In 1999, bees were sampled from several flowering individuals of the leguminous tree, Andira inermis (Papilionoideae), at two sites in northwestern Guanacaste Province of Costa Rica. One site, city outskirts of Liberia, was experiencing steady encroachment of human development, whereas the other was a moderately impacted cattle ranch/wildland area near the small town of Bagaces. A standardized method was employed to sample the bees, which had been used previously in 1996 in the same tree populations at the same two sites (Frankie et al., 1997).

Results of the 1999 samples were compared with those taken in 1996 to examine possible changes in bee diversity and abundance. During this three-year period, El Niño and La Niña climatic events had occurred back-to-back, and this combination of weather patterns provided an opportunity to evaluate possible short-term changes in the bee taxa that use A. inermis as one of their preferred host plants. Other bee host plants at both sites were also surveyed for seasonal flowering, the results of which formed a broader context for assessing the A. inermis bee samples.

The comparison revealed the follwing. 1) The composition of bees had changed with reductions in large bees from 1996 to 1999; most noticeably some anthophorids and especially Centris and Epicharis. Africanized honey bees and smaller bee taxa remained about the same in composition and overall abundance. 2) During El Niño and La Niña years from mid 1997 through early 1999, the flowering of key resources for large bees, especially Byrsonima crassifolia, was substantially delayed (and sometimes reduced) to the point where building and provisioning of bee nests was likely reduced, which apparently led to declines in large bee taxa. Other possible causes of decline, including increasing agricultural development in the region, are also discussed. 3) Intertree variation in attraction of bees to A. inermis was documented in both 1999 and 1996, but persistence of this variable attraction on the same individual trees could be only partially evaluated in 1999. The flowering pattern(s) of individual A. inermis trees has proved to be highly variable within and between the two study sites.