Farm dams facilitate amphibian invasion: Extra-limital range expansion of the painted reed frog in South Africa



Driven by the mobility of organisms and novel habitats created by anthropogenic changes of landscape structure and climate, range expansion can modify the functioning of recipient ecosystems by altering ecosystem processes such as intra- and cross-trophic biotic interactions. We explain the spatial and temporal dynamics of the recent range expansion of painted reed frogs (Hyperolius marmoratus Rapp) in the Western Cape Province of South Africa. We identify the geographical and temporal origin of expansion, quantify extent of occurrence, internal range structure and habitat characteristics of occupied sites. The painted reed frog was introduced into its novel range during 1997 or early 1998, and is now widespread. Highly variable annual spread rates suggest both diffusion-based and human-mediated jump dispersal as drivers of the range expansion. Spatial structure is evident at the scale of the entire novel range, with two distinct populations separated by at least 100 km of unoccupied terrain. Models show that occupancy is limited by summer aridity, low winter temperatures and the absence of fringing vegetation around water bodies. The range structure and the presence of breeding populations in 26% of water bodies surveyed imply that there are further opportunities for range expansion across the network of artificial water bodies available. We infer that, facilitated by human-mediated jump dispersal and the dense distribution of perennial farm dams, painted reed frogs have colonized the winter rainfall region of south-western South Africa and fit the description of ‘urban exploiters’. Unspecialized habitat requirements, rapid spread and high local population sizes further suggest that they could compete with co-occurring endemic frogs and hamper the conservation of these range-restricted amphibians.