Density Anomalies and Regulatory Mechanisms in Land Bird Populations on the Florida Peninsula

Abstract

Pineland bird populations censused at 13 sites in Florida [USA] and on nearby Bahama Islands showed declining density trends southward along the peninsula and a steep rise in the Bahamas. These distributional trends in population density were apparently not paralleled by comparable trends in food supply. In one foraging guild, the pine foliage gleaners, the ratio of foraging biomass to food supply was approximately 4 times as great in the Bahamas as in southern Florida. On the basis that the Florida-Bahama differences in population density in at least the pine foliage insectivore guild cannot be attributed to differences in food levels, it is proposed that the low breeding densities in southern Florida are related to the peripheral position of the area in the geographic ranges of most of the species, and that densities are suppressed in these peripheral zones by the persistent swamping of incipient adaptive mutations there by gene flow from the range centers to the north where populations are adapted to different environmental conditions. This hypothesis is supported by evidence in the literature that undifferentiated species (no subspeciation recognized by taxonomists and hence, presumably, no major barriers to gene flow) have been less successful in maintaining numbers in south Florida than differentiated species. The relatively low densities in northern Florida vis-a-vis the Bahamas are attributed to reduced mean adaptedness in the region, stemming from gene flow between habitat patches in the mosaic of habitat types that characterizes the northern Florida vegetation. The overall density gradients in the winter resident community, roughly paralleling those of the permanent resident (breeding) community, again reflect the position of geographic ranges and range centers in the area. Declining densities toward the range boundaries in these species are attributed to the effect of natural selection in persistently focusing annual migration orientation on core areas of optimal habitat and to the low probability that mutant individuals, potentially adapted to conditions at some peripheral area, will ever reach that area in the face of counterselection in migration orientation. The definitive guild approach can provide access to consumer-resource ratios in natural communities and that comparative studies using this approach may reveal that consumer densities are often maintained at levels well below the resource-determined carrying capacity of the habitat.