Invasive woody plants: a double-edged sword for invertebrate conservation
Uys, Charmaine J.
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While the Cape Peninsula (South Africa) is renowned for its exceptional plant and invertebrate diversity and endemism, extensive alien plant invasion and exotic pine plantations reduce native species richness, and may facilitate alien invertebrate invasions. This study examines the impact of planting and felling pine on litter invertebrate communities, by comparing invertebrate diversity and faunal exchange between pine plantations and two types of native vegetation (Afrotemperate forest and fynbos). Impacts of the worst invasive alien invertebrate (Argentine ant, Linepithema humile) and other alien invertebrate species are investigated. This is one of the first attempts to inventory and quantify impacts of non-ant alien invertebrates in Table Mountain National Park. The entire ground-dwelling invertebrate community was sampled at 31 sites in summer 2008/2009, using soil cores, leaf litter samples, pitfall traps, sugar-baited ant traps, and decayed logs. A total of 112 404 individuals representing 728 species (10 classes and 38 orders), including nine Cape Peninsula endemic and 19 alien invertebrate species, was collected. Pine plantations supported lower species richness and abundance, and different community assemblages, compared to Afrotemperate forest, but similar species richness to fynbos. Pine plantations shared fewer species with fynbos than forest, thus negatively affected fynbos-specialist invertebrates, because afforestation reduces available fynbos habitat. Alien species richness was similar across habitats. Argentine ants, like most other alien species identified, were present in all habitats, but were the most abundant ant species at only four forest, two fynbos and two pine plantation sites. The impact of Argentine ant invasion on native ant communities was evaluated using species richness and community composition analyses, the functional group approach, and species co-occurrence patterns, and provided evidence for displacement, impoverishment, and community disassembly. No clear impacts of the 18 non-ant alien species on the abundance, species richness, or community composition of corresponding native taxa were detected. However, carnivorous molluscs and European wasps (Vespula germanica) require careful monitoring. Using a reiterative process, ant species and functional groups were selected as the best ecological indicators of restoration progress in fynbos following clear-felling of pine. These findings have application to other Mediterranean-type ecosystems impacted by exotic pine.