Reconstructing the range expansion of the European Starling in Southern Africa using a hybrid method of niche modelling and individual based modelling
Janse van Rensburg, M.
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Much emphasis has been placed on modelling species distributions and less so on the distributional dynamics. It is important to not only ask where species occur, but why and how they reached a specific location. Biological invasions provide an ideal natural experiment for studying the drivers and dynamics of spreading. Here, I examine the drivers and dynamics of the spread of the European Starling, Sturnus vulgaris, in Southern Africa. Since its introduction in Cape Town in 1897, this top invasive avian species has rapidly spread across a large extent of South Africa and is continuously expanding its current range. It is, thus, of great theoretical and management value to elucidate the invasion process and identify key environmental drivers of its range dynamics. The aim of this research is twofold. First, I aim to develop realistic suitable habitat maps for the European Starling in Southern Africa based on multiple climatic and geographic variables using two robust methods of species distribution modelling (SDM), namely maximum entropy (MaxEnt) and boosted regression trees (BRT). Second, I aim to, subsequently, develop a hybrid model that runs a dynamic individual-based model (IBM) on these suitability maps predicted from the SDM. This hybrid model provides an excellent opportunity to incorporate a variety of response regimes on how behavioural strategies and dispersal scenarios respond to environmental and geographic features, and as a result leads to a well-rounded study of the species‟ distribution dynamics in the region. Independent atlas data and field collections were used to parameterize and later validate the model. This allowed me to further identify an optimal model through a parameter sensitivity analysis sorting of the contribution of each environmental and behavioural features to shaping the past and current geographical range of European Starling. According to the optimal model, starlings can choose among five sites to locate the one with the highest habitat suitability. They tend to avoid moving through areas with a 300 m elevation barrier, and the dispersal distance per year is limited to below 200 km. At the regional scale, the starlings were able to manoeuvre around mountainous regions and avoided the semi-desert regions of the Karoo. Their distribution was mainly driven by high winter precipitation along the low-lying coastal regions. Future projections of their distribution suggested a continuous range expansion throughout the provinces of the Free State, Gauteng, North West (reaching Mahikeng, the capital of North West, in 2022) and Limpopo (reaching Polokwane, the capital of Limpopo, in 2046) as well as into the neighbouring countries of Mozambique and Zimbabwe (reaching Bulawayo, 2nd largest city in Zimbabwe, in 2062).