AIM To examine whether at a subcontinental-scale ecotonal areas of transition
between vegetation communities are at higher risk of plant invasion.
LOCATION South Africa and Lesotho.
METHODS Using plant data on native and established alien species in South
Africa, we examined the relationship between plant richness (native and alien)
in each grid cell (quarter-degree resolution) in the study area and the distance
of the grid cell to the nearest ecotone between vegetation communities. We
used a residual analysis to estimate each grid cell’s relative invasibility (i.e. susceptibility
to invasion) relative to its ecotone distance. We further explored the
relative importance of ecotones in relation to large-scale environmental variation,
and the importance of ecotonal spatial heterogeneity, in structuring alien
species richness patterns.
RESULTS Both alien and native richness patterns become higher with declining
distance to ecotones, suggesting that transitional environments are more
susceptible to invasion than areas located farther away; however, levels of invasibility
vary across South Africa. The negative relationship between ecotone distance
and alien species richness remained negative and significant for the whole
of South Africa, grassland and Nama-Karoo, after controlling for environmental
variables. Several sources of environmental heterogeneity, which were shown
here to be associated with ecotones, were also found to be important determinants
of alien species richness.
MAIN CONCLUSIONS While most of the current conservation efforts at the regional
and global scales are currently directed to distinct ecosystems, our results
suggest that much more effort should be directed to the transitions between
them, which are small in size and have high native richness, but are also under
greater threat from invasive alien species. Understanding how alien species richness
and invasibility change across transitions and sharp gradients, where environmental
heterogeneity is high, is important for ongoing conservation
planning in a biogeographical context.