Abstract
The ‘Abundant-Centre Hypothesis’ is a well-established but controversial hypothesis stating that the abundance of a species is highest at the centre of its range and decreases towards the edges, where conditions are unfavourable. As genetic diversity depends on population size, edge populations are expected to show lower intra-population genetic diversity than core populations, while showing high inter-population genetic divergence. Here, the genetic implications of the Abundant-Centre Hypothesis were tested on two coastal mussels from South Africa that disperse by means of planktonic larvae, the native Perna perna and the invasive Mytilus galloprovincialis. Genetic structure was found within P. perna, which, together with evidence from Lagrangian particle simulations, points to significant reductions in gene flow between sites. Despite this, the expected diversity pattern between centre and edge populations was not found for either species. We conclude that the genetic predictions of the Abundant-Centre Hypothesis are unlikely to be met by high-dispersal species with large population sizes, and may only become evident in species with much lower levels of connectivity.
Highlights
The Abundant-Centre Hypothesis (ACH) states that species perform better at the centre of their range, where high densities are observed, and that densities decline towards the range edges[1,2]
Another review assessing population genetic structure across the geographical ranges of 115 species of plants and animals showed that, on average, only 64% of studies detected a decline in genetic diversity towards the range margins, there was a geographic bias as the majority of studies concerned the northern range limits of northern hemisphere species[9]
These studies show that species abundance, demography and genetic diversity appear to be more variable than predicted by the ACH, as they are determined by the interaction of multiple fluctuating contemporary environmental drivers plus historical effects[9,10]
Summary
The Abundant-Centre Hypothesis (ACH) states that species perform better at the centre of their range, where high densities are observed, and that densities decline towards the range edges[1,2]. Another review assessing population genetic structure across the geographical ranges of 115 species of plants and animals showed that, on average, only 64% of studies detected a decline in genetic diversity towards the range margins, there was a geographic bias as the majority of studies concerned the northern range limits of northern hemisphere species[9] Taken together, these studies show that species abundance, demography and genetic diversity appear to be more variable than predicted by the ACH, as they are determined by the interaction of multiple fluctuating contemporary environmental drivers plus historical effects[9,10]. We tested the predictions of the ACH for the dominant rocky shore mussels in this region[11], the invasive Mytilus galloprovincialis and the native Perna perna The latter is represented by two distinct regional genetic lineages[12], effectively providing edge and centre populations for three model organisms with similar biology. We hypothesised that centre populations of the three mussels would have greater genetic diversity than edge populations and tested this by comparing genetic diversity for the two population types
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