Following in the footsteps of invasion: comparisons of founder and invasive genotypes of two independent invasions reveal site-specific demographic processes and no influence by landscape attributes on dispersal

Abstract

To understand the success of invasive alien species, it is necessary to evaluate the site-specific eco-evolutionary challenges they face in their new environments. We explored whether the rearrangement of genetic diversity is linked to the invasiveness of Prosopis juliflora by (i) comparing different stages of invasion (founding vs invasive populations) in two invaded areas (Afar Region, Ethiopia and Baringo County, Kenya) to evaluate whether different stages are dominated by different genetic attributes (e.g., characteristic genotypes or levels of genetic diversity) and by (ii) evaluating if landscape features affected dispersal between invasive populations in the two invaded areas. We hypothesised that different invasion stages would have unique genetic characteristics due to either site-specific demographic and/or dispersal dynamics. We also compared the genetic characteristics at an ‘invasive–non-invasive congener’ level by studying the non-invasive P. pallida, introduced to Baringo County, and assessed whether it hybridises with P. juliflora. In the Afar Region, the establishment and spread of P. juliflora were characterised by extensive gene flow that homogenised genetic diversity across all populations. In contrast, in Baringo County, invasive populations had lower genetic diversity than founders, and genetic differentiation was lower between invasive populations than between invasive and founder populations. In both invaded areas, we found no evidence that dispersal was hampered by geographic distance, bioclimatic conditions, or distance to roads, rivers and villages, at least at the spatial scales of our study; indicating frequent long-distance dispersal. Allelic richness was higher in P. juliflora than P. pallida founders and hybrids were mainly planted trees probably resulting from the sympatric cultivation of the two species following their introduction. Thus, management actions on Prosopis invasion in eastern Africa should consider site-specific dynamics occurring during the invasion.