Bats and birds increase crop yield in tropical agroforestry landscapes

Abstract

Human welfare is significantly linked to ecosystem services such as the suppression of pest insects by birds and bats. However, effects of biocontrol services on tropical cash crop yield are still largely unknown. For the first time, we manipulated the access of birds and bats in an exclosure experiment (day, night and full exclosures compared to open controls in Indonesian cacao agroforestry) and quantified the arthropod communities, the fruit development and the final yield over a long time period (15 months). We found that bat and bird exclusion increased insect herbivore abundance, despite the concurrent release of mesopredators such as ants and spiders, and negatively affected fruit development, with final crop yield decreasing by 31% across local (shade cover) and landscape (distance to primary forest) gradients. Our results highlight the tremendous economic impact of common insectivorous birds and bats, which need to become an essential part of sustainable landscape management.

Bats in the Anthropogenic Matrix: Challenges and Opportunities for the Conservation of Chiroptera and Their Ecosystem Services in Agricultural Landscapes

“…However, no effects on plant damage were observed in that study, perhaps as a result of the short duration of the study or release of spiders and other arthropod predators. In Indonesian shade cacao, excluding bats resulted in a 29 % increase in arthropod numbers (Maas et al 2013). Although herbivory did not differ significantly between cacao plantations with different levels of shade or proximities to primary habitats within the landscape, exclosure of bats resulted in a significant decrease in yields, with the effects of bird and bat predation together valued at an astonishing US $730 per ha and year (bat predation was valued at US $520 per ha and year).…”

“…Several studies have considered the effects of agricultural management at landscape scales versus focusing exclusively on farm-level management practices (Estrada et al 1993;Ekman and de Jong 1996;Verboom and Huitema 1997;Numa et al 2005;Faria et al 2006Faria and Baumgarten 2007;FuentesMontemayor et al 2011;Boughey et al 2011;Maas et al 2013). Within agricultural areas, bat activity increases with proximity to natural areas (Estrada et al 1993;Verboom and Huitema 1997;Boughey et al 2011) and in less fragmented landscapes (Fuentes-Montemayor et al 2011;Frey-Ehrenbold et al 2013) or in landscapes with more natural elements such as hedgerows and woodlots (Verboom and Huitema 1997).…”

“…Insectivorous bats have a global distribution and have long been identified as key suppressors of arthropod pests in agricultural systems ). However, surprisingly little evidence exists quantifying the impact of their predation on arthropod populations, plant damage, or its economic value (Boyles et al 2013;Maas et al 2013). Several studies have characterized diets of insectivorous bats (reviewed by Kunz et al 2011), and the recent development of DNA-based methods for dietary analysis provides an unprecedented amount of detail on the composition of bat diets and allows for the identification of individual pest species.…”

“…However, bats capable of gleaning insect prey from substrates exist throughout the world, and their impacts could be monitored via exclosure studies and disentangled from those of birds. This approach has been used fruitfully in the past five years, demonstrating significant increases in arthropod density when bats are absent, in agroecosystems (Williams- Guillén et al 2008;Maas et al 2013), reforestation (Morrison and Lindell 2012), and natural forests (Kalka et al 2008). In Mexican polycultural shade coffee, arthropod densities on coffee plants during the rainy season nearly doubled in the absence of bats, with marked increases in densities of hoppers, katydids, cockroaches, and beetles (Williams- Guillén et al 2008).…”

“…For example, bats in Mexican shade coffee have substantial effects on herbivorous insects (Williams- Guillén et al 2008), while bats in Costa Rican shade coffee had no significant effect on herbivores (Karp et al 2013). In Indonesian cacao agroforestry systems, insectivorous bats strongly contribute to the suppression of many different pest insect groups and crop yield productivity across gradients of local shadetree management and forest proximity within the agricultural landscape (Maas et al 2013). In general, the study sites differ in landscape structure and land use, local farm history and management, habitat dynamics and conversion, intensity of farming practices, and vertebrate insectivore assemblage structure.…”