Nesting-habitat preferences of selected Centris bee species in Costa Rican dry forest

Solitary bees are known to be important pollinators of seasonal dry forest plants in Costa Rica. Recent assessments of the genus Centris (family Anthophoridae) and other solitary bee taxa indicate that populations of these bees have declined by 85-90 percent in this forest over the past 20 yr. Deforestation, wildfires, and conversion of forest land to agriculture are the main reasons for the decline. From 1987 to 1991 we studied the preferred nesting habitats of seven Centris species that specialize in making their nests in preexisting holes in dead trees. Special wooden sampling units, with preexisting standardized holes, were systematically placed in several habitat types in the Lomas Barbudal Biological Reserve, Guanacaste Province. Female bees encountered the holes as they searched for natural holes for nesting. Systematic monitoring of the units over five years, 1987 to 1991, provided a quantitative measure of habitat preference and changes in bee frequency through time. There were sufficient data for statistical analysis of three of the seven species (Centris nitida. C. bicornuta and C. analis). The closed oak forest was the predominantly preferred habitat for the three common Centris species, and this pattern remained constant through time. At least one species (C. nitida) apparently had less strict habitat requirements as it was commonly found nesting in a wide variety of habitat types. Significant increases and decreases in frequencies for individual species through time were recorded; however, there was no significant change in overall bee numbers through time. Results of the analysis, together with a qualitative assessment of the rare Centris species, suggest that the dry forest is actually a mosaic of preferred nesting sites, with each species having specific preferences. This pattern is consistent with other Centris species that have been studied in Costa Rica. The preference for oak forests is significant and suggests that more of this type of habitat should be conserved for Centris refugia.