As dusk falls in Old World forests, flying foxes leave their roosts for a night of foraging for fruits, flowers and the occasional leaf. A familiar sound in these forests is the noisy squabbling of flying foxes gathered in trees with particularly desirable fruit.
Our research demonstrates that these nightly skirmishes over fruit supplies are a critical part of the flying foxes’ role in forest regeneration. When peace reigns – typically because of reduced populations – few seeds are dispersed, despite the presence of fruit-eating flying foxes.
Tropical forests are home to many trees and vines that rely on animals to disperse their seeds, vastly improving germination. Without dispersal, the seeds fall directly from the tree to the forest floor, exactly where seed-eating animals know to look for them. Seeds that survive long enough to germinate face stiff competition from countless other seedlings and understory plants crowded beneath the tree’s canopy.
Clearly, animals that eat fruit and carry the seeds some distance away from the tree, either before consuming the fruit or by swallowing seeds along with the pulp, can sharply increase a plant’s chance of regenerating.
In Western Polynesia, flying foxes are often considered “keystone” seed dispersers: If they were to disappear or for some reason stop dispersing seeds, the forests could be severely altered.
Around 3,000 years ago, when humans first arrived in Western Polynesia, enormous pigeons helped disperse seeds. But these pigeons, which had never before seen humans, or the dogs and rats that arrived with them, were quickly hunted to extinction. That left only flying foxes, smaller pigeons, doves and a few other small birds to meet the forests’ seed-dispersal needs.
Plant species with large seeds were, of course, most affected by the pigeons’ demise, since they became completely dependent on flying foxes for dispersal. In the Western Polynesian country of Tonga, flying foxes are responsible for dispersing almost all seeds with diameters of more than about one inch (28 millimeters). The only other way the seeds of a large, succulent fruit can be dispersed in Tonga today is by a neglectful rat that removes the fruit to eat the seed and then forgets about it.
The preferred food of flying foxes is the juice they extract from fruit pulp. Rather than swallowing fruits whole, flying foxes squeeze out the juices by pressing the pulp between their tongue and upper palate. After swallowing the juice and small seeds, the larger seeds and fibers are spat out or dropped. So to be good seed dispersers, flying foxes must remove the fruit from the tree and fly to another location to process it. This is where the squabbling becomes important.
In Australia, Greg Richards (a member of BCI’s Scientific Advisory Board) watched flying foxes feeding in a mango tree. He noted that when relatively small numbers of flying foxes were feeding together, the bats remained in the tree and dropped all the seeds directly beneath it.
Seed dispersal began only when the tree became full of flying foxes, each defending its own small feeding territory. At that point, a new flying fox could acquire fruit only by “raiding” the tree: flying in, snatching a fruit and promptly departing to eat its prize in a less-contentious environment some distance away. Richards suggested that the flying foxes’ seed-dispersal role may be heavily dependent on gatherings of sufficient numbers to require such raiding behavior.
We investigated this link between flying fox abundance and seed dispersal in the Vava‘u Islands of Tonga. The Tongan flying fox (Pteropus tonganus) is the country’s only remaining species of flying fox; another species went extinct after the arrival of humans. Flying fox population density fluctuates around the Vava‘u archipelago, since the bats regularly travel among the islands and vary their nightly feeding sites.
We watched flying foxes travel between islands on a nightly basis. Research by Sandra Banack in American Samoa indicates these flying foxes can carry large fruit over sufficient distances to disperse seeds among various islands. For two years (1999-2001), we visited several islands that retained some native forest cover and measured both flying fox abundance and the proportion of seeds dispersed from large-seeded trees.
Our results show a very clear link between flying fox abundance and the frequency with which they disperse seeds. More importantly, we found that if flying fox numbers dropped below a certain threshold, virtually no seed dispersal occurred. Although flying foxes were still present on the island and still eating fruits, the trees never became so full of flying foxes that they began removing fruit to eat elsewhere.
For some plant species, flying foxes are responsible for more than 90 percent of seed dispersal, with the balance dispersed by Pacific pigeons, crabs and introduced rats. Flying foxes are also important pollinators. If they show similarly aggressive behavior in flowering trees, their role in this vital aspect of plant reproduction may also be linked to abundance – and face a threshold where cross-pollination between trees virtually stops.
Flying foxes are usually rather abundant in Vava‘u and the rest of Tonga (except after the occasional cyclone), but flying fox numbers have been drastically reduced on some other Pacific islands. In Guam, for example, only one small population of Marianas flying fox (Pteropus mariannus) survives. Some Pacific flying fox populations likely are already too low for the animals to be functioning as seed dispersers, even though flying foxes are present. This could have important consequences for the forests, which often are already threatened, as the composition of plant species shifts in favor of small-seeded plants that can be dispersed by the remaining birds.
Squabbling flying foxes are not confined to the Pacific region. They are found in parts of Africa, through Asia to Australia and the tropical Pacific. Similar feeding behavior has been reported for flying foxes in Malaysia and the Indian Ocean and may be present in many species, including several that are threatened or inhabit diminishing forests.
The long-term conservation of functional rain forests requires not only enough flying foxes to maintain their own populations, there must also be enough of them to ensure the reproduction of the trees they depend on for food. Their essential role in maintaining healthy forests may end long before conservationists realize the problem is upon them. Prompt action to protect declining populations of flying foxes is essential to ensuring the longterm health of tropical forests.
KIM R. MCCONKEY studies frugivory and seed dispersal in Asia and Polynesia, and currently lives in India. DONALD R. DRAKE is an Associate Professor in the Botany Department at the University of Hawaii at Manoa.This research was funded by the Wildlife Conservation Society.