Their voices are unmistakable. From the trees in the Kazanka National Park in Zambia a chirping and chirping sounds like from a huge flock of birds. But the figures that flutter from one branch to the other have leathery wings, a yellowish-gray coat and a fox-like face with big eyes. In one of the largest animal migrations in the world, between October and December, there are up to eight million palm-fly dogs every year –Eidolon helvum– from other regions of Africa into this protected area. As soon as dusk falls, they rise in huge clouds into the sky and flutter to the figs and other trees to fill their bellies with ripe fruit overnight.
Fruit bats can contribute to reforestation of the land
In these fresstrips, however, the bat relatives that are widespread in Africa not only satisfy their own hunger. At the same time they also provide valuable services for their habitat and the people living there. This is the conclusion of a team led by Dina Dechmann from the Max Planck Institute for Ornithology in Radolfzell and Marille van Toor from the Linnaeus University in Kalmar in Sweden in the journal Current Biology.
The researchers have investigated the ecological effects of various palm fly dog colonies in West Africa. Thus, the inhabitants of such a colony can spread hundreds of thousands of seeds in a single night, thereby contributing to the reforestation of deforested land.
For this important job, palmtree dogs are particularly well qualified. Because they are not only very mobile, but with a wing span of up to 76 centimeters and relatively large. And there are huge amounts of it. Many millions of animals are said to flutter through the sub-Saharan regions. “This makes this species one of the most common fruit-eaters in Africa,” says Dina Dechmann. “One colony is made up of thousands, sometimes millions, of members.”
Fruit bats carry seed from the trees miles away
For three such metropolises, she and her colleagues have now simulated the nightly flights of all residents with a new computer model. This not only takes into account findings on the food and flight habits of the animals and observation data from the respective colony. From experiments it was also possible to deduce how long the swallowed seeds stay in the stomach and intestines and when they are excreted.
The average flying fox visits therefore up to three fruit trees per night and carries their seeds up to 95 kilometers far away. However, the services of the flying transporters differ depending on the season. Thus, in the investigated colony in Ghana’s capital Accra during the rainy season, only about 4,000 bats live on, which spread about 5500 seeds per night. In the dry season, however, the flying fox metropolis grows to 152,000 members who carry up to 338,000 seeds in a single night. With their droppings they distribute these on more than 800 hectares of former forest areas, which were cleared between 2001 and 2016.
The animals are hunted hard despite the benefits to the environment
If these areas were once again completely covered with trees, the population would benefit enormously. “For example, it would be possible to use wood and fruit there again, and the rain would not wash out as much nutrients from the soil as it does today,” explains Marille van Toor. Such effects, according to the team’s calculations, could generate additional income of $ 858,000 per year. The protection of the fruit bat colonies should therefore also be worthwhile for economic reasons.
However, it does not look very good for the future of fluttering service providers. “Since there is hardly any other game in Ghana, the animals are hunted there,” says Dina Dechmann. In addition there are the deforestation of forests and neighborhood trees as well as the use of animals in traditional medicine. All this has apparently already led to a significant decline in stocks. Thus, in the colony in Accra formerly ten times as many animals as in recent times. If this trend continues, according to the researchers, this could have fatal ecological consequences and also lead to economic losses.
Bats also use farmers to catch insects
The same applies to the stocks of other flapping animals around the world. The Cave Long tongues flying fox(Eonycteris spelaea)In Southeast Asia, for example, the durian trees pollinate, whose melon-sized fruits are highly prized in the region and traded at high prices – a million dollar business that would not work without the cooperation of nocturnal flower visitors. Just as little as the tequila production in Central America. Because you need the juice of agaves, which are pollinated by different flower bats.
But also the services of insectivores are impressive. After all, they devour an amount of insects in one night, which accounts for about one-third of their own body weight. And that makes them very effective pest controllers. For example, the hundreds of millions of Mexican Bulldog bats develop a tremendous appetite(Tadarida brasiliensis)who live in caves in northern Mexico and the southern United States. It is estimated that farmers in the southwestern United States save half a million dollars in pesticide expenditures every year solely through the activities of these flapping animals. Overall, the insect catchers in the US every year to provide services of more than three billion dollars.
According to other studies, Thailand’s nearly eight million bats averaged a loss of nearly 3,000 tons of rice, worth more than $ 1.2 million a year. In doing so, they are likely to protect food for 26,000 people. And researchers around the world estimate the value of insect killing by bats alone for growing corn more than $ 1 billion.
Trees are protected from bats – with negative consequences for the environment
“For European agriculture, there are only a few such studies so far,” says Christian Voigt of the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin. But they show that nocturnal hunters in Europe have an appetite for pests. In the feces of southern European long-winged bats(Miniopterus Schreibersii)For example, a team led by Antton Alberdi from the University of Copenhagen found DNA from more than 200 species of insects, 44 of which cause agricultural damage. And even in German forests bats clean up under the crawling herbivores vigorously.
Stefan Bhm from the University of Ulm and his colleagues found this out in an experiment on the Swabian Alb and in the Hainich National Park in Thuringia. With nets, they have shielded the stems of ostrich-crowns against the visits of bats and birds and determined three times the damage on the leaves between July and October. In all cases, these trees had larger damaged leaf area and more holes per leaf than comparison trees without a net. “Of course, animals should not be judged solely on their economic benefit,” says Christian Voigt from IZW. For the protection of bats, however, there are reasons enough from a purely economic point of view.