Alien Phytogeographic Regions of Southern Africa: Numerical Classification, Possible Drivers, and Regional Threats

Abstract

The distributions of naturalised alien plant species that have invaded natural or semi-natural habitat are often geographically restricted by the environmental conditions in their new range, implying that alien species with similar environmental requirements and tolerances may form assemblages and characterise particular areas. The aim of this study was to use objective numerical techniques to reveal any possible alien phytogeographic regions (i.e. geographic areas with characteristic alien plant assemblages) in southern Africa. Quarter degree resolution presence records of naturalised alien plant species of South Africa, Lesotho, Swaziland, Namibia and Botswana were analysed through a divisive hierarchical classification technique, and the output was plotted on maps for further interpretation. The analyses revealed two main alien phytogeographic regions that could be subdivided into eight lower level phytogeographic regions. Along with knowledge of the environmental requirements of the characteristic species and supported by further statistical analyses, we hypothesised on the main drivers of alien phytogeographic regions, and suggest that environmental features such as climate and associated biomes were most important, followed by human activities that modify climatic and vegetation features, such as irrigation and agriculture. Most of the characteristic species are not currently well-known as invasive plant species, but many may have potential to become troublesome in the future. Considering the possibility of biotic homogenization, these findings have implications for predicting the characteristics of the plant assemblages of the future. However, the relatively low quality of the dataset necessitates further more in-depth studies with improved data before the findings could be directly beneficial for management.

Plant invasions as a biogeographical assay: Vegetation biomes constrain the distribution of invasive alien species assemblages

“…This was rather surprising as the cluster analysis considered no spatial dimension, but only the co-occurrence of IAPs. Previous studies (Hugo et al, 2012) identified much more diffuse clusters. IAPs in South Africa seem to occupy different regions, co-occurring with a well-defined group of species (indicated by the high number of characteristic species per cluster).…”

“…We explore the biogeography of a subset of invasive alien plants in South Africa and derive several "alien biomes" based on the current alien plant assemblages and associated environmental drivers. This work builds on previous studies by and Hugo et al (2012) who investigated aspects of alien plant species assemblages in South Africa. We provide a more detailed analysis and test several hypotheses to explain the observed biogeographic patterns.…”

A novel phylogenetic regionalization of phytogeographical zones of southern Africa reveals their hidden evolutionary affinities

“…trees, shrubs, herbs and grasses) and similarity in ecological conditions (Low & Rebelo, ; Rutherford, ; Mucina & Rutherford, ), fire characteristics (Archibald et al ., ), and plant species distribution (Linder et al ., , ). Other efforts have examined particular species groups, such as non‐natives (Hugo et al ., ), bryophytes (Van Rooy & Van Wyk, ), flagship and hotspot species (Steenkamp et al ., ) or focused on particular areas (Irish, ). Here, we revisit the definition and delineation of phytogeographical regions (areas where groups of plants occur together in particular climates and form distinct vegetation types; Lomolino et al ., ) within southern Africa using a phylogenetic approach.…”

Drivers of compositional turnover of narrow-ranged versus widespread naturalised woody plants in South Africa

“…Alien biomes in South Africa have been studied using a phytogeographic classification which clusters communities as regions with distinct species composition. In South Africa, the geography of alien phytochoria, or regions inhabited by unique communities of alien plants, was first investigated by Hugo et al (2012); they found eight clusters of communities of naturalised and invasive plant taxa closely associated with biomes and prevailing climatic conditions, but with limited influence of human activities like irrigation and agriculture. Similar results were reported by Rouget et al (2015) using a classification tree.…”