Few corners of the world see the tension between farming and conservation run as high as it does on Sulawesi. Each year, close to two percent of the island's forest is handed over to agriculture, and most of that loss traces back to one crop: rice, the mainstay of the local diet. The sequence is simple — swarms of insect pests spoil so much of the harvest that subsistence farmers, needing to make up the shortfall, clear still more forest for fresh paddy. Habitats disappear and biodiversity drains away as a result, and yet not every species is losing out. A few of the island's insect-eating bats are actually thriving as never before, which matters greatly to farmers and conservationists alike.
This is where Tomás Almeida enters the story. A zoologist at the University of Coimbra, he is devoted to conservation and to bats, and above all to how bats respond to what people do to the land, deforestation especially. His latest study reveals that a whole set of bat species are quietly doing an essential job for Sulawesi's rice growers, feasting on the very insect swarms that would otherwise strip the fields bare. That service, Almeida contends, might lift enough of the financial burden from farmers to make turning forest into fields far less tempting.
Roughly a quarter of every mammal species on Sulawesi is a bat, and more than fifty kinds are found on the island and nowhere else — a count that places it among the most important spots on Earth for keeping this group of animals safe.
Working at the head of an international team, Almeida found that several of the island's own bats have turned the disturbance of their habitat to good use, picking off the insect clouds that hang over the rice. Among them are the Minahasa mouse-eared bat, Reinwardt's long-fingered bat, the lesser sheath-tailed bat and the Celebes free-tailed bat.
'These "winners" are doing Sulawesi a valuable turn at no cost, holding pest numbers in check,' Almeida says. 'Six species of bat, we found, hunt the insects that attack rice, the paddy armyworm and the rice leaf-roller among them. What those pests destroy leaves the island's farmers under intense financial strain, and it is that strain which pushes deforestation along.'
In the journal Farming, Ecosystems and Biodiversity, where the project has now appeared, the team examined how insect-eating bats feed on the belt of farmland fringing Lore Piru National Park, towards the island's south-east.
To work out where the bats preferred to feed, Almeida's team reached for state-of-the-art ultrasonic detectors, capturing more than nine hundred bat 'feeding buzzes' — the quick bursts of echolocation a bat gives off as it closes in on prey — at forty-eight different locations. The guano the animals left behind at each spot was then run through DNA barcoding.
The recordings put the matter beyond doubt: the bats foraged far more over the rice than inside unbroken forest — five times as much above fields on level ground, and fourteen times as much above those on hillsides — plainly favouring these landscapes of human making. Poor drainage and lost nutrients, the team suggests, leave such crops more open to infestation, and where the pests gather the bats follow. The DNA confirmed as much: each of the six species had been feeding on pests of real economic weight. Rice gains most of all, the study found, but the bats were also taking the pests of other crops — the twig-boring beetle that ruins cocoa, the cane leafhopper, the almond seed-wasp and the citrus peel-miner that spoils fruit.
'Elsewhere — in the United States and in northern Italy — the value of bats for keeping pests down has been shown already,' noted Sofia Marques of the University of Porto, who co-wrote the paper. 'On Sulawesi, though, ours is the first study to document it, and here the stakes for farming and conservation are unusually high.'
The islanders may owe their bats on yet another count. Blamed though the animal often is for carrying sickness, the evidence Almeida's team gathered points the other way: Sulawesi's bats consume not only the pests of crops but mosquitoes too — behind malaria, dengue and filariasis — along with the blackflies responsible for river blindness.
The relationship, Almeida stresses, is a complicated one. In times of scarcity, bats become a vital source of protein for local people, and even the children will go after them. Besides roosting in trees, the bats will sometimes settle inside buildings, where they are unwelcome, since the rooms they take over are thought to be left polluted. And yet these same animals are bound up with sacred caves and with the ancestors — beings caught between two worlds — which earns them a weighty place in local folklore. There is a catch, though: even as the bats gain from farming, the deforestation they help to slow is destroying the sites where they roost, which may tell on their numbers over time. 'With a little help,' Almeida says, 'farmers could encourage this give-and-take by putting up bat boxes.'
Higher bat numbers, Almeida and his colleagues are convinced, could lift the size of harvests and help make rural livelihoods sustainable; what is needed now, they say, is further work to measure just how big that benefit is. 'I'm very hopeful,' he adds. 'Offer nature a hand and its recovery only speeds up.'
