When does intraspecific trait variation promote coexistence of competing species?

Background/Question/Methods

Although ecologists and evolutionary biologists have long recognized that phenotypic variation is central to population-level processes, we have only begun to understand how phenotypic variation shapes relationships among the species in a community.  In this study we ask how intraspecific variation in the traits influencing competition might promote (or impede) the ability of an invader to establish and coexist in a competitive community.  We employ a Lotka-Volterra model of competition and examine two classes of biologically motivated trade-offs that mediate the competitive impact of conspecifics and heterospecifics: i) in a response-trait scenario the ability of an individual to grow well in a neighborhood of heterospecifics is negatively correlated with its ability to grow well with conspecifics and ii) in an effect-trait scenario the ability of an individual to impede the growth of heterospecifics is positively correlated with its ability to impede the growth of conspecifics.  Notably, we assume that these tradeoffs are non-linears so that intraspecific variation that is symmetric about the mean trait will impact competition via Jensen's inequlity.  We analyze how variation among individuals, governed by these two tradeoffs, influences the ability of two species to coexist, using Chesson's framework of (stabilizing) niche differences and (equalizing) fitness differences.      

Results/Conclusions

Under the response-trait scenario, any form of variation among individuals that is symmetric about the mean trait value increases the invader's relative fitness by increasing the resident's sensitivity to both inter and intraspecific competition.  Stabilizing niche differences may be enhanced or degraded by intraspecific variation (depending on assumptions); however, increases in the stabilizing mechanism provided by niche differences are always outpaced by the change in the fitness differences.  Under the effect-trait scenario, any symmetric variation will always strengthen stabilizing niche differences and increase the invader's relative fitness at exactly the same rate. 

In both scenarios, intraspecific trait variation provides no benefit to the resident species.  In cases where the invader has lower relative fitness than the resident, intraspecific variation in the resident species may facilitate competitive coexistence; however, in cases there the resident has lower relative fitness, intraspecific variation will serve only to weaken the ability of the resident to persist.   These results suggest that the reduced intraspecific variation expressed in small populations may help to buffer them from extinction.