Study: Tall trees and shade boost bat diversity on Africa’s cocoa farms

Insect-eating bats that prey on pests in cocoa farms prefer farms that retain large, old-growth trees that shade the plantations. Researchers aiming to find a “sweet spot” in agroforestry systems that supports maximum biodiversity surveyed bat diversity on 28 cocoa farms in south central Cameroon. They found the greatest abundance and diversity of bats on farms with 65% or higher shade cover — mimicking natural forest conditions in this region.

“What our study found is that to maintain insectivorous bats, we not only need shade cover but we need that to be provided by very big, old-growth trees,” Diogo Ferreira, a lead author on the paper, published in the journal Biological Conservation, told Mongabay in an interview.

Bats that depend on fruit and nectar, however, prefer high-shade conditions from planted trees, which often provide cocoa farmers with a source of secondary income. Common species on the surveyed farms included avocado, mango, orange and lemon trees, and safou, sometimes known as African plum (Dacryodes edulis), said Ferreira, a Ph.D. student at the Centre for Research in Biodiversity and Genetic Resources (known by its Portuguese acronym, CIBIO).

The researchers surveyed bat diversity on farms with a variety of shade tree cover; they consisted of high shade with more than 65% cover, medium cover between 35 and 65%, and “low shade cocoa” between 20 and 35% cover.

“We propose that if we want to keep both of the bat guilds, we need higher levels of shade cover,” Ferreira said.

For Olivier Honnay, a conservation ecologist at Belgian university KU Leuven, who was not involved in the study, the overall findings confirm that high-shade conditions benefit insectivorous bats in the same way as it does with other species, such as ants, birds and frogs.

“This study is a valuable addition to the existing literature, also because (as far as I know) there have been no studies that have dealt with bat diversity in cocoa agroforestry systems in the Afrotropics,” he wrote in an email.

Finding the shade ‘sweet spot’

“If you could manage farms to get them as close as possible to 65% shade without sacrificing much cocoa yield, you’d nail the appropriate shade level for a biodiversity- or bat-friendly certification,” said Luke Powell, a co-author of the paper and principal researcher with CIBIO’s TROPIBIO program and the University of Glasgow.

Honnay said he agrees that high-shade cocoa agroforestry can contribute to bat conservation, but cautioned against firm conclusions in the absence of baseline comparisons between such farms and natural forest systems. He also added a note of caution over promoting a 65% shade guideline. “This is certainly a very important and robust conservation guideline. However, the key question in this agroforestry system is: what is the effect of such a high shade on cocoa yields and farmer income?”

Past research suggests cocoa yields drop off after 30% shade cover is reached, but the researchers say the presence of bats can bring a host of ecosystem services that can also benefit farmers. In a separate study in Cameroon, Ferreira and his team found birds and bats can help enhance yields by consuming large amounts of pests that would otherwise require pesticides to control, but only when shade cover is high.

However, ecosystem services rarely fully compensate for shortfalls in income, Honnay said. “The best way forward would be to offer farmers a price premium when they maintain high shade levels and big old trees, for example through a certification program,” he said.

It remains to be confirmed which tree species encourage greater bat abundance. An additional aim of the ongoing research is to identify which trees act as “keystones,” enabling a higher diversity of bats on farms. The researchers say further study is needed, but the kapok or cotton tree (Ceiba pentandra var. guineensis) and African oil-nut tree (Ricinodendron heudelotii) may play important roles for bats.

“In future studies, we want to investigate all these different species of shade trees that are associated with the different species of bats,” Ferreira said. Separate research by the same group is investigating which species of birds and bats are the most voracious consumers of cocoa pests by analyzing DNA found in their feces and matching this to bugs, Powell said.

The hope is that by bringing their recent shade findings together with these lines of research, it will be possible to optimize farms with native tree types that provide both shade and homes for those bat and bird species, which in turn are the most effective consumers of cocoa pests.

“If you know that the forest robin, for example, is eating tons of the main cocoa pest — the brown capsid — you can then understand it needs tree species X, Y and Z to build its nest,” Powell said. “Then you’re sort of unlocking that part of the puzzle.”

Bats on the farm beyond Cameroon

The researchers suggest their findings may be relevant for other cocoa-growing countries in Central and West Africa, such as Ghana and Côte d’Ivoire . These two countries produce the vast bulk of the world’s cocoa — at the cost of great swaths of deforestation that continue to expand in some areas. Agroforestry is considered a key approach to slowing and even reversing forest loss linked to cocoa.

“In Cameroon there are still a lot of these high-shade farms, but if you go to Ghana or Ivory Coast we see there is more low-shade cocoa,” Ferreira said. “The same criteria could be applied but we need to let the forest regenerate in these farms.”

In terms of wider bat conservation, Evans Ewald Nkrumah, a researcher at Ghana’s Kwame Nkrumah University of Science and Technology, who was not involved in the study, said cocoa agroforestry could be beneficial, but added that factors other than shade levels and tree type may be equally important to biodiversity on farms.

“Some shaded cocoa farms may have no understory vegetation but high leaf litter cover. In farms like this, some species are likely to use these farms as mere commuting routes to access richer potions of the landscape.” He also pointed to availability of food or roosts and proximity to natural primary forest as other factors affecting a cocoa farm’s suitability as habitat for wildlife.

Social factors, too, could play a role in promoting or suppressing bat biodiversity: “If [farmers] believe the bats are pests, for example, they will eradicate their roost or at the extreme even persecute them for perceived threats,” Nkrumah wrote. Such negative perceptions are a threat to bat species across the globe. Taken together, these environmental and social factors could ultimately influence the species’ abundance on cocoa farms in other areas too.

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Citations:

Ferreira, D. F., Darling, A., Jarrett, C., Atagana, P. J., Sandjo, P. R., Taedoumg, H., … Powell, L. L. (2023). Not all farms are created equal: Shady African cocoa farms promote a richer bat fauna. Biological Conservation, 284, 110191. doi:10.1016/j.biocon.2023.110191

Blaser, W. J., Oppong, J., Hart, S. P., Landolt, J., Yeboah, E., & Six, J. (2018). Climate-smart sustainable agriculture in low-to-intermediate shade agroforests. Nature Sustainability, 1(5), 234-239. doi:10.1038/s41893-018-0062-8

Ferreira, D. F., Jarrett, C., Wandji, A. C., Atagana, P. J., Rebelo, H., Maas, B., & Powell, L. L. (2023). Birds and bats enhance yields in Afrotropical cacao agroforests only under high tree-level shade cover. Agriculture, Ecosystems & Environment, 345, 108325. doi:10.1016/j.agee.2022.108325

Tuneu‐Corral, C., Puig‐Montserrat, X., Riba‐Bertolín, D., Russo, D., Rebelo, H., Cabeza, M., & López‐Baucells, A. (2023). Pest suppression by bats and management strategies to favour it: A global review. Biological Reviews. doi:10.1111/brv.12967

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