Altitudinal patterns in a tropical ant assemblage and variation in species richness between habitats

Abstract

The distribution of ants along an altitudinal gradient from 800 m to 1500 m was studied in southeastern Brazil. Two hypotheses were tested: a) "the altitudinal gradient hypothesis", which predicts that ant species richness decreases with increasing altitude; and b) "the habitat favourability hypothesis", which predicts that ant species richness is higher in mesic habitats than in xeric habitats, independent of altitude. Pairs of mesic and xeric habitats were randomly established and replicated three times at each 100 m of elevation. Mesic habitats were those along washes, creeks and rivers while xeric habitats were those away from water resources. Ants were collected utilizing sardine baits distributed along transects on both vegetation and on soil. The species richness of ants collected on vegetation and soil increased with decreasing elevation. This pattern was found for ants collected on the ground in both mesic and xeric habitats. Also, we found more ant species in mesic habitats (0.64 ± 0.05 ants/sample set) than in xeric habitats (0.45 ± 0.05 ants/sample set)(t = 3.51, p < 0.001). The decreased number of ant species at higher elevations as well as in xeric habitats may be caused by a decrease in habitat complexity that limits the success of ants. Furthermore, at high altitudes harsher habitat conditions and lower temperatures prevail, further limiting the success and establishment of ants.