Diversity, coexistence, and competitive ability: The effect of intraspecific diversity on invasibility and invadability in bruchid beetles

Background/Question/Methods

Recent work has highlighted the role of intraspecific diversity in shaping ecological dynamics. Particularly, it has been suggested that intraspecific and interspecific diversity may mutually support each other. If this is the case, intraspecific diversity is expected to promote mutual invasibility, a necessary component of stable coexistence. Theory predicts that intraspecific diversity influences invadability and invasibility through a number of negatively frequency dependent mechanisms, supporting mutual invasibility, and frequency independent mechanisms, which may promote or suppress coexistence. The effect of increased intraspecific variation may also depend on whether it is in a competitively dominant or subordinate species or both. This study tested how intraspecific genetic diversity affects the ability of a species to invade or repel a heterospecific population through reciprocal invasions. We used 5 phenotypically diverse populations (lineages) of two species of bruchid beetles (Callosobruchus maculatus and C. chinensis) reared on adzuki beans and lentils in petri dishes. Resident populations of varying genetic diversity (1, 3, or 5 conspecific lineages) were invaded by initially-rare heterospecific populations, also of varying genetic diversity, with invasion success measured as the change in invader frequency after three generations.

Results/Conclusions

The pairwise invasions of each heterospecific lineage revealed that the outcome of competition was contingent on lineage identity for both species: mutual invasibility, priority effects, and competitive dominance for both species were observed. Lineages were found to vary along two competitive gradients: a dominant axis of baseline fitness (high larvae survival and fast development) and a secondary frequency-dependent axis of contest versus scramble type competition (female body size and larval survival at high, relative to low, density). Increasing genetic diversity in both species led to increased coexistence through increased resource partitioning with C. maculatus controlling the adzuki beans while C. chinensis specialized on lentils. Increasing diversity in one species (HD) but not the other (LD) only increased coexistence if LD was represented by a competitive lineage, otherwise diversity in HD increased the probability of LD being competitively excluded. These results suggest that intraspecific diversity can, but does not necessarily, increase the likelihood of coexistence among competing species. Future work will elucidate the specific mechanisms promoting or suppressing coexistence in the bean