A lot of coffee is grown on and around Kilimanjaro, which towers almost 20,000 feet in the air. 

The most traditional form of cultivation can be found in the gardens of the Chagga people. In some areas, coffee trees and other crop plants still grow in the shade of banana trees but most coffee is grown on plantations, which still feature a large number of shade trees.

However, Africans reliant on coffee for income have gradually replaced legacy coffee varietals, which prefer shade, with others that are more resistant to fungi and can also grow in the sun.

Yet the coffee may not be better overall, says Professor Ingolf Steffan-Dewenter, a tropical ecologist at the University of Würzburg's Biocenter, because without shade trees, the animal species that pollinate the coffee and eat pests may leave, leading to lesser yield. Bees, birds and bats make a huge contribution to the high yields produced by coffee farmers around Mount Kilimanjaro, according to a new paper.

The aim was to find out whether and how intensified farming can affect the ecosystem. Working with the Biodiversity and Climate Research Centre and the Institute of Experimental Ecology at the University of Ulm, the team conducted experiments in twelve areas on the slopes of Mount Kilimanjaro in Tanzania, located in all three cultivation systems (Chagga gardens, shade plantations and sun plantations).

They used finely woven nets to prevent animals’ access to the coffee flowers or even to entire coffee trees. Then they examined how the presence or absence of the “animal service providers” affects the quantity and quality of the harvest.

If birds and bats are prevented by a net  from feeding on pests on coffee trees, this owers the yield. © A. Classen

They found that where birds and bats had access to the plants, there was almost a ten percent higher fruit set. “We believe that this is due to the fact that the animals eliminate pests that would otherwise feed on the coffee plants” says co-author Julia Schmack (BiK-F, Frankfurt). Reduced leaf damage is supposed to reduce the number of coffee cherries falling from the tree while ripening.

The pollination experiments showed that where bees and other insects, which should be redundant since Coffea arabica is self-pollinating, were evident, the cherries were about seven percent heavier, which contributes to a higher coffee quality.

“So, the effects of pollination and pest control complement each other perfectly; both are important for higher yields,” says Steffan-Dewenter: “Birds and bats provide more cherries; bees and other pollinators ensure better quality.”

Same effect with all cultivation systems

To the surprise of the researchers, intensified farming seems to have no negative effect: the impact of the animal provided services on the harvest was equally good in all three cultivation systems, even in the unshaded plantations.

“We put this down to the mosaic landscape structure on Mount Kilimanjaro with its gardens, forests and grasslands,” says doctoral student, Alice Classen.

Given that much of the landscape is divided into small parcels, pollinators, birds and bats still could find a suitable habitat with nesting places, and from there spread into the plantations.

“However, it is likely that these seemingly stable ecosystem services rest on shaky foundations in the sun plantations,” believe the authors, due to the fact that they registered merely one type of visitor, honey bees, to the blossoms. On the coffee blossoms in the Chagga gardens, however, they additionally recorded wild bees, hoverflies and butterflies.

So if honey bee numbers were to decrease, as they might in climatically unfavorable years, this could reduce the harvest in the sun plantations.

Citation: Classen, A, Peters, MK, Ferger, SW, Helbig-Bonitz, M, , Maassen, G, Schleuning, M, Kalko, EKV, Böhning-Gaese, K&I Steffan-Dewenter (2014): Complementary ecosystem services provided by pest predators and pollinators increase quantity and quality of coffee yields.” Proceedings of the Royal Society B, 10.1098/rspb.2013.3148