Species range dynamics over twenty years across functional groups of Southern African passerines

Raquel A. Garcia (1,2), Guy Midgley (3) , Guillaume Péron (1), Roland Schulze (4), Andrew Skowno (5), and Res Altwegg (1)

1 SEEC – Statistics in Ecology, the Environment and Conservation, Department of Statistical Sciences, University of Cape Town, Cape Town, South Africa

2 Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa

3 Department of Botany and Zoology, Stellenbosch University, Stellenbosch, South Africa

4 Centre for Water Resources Research, University of KwaZulu-Natal, Durban, South Africa

5 South African National Biodiversity Institute, Rondebosch, South Africa

Time series observations of species occurrences provide unique opportunities to test hypotheses about the dynamics of population abundance, species ranges, and species assemblage composition over time. Such data have often been used to show climate-induced range changes for northern hemisphere species, but are less commonly available in the southern hemisphere. In South Africa, Lesotho and Swaziland, volunteer birders have been gathering data on species sightings for the Southern African Bird Atlas Project over two survey phases covering the periods of  1987–1991 (SABAP1) and 2007–present (SABAP2). To account for imperfect sampling, occupancy models have been built for passerine species. Here, we use these occupancy models to examine shifts in passerine species’ ranges, as well local occupancy dynamics, between the two atlas surveys. At the species’ range level, we compare estimated range shifts to the velocity and direction of climate displacement across the species’ ranges between the two surveys. At the grid cell level, we investigate the influence of climatic changes and bush encroachment between the two surveys on local increases and decreases in occupancy. We find that patterns vary across different levels of species’ climate tolerance and marginality, and habitat specialisation. While other factors, such as biotic interactions, also determine species abundances and ranges, here we show the potential relative importance of climate and habitat tracking across species functional groups. Understanding which characteristics of species might render them more vulnerable to global change can enhance our ability to make predictions into the future, particularly for data-poor species.