Birds as Predators in Tropical Agroforestry Systems

Abstract

Abstract. Insectivorous birds reduce arthropod abundances and their damage to plants in some, but not all, studies where predation by birds has been assessed. The variation in bird effects may be due to characteristics such as plant productivity or quality, habitat complexity, and/or species diversity of predator and prey assemblages. Since agroforestry systems vary in such characteristics, these systems provide a good starting point for understanding when and where we can expect predation by birds to be important. We analyze data from bird exclosure studies in forests and agroforestry systems to ask whether birds consistently reduce their arthropod prey base and whether bird predation differs between forests and agroforestry systems. Further, we focus on agroforestry systems to ask whether the magnitude of bird predation (1) differs between canopy trees and understory plants, (2) differs when migratory birds are present or absent, and (3) correlates with bird abundance and diversity. We found that, across all studies, birds reduce all arthropods, herbivores, carnivores, and plant damage. We observed no difference in the magnitude of bird effects between agroforestry systems and forests despite simplified habitat structure and plant diversity in agroforests. Within agroforestry systems, bird reduction of arthropods was greater in the canopy than the crop layer. Top-down effects of bird predation were especially strong during censuses when migratory birds were present in agroforestry systems. Importantly, the diversity of the predator assemblage correlated with the magnitude of predator effects; where the diversity of birds, especially migratory birds, was greater, birds reduced arthropod densities to a greater extent. We outline potential mechanisms for relationships between bird predator, insect prey, and habitat characteristics, and we suggest future studies using tropical agroforests as a model system to further test these areas of ecological theory.

Effects of shade and bird exclusion on arthropods and leaf damage on coffee farms in Jamaica’s Blue Mountains

“…Shade cover was natural-log transformed for analysis, but is depicted on a linear scale in the figure, yielding an asymptotic curve for the regression model. Model curves are shown for two example values of the distance to a non-coffee patch, which was also a significant variable in the model Agroforest Syst (2009) 76:139-148 145 overall arthropods we documented (48-77%) are consistent with other bird exclosure experiments in other temperate (Holmes et al 1979) and tropical systems (Greenberg et al 2000;Van Bael et al 2008).…”

“…The effect of predation was strongest for large arthropods, and for samples taken in summer. A disproportionate effect on large arthropods is expected given birds' preference for Composition did not vary significantly among farms or between summer and fall, so data were pooled for analysis more profitable prey especially in the breeding season (Johnson et al 2005), and is consistent with other bird exclosure experiments (Van Bael et al 2008). The number of birds in Jamaica is much higher in the fall than in the summer due to the arrival of migrant birds from North America (Johnson et al 2005), so our finding that the effect of birds was stronger in summer than fall was unexpected.…”

Birds help plants: a meta-analysis of top-down trophic cascades caused by avian predators

“…This work and some other recent studies (Sekercioglu 2006a, b;Van Bael et al 2008;Whelan et al 2008) have thus shown that birds are beneWcial to plants and form an integral part of ecosystems. In this light, it is worrying that many bird populations are in heavy decline (e.g.…”

“…This is because the separate two-trophic level studies are seldom conducted at the same time or the same place, or with species common to both interactions. A recent review assessed the importance of birds in reducing plant damage mainly in forests and agricultural environments in the tropics (Van Bael et al 2008), and another one documented the topdown cascading eVects of vertebrate insectivores in general (Mooney et al 2010), but as yet there has been no analysis whether bird-driven trophic cascades diVer among climatic areas, including temperate and boreal forests. In addition, plant characteristics which gain beneWts from the presence of birds remain to be explored.…”

Intra and inter-annual variation in seed rain in a secondary dry tropical forest excluded from chronic disturbance

“…Some frugivorous birds feed on insects when they are nesting (Riehl and Adelson, 2008), and since ant density had increased with time of exclusion (ValenzuelaGalván, data not shown) together with the increase in vegetation cover and structure (de la O-Toris, 2009;Alba-García, 2011;Martínez-Garza, 2010), we hypothesize that generalist birds might be attracted to exclosures to forage not only fruits but also ants, therefore increasing seed rain. Further, attraction of insectivorous birds that predate arthropods may have positive effects on vegetation cover (Bael et al, 2008), favouring higher animal visitation to plots. However, further studies are needed to clarify these indirect interactions.…”