Population Variability and Extinction in the Avifauna of a Tropical Land Bridge Island

Abstract

Newly created oceanic islands slowly accumulate species. In contrast, habitat islands created by rising water level or by human reduction of the area of a habitat (for example, by deforestation) support a full complement of local species at their formation. With time these islands lose species, a phenomenon called relaxation (Diamond 1972, Wilcox 1980). Several explanations may account for relaxation in island relicts. (1) As island size decreases, diversity of habitats present decreases. Species dependent entirely or in part on lost habitats disappear (habitat diversity Able and Connor 1979, Wilcox 1980). (2) Islands passively intercept dispersers in proportion to their area. As island size declines, recolonization probabilities decline. Thus, species lost from a small land bridge island are less likely to recolonize (disperser intercept Able and Connor 1979, Martin 1980, Wilcox 1980). (3) Smaller islands have more species with low sizes, a condition which predisposes such species for extinction from random causes (rarity hypothesis; Willis 1974, Terborgh and Winter 1980). Corollaries of the relate bird size and ecological specialization to extinction probability (Willis 1974). (4) On theoretical grounds, species unable to maintain stable populations in the face of environmental vagaries (species with high population variability) may be prone to local extinction (Leigh 1975, 1981). Terborgh and Winter (1980) suggested from qualitative considerations that species dependent on resources likely to be variable and/or patchy in their distribution (especially frugivores and nectarivores) are prone to extinction. Tracking of those resources in space results in locally variable sizes for such species. To my knowledge, no quantitative field data are available to test the population variability hypothesis. To what extent do these hypotheses account for loss of species on Barro Colorado Island (BCI), Panama, a 14.8-km2 land bridge island formed when the Panama Canal was created by damming the Chagras River early in the 20th century? I have used two approaches to explore that question. First, I have examined in detail the avifauna of BCI and its presumed source fauna in the adjacent mainland forest of the Pipeline Road region of Parque Nacional Soberania. Soberania is a 22 000-ha tract of lowland forest on the mainland adjacent to BCI. At its nearest point, BCI is ;200 m from a peninsular extension of that forest, although less-disturbed forest is not so close. As many as 5060 species of forest birds are missing from BCI (Karr 1982), well above earlier conclusions that 15-18 forest species are extinct on BCI (Terborgh 1974, Willis 1974, Wilson and Willis 1975, Willis and Eisenmann 1979). The approach used in earlier estimates of avian extinctions on BCI led to underestimation of extinction rates. Further, the high extinction rate seems to be due to the restricted habitat mosaic of BCI relative to that found on a similar-sized mainland area. Undergrowth and ground species and species associated with foothill forest are especially prone to extinction (Karr 1982). Second, I have conducted a long-term study of undergrowth species in mainland forest near BCI, with the objective of identifying attributes of species prone to extinction. I report those findings here. Birds of forest undergrowth have been sampled periodically since 1968 with mist nets in an area of 2 km2 along the Pipeline Road (centered at Limbo Hunt Club [LHC]) and 9 km east of BCI. A total of 4722 captures of 114 species of birds is available in samples collected from August 1968 through March 1982. The methodology for this study was operation of 12-14 mist nets in the undergrowth of forest for periods of 3-6 d (see Karr [1979, 1981] for details of methodology). All data were collected by me except for 10 monthly samples (826 captures) from 1977 to 1978 collected by Schemske and Brokaw (1981). Net shyness sometimes occurs in areas that are regularly sampled with mist nets. However, sampling in recent years has been limited to two 3-6 d periods each year, and.there is no sign of net shyness with this low sampling frequency. Indeed, 5070W of captures are of birds banded during earlier sampling periods. In this paper I concentrate on two of the four hypotheses described above: rarity and variability. I do not consider the disperser intercept hypothesis since it is incompatible with the empirical observation that the undergrowth forest birds discussed in detail here are very reluctant to cross water gaps. The habitat diversity hypothesis will be discussed only briefly here as it relates to variability since it is the primary focus of another paper (Karr 1982). I assume that the relative abundances of species today in the vicinity of Limbo Hunt Club are similar to those of the bird community of BCI at the time BCI became an island.