Ecosystem services and disservices by birds, bats and monkeys change with macadamia landscape heterogeneity
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The relative importance of ecosystem services and disservices can change with landscape structure in a poorly understood way. We compare the impact of biocontrol, provided by bats and birds, with that of crop raiding by vervet monkeys on yield in South African macadamia orchards. Insectivorous bats and birds are known to feed on macadamia pest insect species, like the macadamia nut borer or the green vegetable bug. Vervet monkeys move into the orchards during the day to feed on premature macadamia nuts. Bats, birds and monkeys benefit from patches of natural vegetation adjacent to orchards. With exclusion experiments (four treatments: day, night, day and night, control) we quantified the relative importance of biocontrol and crop raiding on yield, comparing two different landscape settings of the orchards, a natural and a human-modified. Crop raiding occurred only close to natural vegetation and caused yield losses of about 26%. Biocontrol by bats and birds was higher near natural vegetation, but still significant in human-modified landscapes, at up to 530 m distance to forest patches. Prevented biocontrol through the exclusion of bats and birds resulted in yield losses of up to 60%. Effects of biocontrol by bats and birds (USD ~5,000 ha/year) were economically more important than the losses of crop raiding (USD ~1,600 ha/year). As both are linked to the vicinity of forest patches, the removal of natural vegetation to limit monkey abundances would also limit biocontrol service provision. Synthesis and applications. This study highlights the high economic benefits of biocontrol by bats and birds, which outweighed negative impacts through yield losses caused by crop raiding monkeys. Management practices to prevent crop damage, such as guarding, excluding vertebrates or removal of adjacent natural vegetation, would also limit access for bats and birds and the great economic benefits provided by their biocontrol. Ecosystem services by bats and birds can be promoted by the exposure of artificial roost and nest sites, but research into species-specific preferences is needed. The education of farmers is crucial, as many are unaware of the benefits from birds and bats and the fact that these benefits can outweigh the disadvantages of the monkeys' crop raiding.
- RESEARCH: Taylor, P