This is a guest post from my PhD student Rebecca Peisley, who I am co-supervising with Prof Gary Luck. Rebecca will submit her thesis early next year. She has been working on a really cool project looking at the costs and benefits of bird activity in apple orchards, vineyards and cattle grazing systems across south-eastern Australia; this blog is about her work in apple orchards.
Birds are commonly found in agroecosystems around the world and their foraging activities within crops can result in positive or negative outcomes for producers. For example, birds can help increase saleable yields by preying on insect pests that damage fruit, or removing leftover fruit after harvest, which helps prevent disease and assists in nutrient cycling. However, birds can also contribute to production losses by eating and damaging fruit before harvest, or preying on beneficial insect pollinators.
We cannot then assume that birds are simply ‘good’ or ‘bad’: the same species can in fact be ‘both’. But in our literature review, we showed that most studies of birds in agroecosystems have just considered either costs or benefits separately, which limits our understanding of how birds influence crop yields over spatial and temporal contexts.
In order then to gauge an overall outcome of bird activity, we look at both their beneficial and detrimental activities together in the same crop system and consider the trade-offs that exist between them. For example, the beneficial activity of insectivorous birds preying on pest insects in an apple orchard and reducing insect damage to fruit is traded off against the detrimental activity of the same birds preying on beneficial pollinators resulting in reduced fruit-set.
As part of my PhD research, I am considering how these complex interactions affect crops in south-eastern Australia. In one study recently published in PeerJ, we were the one of the first to consider the overall net outcome of bird activities in apple orchards. We compared the amount of insect damaged apples on branches which birds could access (and therefore predate on insects), with the amount of insect damaged apples on branches where birds were excluded. If birds were helping to control insects, then we would expect less insect damaged apples on the bird accessible branches. However, other frugivorous birds can also consume and damage the apples. Therefore, in this study we asked, what is the trade-off between birds reducing insect damage vs. birds doing damage themselves?
What we found was that the percent of apples that were damaged by insects was more than three times higher when birds could not access branches (18.6% damage) compared to when birds could access branches (5.8% damage). This suggests that, 12.8% less fruit was being insect damaged, thanks to the biological control of insect pests by birds.
We also found that bird damage was actually very low. Only 1.9% of apples had any sort of bird damage.
Therefore, when the costs of bird activity were traded off against the benefits of insect pest control (a 12.8% increase in yield, minus a 1.9% loss due to bird damage), it suggests that birds had an overall positive effect in these apple orchards, contributing to a net increase in saleable yield of 10.9%.
However, no two orchards are exactly the same. Not only are they geographically isolated and each surrounded by different land uses, but they are also subject to different management practices, such as irrigation, chemical spraying and bird scaring. Even within the same orchard conditions vary over time, with differences in rainfall and temperatures between seasons and from year to year. Therefore we cannot expect the activity of the organisms within these variable systems to respond in the same way either.
We found that orchards that were closer to patches of native vegetation, received less chemical sprays, and had less human disturbances had a higher number of insectivorous (beneficial) bird species, and that these orchards also resulted in a higher net benefit (yield). This highlights the importance of recognizing the spatio-temporal, management and ecological differences between orchards and understanding that there is no one-size-fits-all approach to sustainable management. Birds can provide costs or benefits to growers depending on a range of contextual factors including time of season, location, and interactions with other fauna (i.e. invertebrates).
By taking a holistic approach to studying animal activity in crops, we can remove the simplistic ‘good’ and ‘bad’ labels usually placed on species, and instead look at ecosystem function as a whole. This can better inform land managers about implementing strategies which promote the beneficial processes that are essential to the sustainability of agriculture and conservation alike while reducing negative impacts on production.
© Rebecca Peisley 2016