Spatial population structure in a patchily distributed beetle.

Abstract

The dynamics and evolution of populations will critically depend on their spatial structure. Hence, a recent emphasis on one particular type of structure--the metapopulation concept of Levins--can only be justified by empirical assessment of spatial population structures in a wide range of organisms. This paper focuses on Aphodius fossor, a dung beetle specialized on cattle pastures. An agricultural database was used to locate nearly 50 000 local populations of A. fossor in Finland. Several independent methods were then used to quantify key processes in this vast population system. Allozyme markers and mitochondrial DNA (mtDNA) sequences were applied to examine genetic differentiation of local populations and to derive indirect estimates of gene flow. These estimates were compared to values expected on the basis of direct observations of dispersing individuals and assessments of local effective population size. Molecular markers revealed striking genetic homogeneity in A. fossor. Differentiation was only evident in mtDNA haplotype frequencies between the isolated Aland islands and the Finnish mainland. Thus, indirect estimates of gene flow agreed with direct observations that local effective population size in A. fossor is large (hundreds of individuals), and that in each generation, a substantial fraction (approximately one-fifth) of the individuals move between populations. Large local population size, extreme haplotype diversity and a high regional incidence of A. fossor all testify against recurrent population turnover. Taken together, these results provide strong evidence that the whole mainland population of A. fossor is better described as one large 'patchy population', with substantial movement between relatively persistent local populations, than as a classical metapopulation.