Density Compensation in Island Faunas

Abstract

This paper analyzes factors determining the extent of density compensation on islands: i.e., is the summed population density of individuals of all species on islands equal to the summed mainland density as a result of niche expansions and higher abundances of island species compensating for the absence of many mainland species? In addition, a method is described for estimating bird population densities based on analysis of the time dependence or mist—netting yields. Puercos Island in the Pearl Archipelago off Panama has less than one—third as many resident birds species as comparable mainland habitats. Analysis of the Pearl avifauna suggests that about one—quarter of the island species may be relicts of the Pleistocene land bridge and that the remainder are subsequent over—water colonists. The successful colonists are a highly nonrandom sample of the mainland avifauna in such respects as family composition, social structure, and second—growth habitat origin. Song—based censuses and analysis of mist—netting show that Puercos has a slightly density of individuals than the mainland. Niche shifts between islands and mainland, or among different islands, include habitat expansions, wider ranges of vertical foraging strata, abundance increases checkerboard distribution patterns, and decreased morphological variability. Comparison of the present study with other studies shows that summed population densities on islands may be higher than, comparable to, or less than mainland levels, depending upon the particular island, habitat, and group of animals studied. Among factors affecting the extent of density colonists less appropriate to the vacated habitat, tending to lower island densities; and underrepresentation of large species on islands, tending to increase island population densities for a given biomass.

Bird assemblage vulnerability depends on the diversity and biogeographic histories of islands

“…An additional consideration is the possibility that species richness may be associated with species population sizes, which would further link diversity with assemblage vulnerability. The character of this relationship is likely system specific, with species on more diverse islands potentially having smaller population sizes; however, in some systems, increases in density can compensate for increased species richness (26). The limited data available for the birds of Northern Melanesia suggest that the relationship between species richness and population sizes may have a limited impact on vulnerability, because total bird density increases linearly with species richness (27).…”

Species richness and relative abundance of birds in natural and anthropogenic fragments of Brazilian Atlantic forest

“…However, species vary in their responses to forest fragmentation; while several species are extinct, others have benefited from forest fragmentation (Warburton 1997). Species that benefit from forest fragmentation tend to increase their relative abundance in the smallest and more isolated forest fragments principally due to (1) the increase in area of their habitats, as for edge species (Laurance et al 1997), E-mail: llanjos@sercomtel.com.br or (2) the release of competition with the extinction of their competitors, which has been termed density compensation (McArthur et al 1972, Wright 1980.…”

Bird–flower visitation networks in the Galápagos unveil a widespread interaction release

“…Indeed, islands have a high proportion of generalist species showing novel and opportunistic interactions 2,6,7 . We term this niche expansion process 'interaction release' (as one aspect of the wider concept 'ecological release' 8 ), that is, island species wire up novel mutualists and prey as a response to mutualist and food shortage, and to reduced predation risk. To study the generality of this phenomenon at the community level, we adopted a network approach [9][10][11] .…”