Bats help against bad eyes and toothache, fever and hair loss. At least the ancient Egyptians were convinced of this. Hanging over the door, the animals should drive away the very demons that were supposedly responsible for such health problems. And these are far from the only abstruse stories that people have invented about the nocturnal hunters. In medieval Europe, for example, the fluttering animals were considered messengers of the devil and effective ingredients for all kinds of witch potions. But their real talents have long been underestimated. Because of their secretive lifestyle and poor reputation, they have tended to lead a shadowy existence in ecological research until recently. Instead of spending their nights on bats, scientists prefer to focus on animals that are easier to observe, such as birds or bees. "Only recently has there been a shift in perception", says Christian Voigt of the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin. "It is now becoming increasingly clear that bats play an important role in many ecosystems."
Winged string pullers that can become active in the earth’s habitats are, after all, more than plentiful. After all, bats have conquered all continents except Antarctica and are the second largest mammal order with more than 1100 known species. Only rodents have managed to achieve an even greater diversity. In the tropics and subtropics in particular, numerous species flutter through the night, dedicated to a wide variety of lifestyles. Insect hunters are on the move there, as are vegetarians – and, cliche-like, a few vampires who feed on the blood of other animals. But they all influence the other plants and animals in their habitat in their own unique way. And humans can also benefit from this.
This article is included in Spektrum – Die Woche, 01/2016
Insectivores help control pests
"Insectivores, for example, very clearly decimate the populations of their prey.", says Christian Voigt. No wonder, given the huge swarms the nocturnal hunters congregate into in some regions. The Khao Chon Pran cave in Thailand, for example, is home to some 2.6 million free-tailed bats of the species Tadarida plicata. "When they fly out in the evening, it’s a very impressive spectacle", the IZW researcher knows from his own experience. For more than an hour, a huge band of fluttering animals then streams out of the cave. And each of them can eat its own body weight of 12 to 15 grams of prey in one night. In the vicinity of this cave alone, 17.5 tons of insects are collected per night. For Thailand as a whole, this amounts to 20,000 tons per year. A related species in North America, the Mexican bulldog bat, has a similar penchant for mass gatherings (Tadarida brasiliensis). An estimated hundred million of these animals live in caves in northern Mexico and southern USA. Their prodigious appetites prey on more than just insects in their immediate vicinity. Since they can climb to an altitude of 3,000 meters and cover more than 100 kilometers in one night, they can also catch swarms of insects that migrate over long distances.
Scientists have long suspected that the hunger of nocturnal hunters is cash for agriculture. Finally, the bat menu also includes numerous species that have made themselves unpopular as pests. It is estimated that the activities of bulldog bats alone save farmers in the southwestern U.S. half a million dollars on pesticides each year. In total, the insect catchers in the USA are said to provide services worth more than three billion dollars annually. "They also contribute a lot to pest control in Thailand", says Christian Voigt. Finally Tadarida plicata a counterpart of several pointed-headed cicadas, which tend to develop en masse in some years and are feared as rice pests.
What exactly can resident bats do about it? To better assess this, Thomas Cherico Wanger of Stanford University and his colleagues have collected data on bat populations, their activity areas and the number of insects they prey on, on rice fields and their yields, and on the presence and harmful effects of a pointed-headed cicada named Sogatella furcifera gathered. They then fed all this information into a computer model. According to this, Thailand’s nearly eight million bats prevent the loss of nearly 3,000 tons of rice worth more than $1.2 million on average each year. In doing so, they probably protect food for 26,000 people. And these are still conservative estimates, the researchers emphasize. Finally, the animals decimate other pests such as the pointed-headed cicada Nilaparvata lugens, which caused $200 to $240 million in damage to Thailand’s rice crops during a mass development in 1990.
Bats save corn crop
The value of flying pest controllers for corn cultivation has now not only been calculated by scientists, but even proven in an experiment. For this purpose, Josiah Maine and Justin Boyles from Southern Illinois University in Carbondale covered six experimental areas in the cornfields of Illinois with coarse-meshed nets at night, thus excluding the bats but not the insects. As a result, 59 percent more voracious larvae of the cotton bollworm made their way there (Helicoverpa zea) over the plants than on fields without a net. Accordingly, the researchers then recorded 56 percent more damaged kernels per corncob. But such insect attacks also pave the way for infections with dangerous fungi such as mold Aspergillus flavus and Fusarium graminearum, which produce toxic and carcinogenic substances like aflatoxin and fumonisin. In fact, significantly more cobs with fungal infestation and higher fumonisin concentrations were found on the bat-free plots. Since more than a billion tons of corn are produced worldwide each year, such a service is also quite economically viable. Researchers estimate the value of insect destruction by bats alone for growing this arable crop at more than a billion dollars. And that’s not even counting the indirect benefits of suppressing fungal growth.
