Diversity and Conservation of Understory Birds in the Tilarán Mountains, Costa Rica

Abstract

-Using data from 10,726 captures of birds in mist nets, we document the high alpha and beta diversity of the understory avifauna in the Tilaran mountains of north-central Costa Rica. We grouped the capture data from 20 sampling sites into five Holdridge life zones that represent a gradient in elevation and seasonality of precipitation. Despite the limited size of the study area (200 kM2) and the relatively small differences in elevation (700 to 1,700 m), major differences were found among each of the five life zones. We recorded a total of 235 species, with species richness increasing downslope on both the Pacific and Caribbean sides of the range. Rarefaction curves showed that the elevational zone of 650 to 750 m on the Caribbean side (Wet Premontane Transition Forest) was the most diverse per unit of mistnetting effort and that the Lower Montane Rain Forest on the continental divide (1,500 to 1,700 m) was the least diverse. Adjacent life zones had similarity indices ranging from 0.30 to 0.47; beta diversity for the region was 0.45. These measures are indicative of a high species turnover among our samples as delimited by Holdridge's life zone system, suggesting a concordance between the life zone system and the distribution of the region's diverse understory avifauna. Elevational migrants made up similar proportions of the avifauna in all five zones, whereas the diversity of long-distance migrants was inversely related to elevation. The distribution of species of conservation concern did not track the pattern of overall diversity. Our results provide several lessons for land managers and present a strong case for the need to preserve large areas with elevational diversity in mountainous regions of the tropics. Received 22 September 1997, accepted 2 April 1998. ALTHOUGH ORNITHOLOGISTS have long been aware of the rapid turnover of bird species across elevational gradients in the Neotropics, quantitative studies of fine-scale distributional patterns are available for very few geographic regions (Terborgh and Weske 1975). Such studies allow us to examine the faunistic composition of communities in different elevational habitats, turnover in species composition among habitats (or beta diversity, sensu Whittaker 1972), and to evaluate the conservation status and determine area requirements for conserving montane biodiversity. The ongoing and projected crisis status of tropical forest conservation demands that we gather this information as rapidly as possible (Diamond 1985, Stotz et al. 1996). 4Present address: Latin America and Caribbean Division, The Nature Conservancy, 1815 North Lynn Street, Arlington, Virginia 22209, USA. E-mail: byoung@tnc.org 5Present address: Monteverde Conservation League, Apartado 10581-1000, San Jose, Costa Rica. We compiled data from a series of mist-netting projects to produce estimates of bird diversity in the Monteverde reserve complex, a composite of private reserves in the Tilaran mountains of north-central Costa Rica. The reserve complex has a rich history of ornithological investigation over the last 25 years, but few studies have addressed the distribution of species across an elevational gradient (Young and McDonald 1998). Previous distributional studies of birds have focused on individual species or restricted guilds (Feinsinger 1977; Tramer 1979; Tramer and Kemp 1980, 1982; Stiles and Smith 1980). Treatments of the entire Monteverde avifauna are currently restricted to observation check-lists (Stiles 1983, Fogden 1993). Our goal was to use a compilation of capture data to describe and characterize the diversity and distribution of the understory avifauna between 700 and 1,700 m on both slopes and the crest of the Tilaran range. We analyzed the distributions of the entire understory avifauna and a subset that included only threatened spe-