Ants, altitude and change in the northern Cape Floristic Region
van Niekerk, A
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Aim Climate-modelling exercises have demonstrated that the Cape Floristic Region is highly sensitive to climate change and will apparently lose much of its northern limits over the next few decades. Because there is little monitoring of diversity in this area, ant assemblage structure was investigated within the main vegetation types in the Greater Cederberg Biodiversity Corridor. In particular, we sought to determine how ant assemblage structure differs between the main vegetation types, how restricted ants - and in particular the major myrmecochores - are to the major vegetation types, and which environmental variables might underlie differences in the ant assemblages and in the specificity of species to particular areas. Location Northern Cape Floristic Region, Western Cape, South Africa. Methods Sampling was undertaken during October 2002 and March 2003 across an altitudinal gradient ranging from sea level (Lambert's Bay) to c. 2000 m a.s.l. (Sneeukop, Cederberg) and down again to 500 m a.s.l. (Wupperthal) in the Western Cape, South Africa. Pitfall traps were used to sample ants at 17 altitudinal bands, stretching over three vegetation types (Strandveld, Mountain Fynbos and Succulent Karoo). Biotic and abiotic environmental variables were collected at each sampling site. Generalized linear models were used to determine the relationships between species richness, density, abundance and the abundance of the major myrmecochores, and the environmental variables. Redundancy analysis was used to determine the relationship between ant assemblage structure and the environmental variables. The Indicator Value Method was used to identify characteristic ant species for each vegetation type and altitudinal site. Results Temperature explained significant proportions of the variation in species density and abundance, and, together with area and several vegetation variables, contributed significantly to the separation of the assemblages in the major vegetation types and biomes. Four major myrmecochores were identified [Anoplolepis sp. (cf. custodiens), Anoplolepis sp. (cf. steinergroeveri), Camponotus niveosetosus, Tetramorium quadrispinosum]. The abundances of the two Anoplolepis species were related to vegetation variables, while the abundance of the other two species showed opposite relationships with temperature variables. Fourteen ant species were characteristic of certain vegetation types and altitudes. Several of these species contributed to the differences between the assemblages. Main conclusions There are likely to be substantial and complex changes to ant assemblages as climates change in the northern Cape Floristic Region. Moreover, the importance of ants for ecosystem functioning suggests that these responses are not only likely to be a response solely to vegetation changes, but might also precipitate vegetation changes. The changes that are predicted to take place in the next 50 years in the Cape Floristic Region could be substantially exacerbated by such synergistic effects, which have major implications for long-term conservation plans. Ongoing monitoring of this transect will reveal the nature and pace of the change as it unfolds.