Mapping the potential habitat suitability and opportunities of bush encroacher species in Southern Africa: a case study of the SteamBioAfrica project

Abstract

Senegalia mellifera (Benth) Seigler & Ebinger., Dichrostachys cinerea (L.) Wight & Arn. and Terminalia sericea Burch. Ex DC., are three important bush encroacher species that contribute to the well-known ecological process named “thicketization” in Southern Africa. This issue has persisted for many years, impacting species distribution, plant communities, soil, and fauna dynamics. According to climate change projections, Southern Africa is expected to become drier and warmer in future scenarios, creating favourable conditions for proliferation of bush encroacher species. MaxEnt is a general-purpose machine learning method widely utilized in various ecological and biological scenarios to predict the potential suitable habitat of species. This is achieved by incorporating presence-only occurrence records and bioclimatic, and topographic variables. The analysis was performed in a Geographic Information System based on the current and future suitable areas for the respective Shared Socioeconomic Pathways (SSP) scenarios according to INM-CM5-0, UK-ESM1-0-LL and MPIES-M1-2-HR climate models. This was done to assess the potential effects of climate change on the distribution patterns of bush encroacher species. Model performance was assessed by the area under the curve (AUC) of the receiver operator characteristic (ROC), with 0.836, 0.822 and 0.738 as results to Terminalia sericea, Dichrostachys cinerea and Senegalia mellifera respectively. The current results show that Senegalia mellifera presents a habitat suitability of 56% (1,460,353 km²) of the total area, while Terminalia sericea and Dichrostachys cinerea have suitability over 37.9% (996,168 km²) and 43.9% (1,154,645 km²) of the area, respectively. These findings indicate that precipitation and temperature variables are the most important factors in explaining the spatial distribution of the bush encroacher species, predicting a future increase between 8–29.4%, 2.8–24%, and 3–24.2% for Senegalia mellifera, Terminalia sericea, and Dichrostachys cinerea respectively. Furthermore, each species has its own set of important variables and different ecological behaviour patterns. These results imply that an improved understanding of the response of woody species to a changing climate is important for managing bush encroachment in savanna ecosystems. Based on our analysis, Senegalia mellifera is currently suitable in 56% of the total area; while 43.9% of the total area is suitable for Dichrostachys cinerea, and 37.87% of the total area is currently suitable for Terminalia sericea. In the future scenarios, the habitat suitability increases for all three species compared to the current state. The unsuitable areas decrease in all the proposed scenarios with respect to the present, predicting a shrub expansion in the future throughout southern Africa. Furthermore, our results imply that Senegalia mellifera is the most potentially affected by bush encroachment in future changes, with a larger distribution area in future scenarios. These findings are supported by many studies, which indicate the probable increase of woody cover and loss of grasslands. Temperature and precipitation patterns are the main drivers behind the distribution of these bush encroachers, increasing or decreasing the competitiveness of these species according to these variables and their phenotypic plasticity. The increase in habitat suitability occurs throughout the case study, but there is a clear trend of shrub expansion towards the south of the case study. The change maps show a pattern of shrub movement towards South Africa and Botswana. This would fit with climate predictions for South Africa, which predict a decrease in rainfall in the east and north of the country, being more suitable for shrub competitive species.