Supporting sandy beach conservation through comparative phylogeography: The case of Excirolana (Crustacea: Isopoda) in South Africa

Abstract

Sandy beach ecosystems are ecologically and culturally important, yet they face a wide range of anthropogenic stressors. Their constituent biodiversity is usually poorly resolved, with numerous instances of cryptic species described. In addition, there are few studies on the spatio-temporal patterns of evolutionary diversity, although these are vital for unravelling historical and contemporary processes acting to shape biodiversity, with measures of molecular diversity and differentiation providing important insights towards the conservation and management of beach ecosystems. Here we utilise a comparative phylogeographic approach to describe the mtDNA COI variation of two co-distributed Excirolana (Crustacea, Isopoda) sister-species in South Africa. We found that both species showed medium to high population genetic structure, with many populations isolated from each other. In addition, we describe two deeply divergent lineages for both species in the eastern parts of their ranges, which may constitute two additional Excirolana species in South Africa. We show that populations on the range edge for both species have lower genetic diversity than in the core. Our results on the divergent patterns of molecular diversity, in addition to the high levels of population divergence, highlight the need for the inclusion of molecular indices into conservation planning. This ensures safeguarding not only patterns of species occurrence, but also genetic diversity, critical to maximising resilience of sandy beach species in a changing world.