Species Richness and Pollen Loads of Hummingbirds Using Forest Fragments in Southern Costa Rica1

Abstract

In many areas of the humid tropics, relatively small patches of forest is the habitat most commonly available for forestdwelling organisms. To assess resident hummingbird use of forest fragments near the Las Cruces Biological Station in southwestern Costa Rica, we mist-netted birds in five fragments (0.3‐20 ha; ca 1300 m elev.) and the station’s 226ha forest preserve (ca 900‐1280 m elev.). From January 1994 through mid-March 1999, we recorded 1069 captures of 21 hummingbird species during 49,900 net hours. Species richness, diversity indices, and capture rates increased asymptotically with patch size, as expected. We captured 16 hummingbird species in both the largest fragment (20 ha) and in the nearby forest preserve. Non-forest hummingbirds did not occur more frequently in the fragments than in the preserve, and all fragments supported a mixture of forest-interior and canopy-dwelling hummingbird species, along with a diverse group of hummingbird-pollinated plants. Phaethornis guy was common at all sites and visited .13 plant species in the fragments during the 1998 dry season (based on analyses of pollen collected from the birds’ heads). In contrast, Eutoxeres aquila was found at only the largest three sites (10, 20, and 226 ha) and carried pollen from only one plant species during the 1998 dry season. Lampornis castaneoventris, a high-elevation hummingbird species, was far more common in the fragments than in the preserve, while three species typically found at lower elevations were rare in the fragments and common in the preserve. These distribution patterns could be influenced by local climatic differences because the fragments are higher and receive more cloud mist and annual precipitation than the preserve. Our data show that pollen loads on hummingbird species vary, and this has implications for gene flow among hummingbird-pollinated plants. Although our results are consistent with previous reports that hummingbird species may be less affected by deforestation than insectivorous avian species such as army ant followers, it is important to note that from the perspective of the genetics and demographics of hummingbird-pollinated plants, the important issue is whether appropriate pollinator species are able to move plant propagules among subpopulations of plants in fragmented landscapes. We conclude that the biodiversity of tropical forest fragments as small as 10 to 20 ha is high enough to warrant long-term preservation of these areas, especially if they are located near larger tracts of forest.