FOREST BIRD COMMUNITY STRUCTURE IN CENTRAL PANAMA: INFLUENCE OF SPATIAL SCALE AND BIOGEOGRAPHY

Abstract

Historical and biogeographic contexts can play important, yet sometimes overlooked, roles in determining structure of local communities. In particular, few examinations of historical influences on patterns of species richness and relative abundances in tropical communities have been conducted. In part, that gap in our knowledge has been caused by a paucity of data on tropical communities, even for relatively well-studied taxa such as birds. In the Neotropics, only two sites, a 97-ha plot in lowland Peru and a 100-ha plot in French Guiana, have been inventoried on a spatial scale sufficient to estimate population densities for a majority of resident bird species. Results from those studies revealed extremely similar species richness, community biomass, and patterns of relative abundance. A third site in lowland Panama was originally censused in 1968–1969 and has often been compared with many other tropical and temperate sites. Results from Panama suggested an exceptionally different community structure from that observed at the Amazonian sites. Informative comparisons among sites have been hampered, however, by differences in sampling protocols. The Panama site was sampled on a much smaller spatial scale (2 ha) than the two Amazonian sites. To improve comparisons, we censused a 104-ha area (the Limbo plot) encompassing the original 2-ha Panama study area and used several census methods, including those used at the Amazonian sites. As expected, spatial scale had a strong effect on estimates of species richness. We detected 252 species on the Limbo plot, compared with 161 detected on the original 2-ha area. Estimates of total individual birds per 100 ha were similar, but estimates from the original study were based on densities measured for one-third fewer species than we measured on our larger study area. Of the 53 species for which both Panama studies estimated population densities, a significant number of estimates were higher in the original study. Thus, the small spatial scale of the original study apparently led to inflated density estimates. The primary cause of disparities appeared to result from undersampling in the smaller plot of many species with patchy distributions and large territory sizes. Compared with Amazonian communities, the Panama community had far fewer rare species. Although 33% of species in Amazonian sites had densities of ≤1 pair/100 ha, only 17% were equally rare in Panama. Furthermore, eight species in Panama were, by tropical standards, “superabundant,” attaining densities as high as 212 breeding individuals/100 ha; the most abundant species in Amazonia barely reached one-third of that number. In total, those eight species accounted for 36% of all individuals at Limbo. The median abundance at Limbo was 7 pairs/100 ha, vs. 2.5 pairs/100 ha in Amazonia. Consequently, the total number of birds on the Limbo study area was nearly twice that found in Amazonia, despite species richness being only three-fourths as great. We conclude, first, that spatial scale has indeed had an important effect on the characterization of the Panama bird community. The intrinsically patchy distributions of most forest-dwelling bird species raise the need for large-scale censuses. Second, the Panama community, compared with the two Amazonian sites, has a fundamentally different organization; it hosts nearly twice as many individual birds and is distinctly less dominated by rarity. Similar patterns of community structure appear to be present within tree and mammal communities as well. Therefore, results from the Amazonian studies cannot be generalized to all lowland Neotropical communities. We attribute differences in community structure primarily to differing biogeographic histories. The lower species richness and the greater number of total birds present in Panama appear to derive, at least in part, from two important factors: an area effect linked to the location of Panama on a narrow isthmus, and the repeated history of disturbance on multiple temporal scales in Panama.