Predator-mediated coexistence of calanoid copepods in a spatially heterogeneous environment: a numerical simulation model

Abstract

Literature data on the life history and ecology of two sympatric calanoid copepod species, Eurytemora affinis and Acartia tonsa, were analyzed for density-dependent processes and for processes promoting direct and indirect interactions between the two populations. The impact of these processes on the competition and coexistence of the two copepod species was examined with a small strategic model which could be solved analytically, and a detailed numerical simulation model integrating comprehensive experimental evidence with theoretical concepts about species coexistence. In the strategic model considering the major interactions between the two populations, no stable coexistence was achieved within a reasonable parameter range under steady state conditions. However, in the numerical simulation model, which included in addition to many biological details the spatial heterogeneity of the biotope, both species coexisted for a wide and sensible parameter range. Model results suggest that in the present case study, competition is mainly caused by common predators (i.e. carnivorous zooplankton), but not by utilization of common resources. Competition for common resources is weak and diffuse, because of numerous other herbivores. Predation of A. tonsa on nauplii of both species enhances intra-specific and interspecific competition at high abundances but despite being species-specific it does not support coexistence within a reasonable parameter range. Non-selective predation by carnivorous zooplankton depending on the combined abundance of the two prey species appears as the main factor promoting competition between copepod species. At the same time, it is — at least in the model — the most important mechanism of coexistence if the prey species are spatially heterogeneously distributed and the predation pressure relies on the local prey density. Revealing this mechanism of coexistence allows a better explanation for the observed spatial distribution of estuarine copepod species.