The Conservation Values of Bees and
Ants in the Costa Rican Dry Forest
S. Bradleigh Vinson, Sean T. O'Keefe, and Gordon W. Frankie
Over the past 30 years of solitary-bee studies in the Costa Rican dry forest we have observed a steady decline in native solitarybee populations. In 1972 bee diversity was surveyed from a small population of the fabaceous tree Andira inermis, at a site just south of the town limits of Liberia (see maps in chapter 1). At that time about 70 species were collected (Frankie et al. 1976). In 1989 we casually sampled bees again from several A. inermis trees in the vicinity of Liberia and found only 37 species (Vinson et al. 1993). Another sampling at the original Liberia site in 1996, during a year of robust A. inermis flowering, revealed that diversity had declined from a high of 70 species in 1972 to only 28 species. Further, overall numbers of bees had declined by slightly more than 90 percent (Frankie et al. 1997).
What brought on this decline? Our data suggest four major factors. The first is habitat destruction, caused primarily by agricultural development and urbanization, between the towns of Liberia and Cañas (see maps in chapter 1). The second factor is the introduction of jaragua grass, Hyparrhenia rufa, from Africa. The third factor, often linked to the second, is human-caused fire. Jaragua grows 2–3 m higher than the short native grasses, becomes highly flammable during the dry season, and burns very hot. Jaragua is also fire-adapted and readily resurges after a fire. It is fanned by dry-season trade winds that allow fire to burn well inside the dry-forest edge, which opens the forest to further jaragua invasion. Besides slowly converting forest to grassland, fires sometimes invade deep into the forest, resulting in loss of deadwood that is used by nesting anthophorid bees including some Centris (Frankie et al. 1988) and several Xylocopa species. Deadwood-nesting species were rare in the 1996 captures (Frankie et al. 1997). Fire also leads to other problems for bees. Important resource trees such as three Tabebuia species, Dalbergia retusa, Caesalpinia eriostachys, and Cochlospermum vitifolium are resistant to