Abstract
AbstractSignificant progress has been made in providing guidelines and recommendations for assessing the ecological niche, stage of invasion, and probability of invasive alien plant species (IAPS) potential distribution in space and time. We followed these recommendations by developing and comparing ordination and species distribution models (SDMs) of two important woody IAPS in Eastern Africa,Prosopis julifloraandLantana camara, and interpreting the results to inform IAPS management. The two species differ in their invasion history in Eastern Africa; whileL. camarawas widely introduced there in the 19th century,P. juliflorawas only planted at selected locations in the 1970s and 1980s. For the SDMs, machine learning algorithms were used to generate one ensemble model each forP. julifloraandL. camara. For ordination, we used bioclimatic variables, performed a principal component analysis, and compared the native and global niches of the species with the Eastern African niche. Niches varied substantially depending on the percentage of marginal climates excluded from the models. Additional analysis of the local niches surrounding the originalP. julifloraplantations showed that they are complementary, which may have led to an overestimation of regional niche filling. While niche expansion was absent or small depending on the percentage of marginal climates excluded, analysis of the stages of invasion suggested thatP. julifloramay have started to adapt to novel climatic conditions and thatL. camarais approaching a pseudo‐stable equilibrium in Eastern Africa. The SDMs showed that large areas in Eastern Africa that have not yet been invaded byP. julifloraare suitable or will become suitable with climate change. ForL. camara, the global SDM predicted a considerably larger suitable area than the Eastern African one, raising uncertainty about the areas to be included in a regional management strategy. Thus, combining ordination and SDMs and integrating a geographic component into ordination is useful in assessing IAPS invasion stages and potential niche shifts, and the results help inform IAPS policy and management. The combined approach can also serve to guide experimental studies addressing divergences between results generated with the different approaches.
Highlights
Invasive alien plant species (IAPS) are considered to have serious impacts on ecosystem functioning, the supply of ecosystem services, and human well-being (Vila et al 2011, Simberloff et al 2013, Vila and Hulme 2017)
For the Eastern African niche of L. camara, the requirements were met by the two BRT models and the two RF models, with true skill statistics (TSS) ranging between 0.57 and 0.66 and area under the curve (AUC) between 0.98 and 0.99
Looking at future climate scenarios, our models suggest a small expansion of the Eastern African niche of L. camara only when at least 10% of marginal climates are excluded; this is coupled with a shift of the niche toward higher annual and mean diurnal temperature ranges and higher precipitation during the warmest quarter
Summary
Invasive alien plant species (IAPS) are considered to have serious impacts on ecosystem functioning, the supply of ecosystem services, and human well-being (Vila et al 2011, Simberloff et al 2013, Vila and Hulme 2017). It should be managed according to a spatially explicit management strategy that sets different control objectives for areas with different invasion levels (Grice et al 2011, Terblanche et al 2016, Shackleton et al 2017b). Developing such a strategy requires a holistic understanding of a species’ stage of invasion and the invasion process, including factors affecting its local abundance, spread, and impact. To develop a spatially explicit management strategy, it is essential to assess (1) the current and projected future geographic distribution and abundance of the target IAPS, and (2) its ecological niche in the invaded range in comparison with the global or native niche under current and projected climates
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