Tropical Wet Forest Treefall Gaps and Distributions of Understory Birds and Plants

Abstract

I examined the patch dynamics of understory birds and fruiting plants in treefall gaps and intact forest sites to determine the importance of gaps in structuring a Costa Rican tropical wet forest community. Understory birds and fruiting plants were censused monthly for 12 mo in 13 gaps and 13 intact forest sites. Of species with adequate sample sizes, 40% of bird species (17 of 42) and 30% of fruiting plant species (10 of 33) were found significantly more often in gaps than in intact forest. In contrast, only 5% of the bird species (2 of 42) and no plant species were found significantly more often in intact forest sites. Frugivorous and nectarivorous bird species were especially common in gaps. Birds of second—growth and forest edge were proportionately more abundant in gaps than were birds of forest interior. Canopy birds were not prevalent in gaps. Plants in gaps tended to produce more fruit over a longer period of time than conspecifics fruiting under a closed canopy. Also, large gaps had higher densities of fruiting plants and fruit—eating birds than small gaps. The high diversity and long fruiting periods of plants in gaps suggest that gaps may be important sources of fruit during periods of fruit scarcity. Given the high density of fruit—eating birds in gaps, seed deposition patterns generated by these birds are probably nonrandom. In particular, seeds may be dispersed into or around the periphery of gaps more often than into closed understory sites. Thus, the distributional patterns of understory fruits and frugivores may reinforce each other; fruiting shrub density affects the distribution of fruit—eating birds, which then influences the distribution of fruiting plants and birds in the next generation.