Trade-Offs and Coexistence: A Lottery Model Applied to Fig Wasp Communities

Abstract

Ecological communities in which organisms complete their life cycles on discrete ephemeral patches are common and often support an unusually large number of species. Explaining this diversity is challenging for communities of ecologically similar species undergoing preemptive competition, where classic coexistence mechanisms may not readily apply. We use nonpollinating fig wasps as a model community characterized by high diversity and preemptive competition to show how subadditive population growth and a tradeoff between competitor fecundity and dispersal ability can lead to coexistence. Because nonpollinator species are often closely related, have similar life histories, and compete for the same discrete resources, understanding their coexistence is challenging given competitive exclusion is expected. Empirical observations suggest that nonpollinating fig wasp species may face a trade-off between egg loads and dispersal abilities. We model a lottery in which a species' competitive ability is determined by a trade-off between fecundity and dispersal ability. Variation in interpatch distance between figs generates temporal variability in the relative benefit of fecundity versus dispersal. We show that the temporal storage effect leads to coexistence for a range of biologically realistic parameter values. We further use individual-based modeling to show that when species' traits evolve, coexistence is less likely but trait divergence can result. We discuss the implications of this coexistence mechanism for ephemeral patch systems wherein competition is strongly preemptive.

Towards the general mechanistic prediction of community dynamics

“…The coexistence of competitors in ephemeral patch communities is widely assumed to be dominated by the stabilising mechanism of intraspecific vs. interspecific aggregation (Chesson, 2000a; Duthie, Abbott, & Nason, , ). In general, when conspecifics aggregate within or among patches, intraspecific competition increases relative to interspecific competition, facilitating coexistence.…”

“…In general, when conspecifics aggregate within or among patches, intraspecific competition increases relative to interspecific competition, facilitating coexistence. Mechanisms causing conspecific aggregation include the behaviour of females ovipositing onto patches in clutches (Takahashi, ), and variation among species in patch attractiveness or accessibility, which generate spatial and temporal heterogeneity in species distributions (Chesson, 2000a; Duthie et al., ; Heard, ). The mechanisms modulating aggregation may be directly linked to hard functional traits, such as individual longevity, wing loading, egg load and larval feeding rate, enabling the prediction of community dynamics (e.g.…”

“…Mechanisms causing conspecific aggregation include the behaviour of females ovipositing onto patches in clutches (Takahashi, 2007), and variation among species in patch attractiveness or accessibility, which generate spatial and temporal heterogeneity in species distributions (Chesson, 2000a;Duthie et al, 2014;Heard, 1998).…”

How to be a fig wasp parasite on the fig–fig wasp mutualism

“…Fig species with larger syconia and larger numbers of flowers may provide greater opportunities for niche specialisation, supporting greater NPFW species diversity. There may be trade-offs between life history traits such as fecundity (eggs matured at eclosion) and dispersal ability assuming that wing loading (ratio of body mass of the insect to wing area) correlates with intrinsic dispersal ability [50] which is the distance that wasps can fly fuelled by their own reserves. Since fig wasps can also be wind-dispersed, their realised dispersal distance would be the combined effect of intrinsic dispersal ability and wind-aided movement.…”

“…Therefore, proovigenic wasps with higher numbers of mature eggs at eclosion may be expected to have lower intrinsic dispersal ability compared to synovigenic wasps. These tradeoffs between life history strategies and intrinsic dispersal ability might therefore affect the realised dispersal distance of NPFWs with different egg maturation strategies, which when coupled with the spatiotemporal variation in availability of syconia suitable for parasitism and the physiological tolerance of wasps may structure local NPFW communities [50,51]. Parasites vary in their costs on the fig-fig wasp mutualism [52][53][54] and such costs may also contribute to NPFW community structure.…”