Bats are also very interesting allies for other branches of agriculture. Bea Maas of the University of Vienna and her colleagues, for example, have blocked both bats and diurnal birds from accessing cocoa plantations in Indonesia. As a result, not only the plant-eating insects became more frequent, but also the fruits developed worse – which in the end led to almost one third less harvest. "Such studies do not yet exist for European agriculture", says Christian Voigt. He suspects similar effects on canola and several other important crops in this country. In German forests, at least, the nocturnal hunters are cleaning up among the crawling herbivores. This is what Stefan Bohm from the University of Ulm and his colleagues found out in another experiment with nets. They screened the crowns of English oaks on the Swabian Alb and in the Hainich National Park in Thuringia against visits by bats and birds and determined the damage to the leaves three times between July and October. In all cases, these trees had a larger damaged leaf area and more holes per leaf than comparison trees without netting. The nibbling vegetarians were particularly hard on the oaks in the Swabian Alb – even though the Hainich, with its warmer and drier climate, actually offers the more insect-friendly conditions. On the other hand, there are more bats and birds living in the national park, which obviously makes life easier for the trees.
Fertilizer from the gut
And it also has an amazing effect in tree cavities, Christian Voigt and his colleagues have found in an Atlantic lowland rainforest in Costa Rica. This is where the imposing forest almond grows (Dipteryx panamensis), whose branches protrude above the canopy and often have large holes at the base of their trunks. "But these are very popular roosts for many different species of bats", explains Christian Voigt. The animals use these burrows not only as daytime roosts, but also as toilets. So perhaps the tree can benefit from the nutrients its inhabitants drop? To find out, the IZW team set out on the trail of nitrogen. A heavier version of this element, which chemists call N-15, naturally accumulates in bat droppings. With their help, it is therefore possible to track where the nutrients from the guano end up. However, the researchers were initially disappointed in their analyses: the soil in the bat burrows did not contain more N-15 than that around the tree. So they had to shelve the theory of the fertilizing fluttering animals?
Another experiment did not confirm this fear. When the animals lived and digested in artificial quarters, their nutrient traces were also found on the ground. "Apparently, nitrogen in a living tree is carried away so quickly that it doesn’t accumulate in the cavity in the first place", concludes Christian Voigt. Fine roots seem to pick it up and transport it up the trunk and into the crown. The researchers were even able to trace this pathway into the seed coats of forest almonds: If these came from trees with bat cavities, they contained significantly more N-15 than in specimens without fluttering inhabitants. Vampire bats, which live on the blood of other animals, have proven to be particularly effective fertilizer producers. "They eat a very high protein diet and therefore have more nitrogen in their feces.", explains Christian Voigt. They carry this from their "restaurants The bats migrate to their roosting sites in the forest and thus act as flying nutrient transporters. "In some trees, hundreds of these animals congregate, covering the floor of their burrow with a black puddle of digested blood", reports the IZW researcher. This nitrogen boost can be very useful, especially for trees in the tropics. This is because nutrients such as nitrogen and phosphorus, which are important for plant growth, are chronically scarce in the soils there. The researchers therefore assume that in tropical forests many other large tree species with cavities in their trunks also benefit from the services of the fluttering animals.
"For the trees in Germany this is probably not the case", says Christian Voigt. For one thing, the local soils are much richer in nutrients than those of the tropics, so an additional fertilizer boost does not bring such great benefits. Most importantly, Central European bats usually do not live in openings at the base of the trunk, but in woodpecker cavities higher up. There, however, the tree has no roots with which to absorb the nitrogen.
Bats disperse seeds and pollinate plants
In general, the performance of tropical and subtropical fluttering animals seems to be more diverse than that of their colleagues in temperate latitudes. This is due to the fact that, in addition to insect hunters, numerous vegetarians are also on the move there. Some 250 species eat fruit or nectar from more than 500 trees and shrubs. In Africa and Asia the flying foxes have specialized in this lifestyle, in the New World the leaf-nosed bats. When such an animal has a stomach full of fruit, it often flies long distances before excreting the remains of its meal again. Bats are therefore important seed dispersers. They are very effective in ensuring that young plants can germinate even at great distances from their parent tree. Flying foxes, for example, do not only distribute mango seeds in the landscape. About a third of Africa’s economically important timber belongs to species that rely on the services of these nocturnal fruit harvesters. Especially in the New World, bats even contribute to the faster re-growth of forest on cleared areas. Many leaf-nosed bats have a soft spot for the fruits of pioneer plants, such as figs or ant trees.
Other fluttering vegetarians, on the other hand, show a special talent for pollinating plants. After all, they can carry large amounts of pollen over long distances. The flowers that have adapted to these visitors usually open at dusk. They are usually large and produce a lot of nectar, typical are also a white or pale yellow color and a musky scent. The associated bats usually have long, narrow snouts, delicate jaws, relatively few teeth and long tongues. One of the most economically important species in this group is the cave long-tongued bat (Eonycteris spelaea) in Southeast Asia. The bat pollinates the durian trees, whose melon-sized fruits are highly valued and traded expensively in the region – a multi-million dollar business that would not function without the cooperation of the nocturnal flower visitors. Neither is the tequila production in Central America. For this you need the sap of agaves, which are pollinated by different flowering bats. "Of course, one should not judge animals only by their economic utility", emphasizes Christian Voigt. But there are enough reasons for the protection of bats from a purely economic point of view. Even if they do not help against fever and hair loss.