Implications of Climate Change for the Distribution of the Invasive Grass Genus Nassella in South Africa and Lesotho

Three South American Nassella species ( N. neesiana, N. tenuissima and N. trichotoma ) have naturalized in South Africa but have not yet been reported in Lesotho. Of these, N. tenuissima and N. trichotoma have been recognized as serious invaders since the 1970s. Assessing their current and future distributions is crucial for understanding the threats they pose in both countries. This study addressed the following questions: 1) What key bioclimatic variables control the geographical distribution of these species? 2) What is the current potential distribution range? and 3) What is the future potential distribution under three carbon-emission scenarios using estimates for the period 2071–2100? Nine non-collinear predictors from the CHELSEA database were selected for Maxent species distribution models. These models were projected under future climate scenarios, incorporating three Shared Socioeconomic Pathways (SSPs) from five global circulation models. Results showed that temperature-related variables, particularly mean diurnal range, were critical for all species, while precipitation in the driest month also influenced N. trichotoma . Results suggest that all three species can potentially occupy a greater area than they currently occupy. While highly suitable area is predicted to contract for all species, the Maloti-Drakensberg region—including in Lesotho—is predicted to become increasingly suitable. This poses a severe risk to the endemic-rich biodiversity and rangeland productivity. Much of Lesotho is affected by overgrazing and other disturbances which favor Nassella establishment. Proactive surveillance directed at early detection of Nassella incursion into Lesotho should be given high priority.