Habitat Fragmentation, Native Insect Pollinators, and Feral Honey Bees in Argentine 'Chaco Serrano'

Abstract

In subtropical dry forest (Chaco Serrano) of Tucuman province, northwestern Argentina, we assessed responses of the flower—visitor assemblage to habitat fragmentation by monitoring insect visits to two spring—flowering tree species and by sampling insects with yellow pan traps. Each of four study sites contained a tract of continuous forest, one large (>2.2 ha) forest fragment, and one small (<1 ha) fragment, with fragments isolated for 5—20 yr preceding the study. During its respective flowering peak each tree species examined, Prosopis nigra (Mimosoideae) and Cercidium australe (Caesalpinoideae), dominated the sites' entomophilous flora. Results indicate that flower—visitor assemblages respond to landscape features on the scale of hectares; specifically, forest fragmentation in the Chaco Serrano leads to an insect flower—visitor fauna increasingly dominated by the exotic honey bee (Apis mellifera). 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—flowering 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 of honey bee visits tended to increase in complementary fashion, such that the total frequency of insect visits to flowers of either plant species varied little with fragment size. Frequencies of visits by honey bees and those by native insects were also negatively correlated across individual trees. In both plants, visits by native insects were most consistent (varied the least among plants or over time) in large forest tracts, whereas honey bee visits to C. australe were most consistent in small fragments. In pan trap samples the relative frequency of honey bees increased with decreasing fragment size. Native flower—visitors sampled by pan traps increased in numbers and taxon richness both with increasing patch size and as spring progressed. Thus, fragmentation of the Chaco Serrano appears to (a) affect native flower—visitors adversely and to (b) facilitate honey bees' access to floral resources. It is not clear that these two effects are directly related to each other, however.