles of populations.  

Results/Conclusions

We found plasticity promotes stability and persistence more than genetic diversity; this implies that if one is wants to accurately predict community dynamics, not only must one measure the range and distribution of traits within a community, one must also determine the source of those traits.  Our results show that while plasticity, generally, stabilises dynamics, the trophic location of genetic diversity determines whether it stabilises or destabilises dynamics.  We also found that variation at the herbivore level (second highest) promotes stability and persistence more than variation at the autotroph or two levels.  We propose that this phenomenon should apply to larger food webs and that with all other things being equal, variation at the penultimate level, regardless of food chain length, will have the greatest impact on stability and persistence, and therefore, the potential for rescue.   Our study adds to the understanding of rapid adaptive phenotypic change by showing how predictions of the effects of variability are sensitive to the trophic location of variation.