Volume 31, Issue 2, Summer 2013

Flying Foxes Rebuild Madagascar’s Tattered Forests

Do seeds do better when they pass through bats?

By Ryszard Oleksy

The island of Madagascar is losing its forests at a perilous pace, as vast areas are cleared for timber, charcoal production and agriculture. The loss of these natural ecosystems threatens not only the nation’s remarkably diverse and often unique wildlife, but also the well-being of its human residents, who depend on the resources of their forests. Little is being done officially to reverse this trend.

Yet nature provides a powerful tool for helping to regenerate fragmented forests. Fruit bats, which feed on the fruits of many plants, as well as the leaves, pollen and nectar, scatter the seeds over wide areas, notably including cleared lands. At least 300 plant species are known to rely on Old World fruit bats for seed dispersal.

The Madagascan flying fox (Pteropus rufus) is potentially a critical seed disperser in Madagascar, an island of some 228,000 square miles (590,000 square kilometers) off southeastern Africa. But this important species, found only on Madagascar, is listed as Vulnerable by IUCN because of sharply declining populations. The flying foxes have virtually no legal protection and are, in fact, listed as a game animal under Malagasy law. They are hunted and persecuted throughout the island.

Reliable information about the Madagascan flying fox and its contributions to restoring damaged forests should, I hope, convince local people and government officials to protect these beleaguered bats.

After two years of research, with support from BCI Student Research Scholarships, my results with bats carrying GPS tags demonstrate the importance of these bats as long-range seed dispersers. And a series of experiments designed to simulate natural conditions finds that bat-processed seeds – those that are eaten by bats, pass through the gut and are dispersed in droppings – are significantly more likely to germinate than seeds that come directly from fruit.

More details will emerge as I continue analyzing data to complete my Ph.D. research. The study is being conducted in collaboration with Berenty Private Reserve, where a large Madagascan flying fox roost is found in the remaining gallery forest.

A few previous studies suggest that passage through a bat’s gut may improve seeds’ germination (sprouting) rates, but clear conclusions have been elusive. I tested this hypothesis by experimentally comparing the germination success of seeds taken from bat feces with those from ripe fruit, under progressively more natural conditions.

We tested a total of 2,400 seeds of fig trees (Ficus grevei) that are found only on Madagascar. The seeds were distributed in 120 Petri dishes, 20 seeds per dish, under three different conditions. Both seeds extracted from bat droppings and unprocessed seeds taken from fresh fruits were grown on plain filter paper; on soil that was treated to remove any organisms; and on untreated soil. The dishes were arranged randomly on a shaded, outdoor table and watered on the same schedule.

By far the most dramatic effect was among seeds grown on paper, without a soil substrate. Among bat-processed seeds, an average of 11.7 seeds (out of 20) germinated in each Petri dish. Seeds extracted from ripe fruits showed a germinate average of only 3.3 per dish. Statistical analysis showed this to be a highly significant impact.

This difference might be caused by small amounts of feces that likely remain on the processed seeds and act as fertilizer to promote germination. Considering that bats defecate in flight and scatter seeds not only on the ground but also on vegetation, bits of manure around the seeds may very well increase the chances for germination and seedling establishment.

We also sterilized some seeds with an ethanol solution to prevent fungal or bacterial infection. Bat-processed seeds had slightly less germination success when sterilized, while unprocessed seeds showed a sharp increase, indicating that processed and unprocessed seeds respond differently to sterilization. The reason for this response is not yet clear, but it indicates that bat-processed seeds undergo substantial changes while passing through a bat’s gut.

We found no significant differences in germination of seeds sown on the soil treatments or between bat-processed and unprocessed seeds in treated soil. Generally, germination success on the soil treatments was lower than on filter paper.

Part of the explanation may be that Ficus grevei is a “strangler tree” – one that will initiate its germination on the bark of another tree, and then gradually send its roots to the ground. Dispersal by flying bats increases the opportunities for seeds to be deposited on other trees, and the residual manure provides a good start for germination. Therefore, the soil-substrate experiment may not be relevant to the ecology of this plant.

We also sowed sets of processed and unprocessed seeds on the ground, fully exposed to the environment. We found no successful germination among these seeds, most of which were carried away by ants. But the ants may, in fact, help disperse seeds from accumulated bat droppings. They may take seeds to their nests, which provide anchorage and a moist and nutrient-rich environment.

Gauging the Madagascan flying fox’s ability to distribute seeds around the landscape requires knowledge of “gut retention time” – how long it takes from when a bat eats the fruit until the seeds are expelled in droppings.

We captured nine bats and kept them in individual cages for three nights. The bats usually refused to eat during the first night. Beginning on the second night, each bat was given about seven ounces (200 grams) of sliced bananas. Some contained seeds from ripe fig fruits, while others were left plain as a control. Food consumption and droppings were recorded every 30 minutes, and the seeds contained in droppings were counted. The bats were released on the fourth night.

Seed-transit times are usually quite short, less than 30 minutes, but some seeds were occasionally retained in the gut for 24 hours or more. Bats generally disperse most seeds within about three hours after eating, but smaller numbers of seeds may be retained much longer.

We also attached GPS tags on 18 flying foxes to track their travels during nighttime foraging. Only bats that weighed at least 1.3 pounds (600 grams) were tagged, which involved gluing the device to a shaved area of skin on the bat’s back. We collected a total of 101 nights of data from 15 bats. The other three either lost their tags or could not be located. We excluded the first night after tag-attachment from our analysis, leaving 86 nights of data. The tags recorded each bat’s location at 2.5-minute intervals from 6 p.m. to 6 a.m.

This effort produced a great deal of data on bat movements and habitat preferences. The home range of the whole tracked population was mainly composed of endemic “spiny forest,” but these semiarid areas provide very few feeding options. Thus the bats focused their foraging on both working and overgrown sisal plantations and on small, remnant patches of native forests along a river. Sisal is a succulent agave species, originally imported from the New World, that yields a fiber used in twine, ropes and other products. The plantations offer easy access to pollen and nectar year round.

Gallery forests, the most natural and productive habitat available in the area, were a strong second choice for foraging, with an abundance of fruiting tamarind trees and some large fig trees that provide food resources during the dry season.

We found that a bat can travel as far as six miles (10 kilometers) from a feeding site in just 12 minutes – with a maximum in our study of almost 12 miles (19 kilometers) in 84 minutes. And we documented that a single bat can, in one night, disperse seeds from three fig trees in three different directions. Clearly, the Madagascan flying fox is an extremely efficient long-distance seed disperser.

The GPS data indicate these bats frequently crossed cleared areas and agricultural fields. Considering their ability to disperse seeds while flying and the germination advantage of bat-processed seeds, bats may well be key species for introducing new trees into open areas and small forest patches that would rarely attract other frugivores. Thus, this initial growth may provide new feeding grounds for other fruit-eating mammals and birds, which then add their own contributions to the regrowth of damaged forests.

In this way, my results suggest, Madagascan flying foxes may serve as “stepping stones” that begin the forest-regeneration process and attract other seed-dispersing animals to build on that foundation to protect wildlife biodiversity and the economic health of local people.

RYSZARD OLEKSY is a graduate student working toward a Ph.D. from the School of Biological Sciences, University of Bristol, in the United Kingdom.

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