HABITAT FRAGMENTATION, NATIVE INSECT POLLINATORS, AND FERAL HONEY-BEES IN ARGENTINE CHACO SERRANO

In subtropical dry forest (Chaco Serrano) of Tucuman province, northwe stem Argentina, we assessed responses of the flower-visitor assemblage to habitat fragmentation by monitoring insect visits to two spring-fl owering tree species and by sampling insects with yellow pan traps. Ea ch of four study sites contained a tract of continuous forest, one lar ge (> 2.2 ha) forest fragment, and one small (< 1 ha) fragment, with f ragments isolated for 5-20 yr preceding the study. During its respecti ve flowering peak each tree species examined, Prosopis nigra (Mimosoid eae) and Cercidium australe (Caesalpinoideae), dominated the sites' en tomophilous flora. Results indicate that flower-visitor assemblages re spond to landscape features on the scale of hectares; specifically, fo rest fragmentation in the Chaco Serrano leads to an insect flower-visi tor fauna increasingly dominated by the exotic honey bee (Apis mellife ra). Bees as a group made >90% of observed flower visits to both plant species. The honey bee alone made 82% of all visits to the early-flow ering tree species P. nigra and 44% to the later flowering C australe. Frequency and taxon richness of native flower-visitors at both plant species declined with decreasing forest-fragment size, but frequency o f honey bee visits tended to increase in complementary fashion, such t hat the total frequency of insect visits to flowers of either plant sp ecies varied little with fragment size. Frequencies of visits by honey bees and those by native insects were also negatively correlated acro ss individual trees. In both plants, visits by native insects were mos t consistent (varied the least among plants or over time) in large for est tracts, whereas honey bee visits to C australe were most consisten t in small fragments. In pan trap samples the relative frequency of ho ney bees increased with decreasing fragment size. Native flower-visito rs sampled by pan traps increased in numbers and taxon richness both w ith increasing patch size and as spring progressed. Thus, fragmentatio n of the Chaco Serrano appears to (a) affect native flower-visitors ad versely and to (b) facilitate honey bees' access to floral resources. It is not clear that these two effects are directly related to each ot her, however.