Countryside biogeography: Use of human-dominated habitats by the avifauna of southern Costa Rica

Understanding the multifaceted relationship between biodiversity and landus e intensity is key to conservation policy. To begin to characterize this re lationship in a tropical region, we investigated the bird fauna in an agric ultural landscape in southern Costa Rica. Landsat Thematic Mapper (TM) data show that about 27% of the land remains forested in the 15 km radius study region encompassing our sites. The rest was cleared about 40 yr ago for re latively small-scale coffee and cattle production, intermixed with other cr ops. Our goals were to: (1) compare the composition of the avifauna found i n forest-fragment and open habitats of the countryside; (2) assess the faun al change that has occurred since deforestation: and (3) provide a baseline for future comparisons. We surveyed the avifauna of eight forest fragments (0.3-25 ha) and 13 open- habitat sites (1.0 ha each) in the agricultural landscape. The pre-deforest ation avifauna was approximated by the long-term bird list for the largest forest fragment (Las Cruces, LC; 227 ha) in the study region. We assumed co nservatively that a species recorded in LC but not detected elsewhere occur red only in LC. Of the 272 locally extant bird species considered in this s tudy, 149 (55%) occurred in forest habitats only. There was a significant p ositive correlation between forest fragment size and species richness for t hese forest birds. Of the remaining 123 species, 60 (22% of the total) occu rred both in forest and open habitats. Sixty-three species (23%) occurred i n open habitats only; the three nonnative species (1%) are in this group. B ased on comparisons with larger forest tracts outside of the study region, it appeared that between 4 and 28 species (1-9% of the possible original to tals) have gone locally extinct since deforestation began. The avifauna of open habitats was similar throughout the study region and did not vary with proximity to extensive forest. A substantial proportion of the native bird fauna occurs in a densely (huma n) populated, agricultural landscape almost a half-century after extensive clearance. There are, however, cautionary messages: (1) the common occurren ce of forest birds in human-dominated countryside (including both forest-fr agment and open habitats) does not necessarily imply that these species mai ntain sustainable populations there; (2) about half of the species have lit tle prospect of surviving outside of the forest; and (3) ongoing intensific ation of land use may greatly reduce avian diversity in countryside habitat s. Nonetheless, countryside habitats may buy time for the conservation of s ome species; at best, they may even sustain a moderate fraction of the nati ve biota.