Scientists at the University of York have found that oil palm plantations, which produce oil for commercial use in cooking, food products, and cosmetics, may act as a barrier to the movement of butterflies across tropical landscapes.
The study, conducted in Borneo, Southeast Asia, highlights the importance of maintaining connected networks of rainforest to support and conserve biodiversity.
To make way for oil palm plantations, large areas of lowland rainforests are cut down, which is thought to have an impact on the movement of rainforest species across these landscapes. Until now, however, there was little known about insect movement behaviour at rainforest-plantation boundaries.
The team, in collaboration with the Universiti Malaysia Sabah, examined the ability of butterflies to cross rainforest boundaries in relation to particular characteristics, such as wing size; the number of food plants their caterpillars can feed on; where in the world populations can be found; and the presence or absence of food plants within oil palm plantations.
Sarah Scriven, PhD student and lead author of the research paper, said: "It is essential that rainforest species, such as fruit-feeding butterflies, move through the forest freely in order to search for food sources, suitable areas for breeding, and mates. By breeding with individuals in neighbouring habitats, genetic diversity and stable populations can be maintained.
"In order to understand whether oil palm plantations have any impact on butterfly movement, we carefully numbered the wings of large butterflies, and colour coded smaller butterflies with small dots."
Using a grid system spanning several boundaries of the Borneo rainforest and oil palm plantations, Sarah set up a number of butterfly-friendly banana food traps to see how many of the numbered and colour-coded species moved from the forest into the plantation.
The study showed that small butterflies were more likely to cross from rainforest into oil palm plantations compared to larger butterflies. This was because the plants that caterpillars from small butterflies feed on are often grass, which grows in the plantation habitat. These smaller species can potentially breed in oil palm plantations.
Professor Jane Hill, project supervisor at the University of York, said: "Larger butterflies were over two times less likely than small butterflies to move into oil palm plantations. This is because their larval food sources, largely rainforest herbs, shrubs and trees, do not grow in the plantations.
"Our results, therefore, suggest that oil palm plantations may act as barriers to the movement of forest-dependent butterflies, which highlights the importance of conserving existing forest areas that form corridors linking forest reserves."
The researchers also point out that movement across boundaries will become more important in the context of future climate change. If forests become warmer, certain species will need to move across plantations in search of cooler locations at higher elevations.
Co-author, Dr Colin Beale, also from the Department of Biology at York, said: "Oil palm provides a valuable crop to many farmers in the tropics, but conversion of rainforest to oil palm plantations results in a dramatic change in habitat structure, making plantation habitats unsuitable for many rainforest species.
"Our study adds to the growing understanding that leaving a connected network of forest is the best way to allow forest species to move around the landscape."
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Sarah A. Scriven et al, Barriers to dispersal of rain forest butterflies in tropical agricultural landscapes, Biotropica (2016). DOI: 10.1111/btp.12397