Abstract:
This thesis addresses questions within the research fields of invasion biology and spatial
ecology, with a focus on species distribution patterns, biogeographical regions and ecological
transition zones, or ecotones. More specifically, species distribution patterns in alien plants at
large spatial scales using atlas data, and invertebrate patterns making use of field data
collected at a smaller scale (total extent ca. 30 km). First I show that alien plants form largescale
geographically differentiated species assemblages in southern Africa (i.e. South Africa,
Lesotho, Swaziland, Namibia and Botswana). I demonstrated this by mapping and describing
several alien phytogeographic regions at a quarter-degree spatial resolution, and further
suggest possible environmental and human-caused determinants of each of these regions.
Second, at the same spatial resolution (for South Africa and Lesotho combined, and each of
the plant biomes), I show that relatively higher levels of alien plant species richness occur at
or near to ecotones, compared to areas that are spatially further away from these ecotones and
that typically represent the core habitat of the ecoregions examined. This finding remained
valid after taking into account the underlying positive relationships between alien plant
richness and energy availability. I also suggest that it is the relatively higher environmental
heterogeneity at ecotones (represented here by spatial variation in altitude, rainfall and
geology) that promote high alien plant richness. Third, at a smaller spatial scale I report
several examples of change in beetle and spider species composition across a savannagrassland
ecotone in the west of South Africa’s Free State Province, with the ecotone itself
supporting comparatively lower levels of species richness and abundance. This contrasts with
a popular assumption that ecotones are characterised by high species richness. Data gained
from long-term intensive sampling is preferable for ecological studies, but not always
available or practical to acquire; however with the three studies in this thesis I show that data
from existing species atlases and feasible short-term surveys can be successfully applied to
answer a variety of ecological questions.
