Print PageTropical rainforest understory insectivorous birds are highly vulnerable to forest fragmentation, with declines often attributed to direct effects of decreased patch size, notably food limitation given their highly specialized dietary habitats. Yet this guild has nearly disappeared from larger fragments, e.g., 1600ha La Selva Biological Station, while persisting at smaller nearby reserves, e.g., 354ha Reserva Tirimbina. Fragmentation can alter remnant mammal communities dramatically, which in turn can have dramatic consequences on plant, arthropod, and bird communities. La Selva has experienced a dramatic increase in the abundance of collared peccaries (Pecari tajacu), a mid-sized herbivore, since the 1970s, concurrent with understory insectivorous bird declines. Our research supports an indirect-effects hypothesis with three components: (1) Collared peccaries are locally suppressing liana growth and regeneration, (2) Understory insectivorous birds forage selectively in dense liana tangles, and (3) Understory insectivore declines correspond with loss of preferred liana tangles. Research was conducted at seven sites spanning a collared peccary density gradient from 0 - >14.3 peccaries/sq. km.: La Selva and Tirimbina (Costa Rica); Refugio Bartola (Nicaragua); and Barro Colorado Island, Limbo Hunt Club, Gigante Peninsula, and Carretera 25 (Panama).
Results/Conclusions
Across all sites, vine, liana, and aerial leaf litter density and cover were inversely correlated with collared peccary density. The avian focal species, Checker-throated Antwren (Epinecrophylla fulviventris), Dot-winged Antwren (Microrhopias quixensis), and Ruddy-tailed Flycatcher (Terenotriccus erythrurus), all selected foraging sites with high liana density and cover. Liana tangles are less frequent at sites where understory insectivores have declined, including La Selva which retains only 0-30% of these species’ preferred microhabitats. Direct food-limitation is unlikely as arthropod abundance was significantly greater at La Selva, where the birds have declined, than Bartola, with an intact avian community. This is the first study to our knowledge to show that understory insectivorous birds are vulnerable to loss of foraging microhabitats. This research highlights the important role of indirect effects in rendering even large forest tracts unsuitable for remnant species. Importantly, these findings suggest that reserves are not all equal: setting aside land alone – without intact mammal communities or management to preserve key components of vegetation structure – is insufficient to preserve some fragmentation-sensitive species.
