| Understory insectivorous birds are declining in tropical rainforests globally, for unknown reasons and with unappreciated consequences. In this study I tested two novel hypotheses investigating trophic and non-trophic downgrading involving insectivorous birds: (1) Interaction cascades initiated by collared peccaries (Pecari tajacu) cause declines of understory insectivorous birds, tested using Checker-throated Antwren ( Epinecrophylla fulviventris), Dot-winged Antwren (Microrhopias quixensis), and Ruddy-tailed Flycatcher (Terenotriccus erythrurus ); and (2) insectivorous bird declines cascade to affect arthropods and plants, with compensation by bats. I worked at six field sites in Costa Rica, Nicaragua, and Panama with collared peccary densities ranging from 3--14 / sq km and with experimental mammal exclosures at three sites. Collared peccaries reduced leaf litter depth, vegetation density, liana cover, and forest cover at La Selva Biological Station, Costa Rica (highest peccary density) relative to experimental mammal exclosures, and reduced vine density relative to exclosures at La Selva, Barro Colorado Island and Gigante Peninsula, Panama (moderate peccary density). Percent palm cover decreased with peccary density across six sites. Focal bird species preferentially selected dense liana tangles as foraging microhabitat. Collared peccaries reduced liana tangle frequency relative to experimental exclosures across sites, implicating them in avian declines. Insectivorous birds and bats limited arthropods and herbivory within experimental mammal exclosures, but peccaries reversed the trophic cascade by indirectly increasing predatory arthropod densities. Birds limited herbivory, and birds plus bats limited arthropod densities at Refugio Bartola, with an intact insectivorous bird community. At La Selva, where insectivorous birds had declined, bats limited herbivory and birds actually increased arthropod densities. Bats appear to compensate for insectivorous bird declines at La Selva, but only partially: arthropod densities and herbivory rates were >70% greater at La Selva than Bartola. Novel insights from this research: Foraging microhabitat is critical for avian persistence, native consumers like peccaries can initiate interaction cascades at both reduced and increased densities in rainforest understory, vertebrate consumers disrupt bird-arthropod-plant interaction cascades, and insectivorous bats partially compensate following bird declines. These results highlight the often unpredictable cascading effects that result from rainforest fragmentation, and support the call for an ecological network approach to global change research and conservation. |
