Birds defend trees from herbivores in a Neotropical forest canopy

Abstract

Most forest birds include arthropods in their diet, sometimes specializing on arthropods that consume plant foliage. Experimental tests of whether bird predation on arthropods can reduce plant damage, however, are few and restricted to relatively low-diversity systems. Here, we describe an experimental test in a diverse tropical forest of whether birds indirectly defend foliage from arthropod herbivores. We also compare how the indirect effects of bird predation vary with different levels of foliage productivity in the canopy vs. the understory. For three Neotropical tree species, we observed that birds decreased local arthropod densities on canopy branches and reduced consequent damage to leaves. In contrast, we observed no evidence of bird-arthropod limitation on conspecific saplings in the less productive understory of the same forest. Our results support theory that predicts trophic cascades where productivity is high and suggest that birds play an important role in Neotropical communities by means of their indirect defense of some canopy tree species.F or decades, ecologists have debated the circumstances under which a predator limits its prey's consumption of organisms in lower trophic levels (i.e., a predator-driven trophic cascade) (1-4). One body of theory predicts such cascades will occur in terrestrial systems with high plant productivity (5, 6). Opposing theory predicts that cascades will not occur in terrestrial systems and only in low-diversity aquatic or marine systems (7). Proponents of the latter theory suggest that higher diversity in terrestrial systems leads to diffuse food webs, rendering trophic levels nonexistent (7). Field experiments demonstrate that insectivorous birds can limit arthropod abundances and decrease damage to plants, but these tests have been conducted in settings with relatively low tree species diversity such as temperate forests (8-11) or agricultural systems (12, 13). Along with high tree species diversity, tropical forests support a high diversity and biomass of leaf-chewing arthropods (14) as well as a high biomass of birds that consume them (15). In a lowland forest of Panama, we used canopy crane access (16) to test the hypotheses that (i) birds limit arthropod densities and consequent herbivore damage, and (ii) the effects of bird predation are strongest where foliage production rates are high.For one year, we observed how the local density and taxonomic composition of the arthropod community responded to the absence of bird predation and also assessed changes in herbivore damage. We estimated and compared these quantities on control branches͞saplings where birds had access to foliage and on branches͞saplings in experimental exclosures where foliage was inaccessible to birds. A within-site comparison of canopy branches and conspecific understory͞edge saplings allowed us to investigate the effects of bird predation across a 3-fold vertical gradient of foliage production. MethodsA canopy crane provided forest canopy access in a dry, semideciduous lowland tropical for...

Edge Responses of Tropical and Temperate Birds

“…The biological control of herbivorous insects by birds may be of value in both anthropogenic and natural forest settings (Tscharnt�e et al 2008). However, although it is clear that insectivorous birds play an important role in controlling outbrea�s of herbivorous insects in agroforests (Perfecto et al 2004), there are fewer studies with comparable results in natural forest stands (Van Bael et al 2003, Se�ercioglu 2006. Generally, insect herbivores inflict high damage in both the canopy and understory of forest stands that lac� avian insectivores (Van Bael et al 2003, Van Bael and Brawn 2005, �unham 2008); this ability of birds to control herbivorous insects complements that of other insectivorous predators (Greenberg et al 2000).…”

“…However, although it is clear that insectivorous birds play an important role in controlling outbrea�s of herbivorous insects in agroforests (Perfecto et al 2004), there are fewer studies with comparable results in natural forest stands (Van Bael et al 2003, Se�ercioglu 2006. Generally, insect herbivores inflict high damage in both the canopy and understory of forest stands that lac� avian insectivores (Van Bael et al 2003, Van Bael and Brawn 2005, �unham 2008); this ability of birds to control herbivorous insects complements that of other insectivorous predators (Greenberg et al 2000). As we discuss in more detail below, understory insectivores and frugivores are predicted to decline with increased disturbance and fragmentation of the tropical forest (Şe�ercioğlu et al 2002); thus, the potential implications of the loss of birds of these two �ey trophic guilds for tropical forest productivity must be carefully examined.…”

Birds foraging for fruits and insects in shrubby restinga vegetation, southeastern Brazil

“…However, foraging behavior of birds may influence their effectiveness as dispersers (Sorensen 1984, Schupp 1993, Witmer & Van Soest 1998. Similarly, insectivorous birds are important components of ecosystems as they may control populations of insect herbivores on certain plant species (Van Ball et al 2003); their preferences for some foraging microhabitats generally determine what species or type of prey are eaten (Wolda 1990).…”

Demonstration of the potential of environmental DNA as a tool for the detection of avian species

“…For example, in forest ecosystems, birds can contribute to maintenance of the tree community by seed dispersal and pollination, and to the reduction of herbivory by predation upon insect herbivores [15][16][17]. However, recent increases in anthropogenic impacts on ecosystems, e.g., urbanization and habitat fragmentation, drive substantial declines in bird species diversity [18,19], which could have impacts on the ecological functions of birds.…”