Bats are under threat in many areas of the world. Some have declined to such an extent that the numbers remaining may not be enough to sustain the species. Some species, such as the Guam flying fox (Pteropus tokudae), the Puerto Rican flower bat (Phyllonycteris major) and the New Zealand greater short-tailed bat (Mystacina robusta), have become extinct in recent times.
Species most at risk are those living in isolation and those living at low densities: any changes to their environment could wipe out the species. Some fruit bats on isolated Pacific and Indian Ocean islands, such as the Comoro black flying fox (Pteropus livingstonii) and the Marianas flying fox (Pteropus mariannus), are at great risk of extinction. Some microbats living in one small area, such as Kitti's hog-nosed bat (Craseonycteris thonglongyai) on the Thai-Burmese border, are also threatened.
This matters not just because bats are fascinating creatures in their own right. Our increasing familiarity with the term biodiversity reflects the importance biologists place on this aspect of conservation. They are realising that the more diversity there is in animal and plant populations, then the healthier is the environment. If animals, such as microbats, that are at the end of their food chains become extinct, then the creatures that they preyed on will increase in number and compete with others for food and space, so upsetting other food chains.
The fruit bats and some microbats have an even more direct effect. By pollinating plants and dispersing their seeds in the course of feeding, they ensure the survival of many species of trees and other plants, and the recolonisation of cleared forest. Furthermore, a single insect-eating bat may eat hundreds of insects a night, and there are few other nocturnal insect-eaters. Most birds, for instance, feed in daytime on a different set of day-flying insects. Take away a small roost of a hundred bats and there will be a large number of extra insects flitting around the area each night. So bats help to keep a balance in the natural world. They are also a great help to humans as farmland, gardens and house timbers all suffer attacks from insect pests and bats are instrumental in keeping down their numbers.
Human activities have by far the greatest influence on bat populations. Deforestation is killing off many thousands of individual bats and, in some cases, wiping out entire species. Even when some trees are left, they may be exposed to storms and blow over. The soil is soon washed away and the plants that sustain bats by providing insects or fruit can no longer grow. Roost sites are being chain-sawed away.
The human impact on the landscape is now immense. Woodlands, fields and other habitats are dug up and replaced by roads and cities, and this affects the bats' feeding or roosting needs.
In the developed world, agriculture has been highly mechanised and insecticides are widely used to produce high crop-yields. Some of these poisons are ingested by bats and may affect their life expectancy or reproductive success. Researchers have evidence that these poisons can be laid down within the bats' bodies in the fat reserves used for winter hibernation, so they are like ticking bombs. The bat survives only until it starts to burn up the fat.
Generally there are far fewer insects around now than before the large-scale use of these poisons, so there is less food for the bats. It is likely that the population crashes of free-tailed bats in Mexico were due to the introduction of these powerful modern insecticides into the ecosystem. In the UK such a pesticide (lindane) was widely used in attics to kill wood-boring beetles, and it was also found to kill any bat that came into contact with the treated timbers for at least 2 years after the spraying. Thousands of roofs used to be treated annually, but now a safer pesticide (permethrin) is used due to the pressure from conservationists and changes to the law.
Each month, local people on one of the Solomon Islands take about 1,000 small bats, as well as 100 fruit bats, for food from just one cave. (Image: Phil Richardson)
Direct causes of death usually have less of an impact. Some native people eat bats as part of their traditional diet. With a stable human population, this is usually sustainable, but with modern changes to healthcare, many populations of people are rising dramatically and the bats cannot cope with the greatly increased level of killing.
Disease can wipe out local populations of bats. Little is known about diseases afflicting bats, and few have so far been identified, but they are thought to have had a major effect on some of the fruit bat populations. Roosting colonially, the whole population is likely to be affected. The endemic Solomons flying fox, Pteropus rayneri, was surveyed on Choiseul in 1995, and its population was found to have decreased from 50,000 to 15,000 in just 10 years. A disease thought to have been brought into the island by domestic animals was believed to be one of the main reasons for this serious population crash.
There are few natural predators of bats. Among the birds, there are some raptors that specialise in catching bats emerging at dusk and some opportunistic owls too. The bat falcon, Falco rufigularis, of the New World and the bat hawk, Machaerhampus alcinus, which occurs from Africa right across to New Guinea, wait outside cave roosts at dusk. Their agile flight enables them to catch emerging bats. Also waiting may be owls, but these are mostly more adept at catching ground mammals and insects flying more slowly and straighter than the bats.
Domestic cats can hear the high-frequency calls of bats and may lie in wait on a high roof for the bats to follow their usual flight path, then snatch them out of the air. Some snakes will lie in wait in the cave entrance and reach out to grab a passing meal. Locally, such predation can be devastating, as on the Pacific island of Guam, where the introduced brown cat snake, Boiga irregularis, has decimated bat populations.
One of the most effective steps people can take to help microbats is to increase the number of roosting sites available to them. This can be done by installing bat roosting boxes. These are similar to bird nest boxes, but with a narrow entrance underneath rather than a hole in the front. The size and design need to be tailored to different species, but as long as the bat can crawl from the entrance upwards and hang in a sheltered, dry place in darkness at the top, then it should be suitable. Bat boxes are of particular importance in areas such as coniferous woodland, or deciduous woodland where mature trees with holes and hollows are cut down, which contain few or no natural roosting places.
Habitat management is starting to happen on a large scale in state-controlled woodlands, where maintenance of firebreaks throughout the wood is linked to insect conservation. Mowing is carried out at certain times linked to insect breeding cycles, and the edges of firebreaks are allowed to grow up a little to add extra diversity. Insect numbers increase and the bats can obtain plenty of food along these avenues. Tree planting is increasingly carried out according to the principles of conservation, rather than simply to provide a commercial resource, so native trees are being used as replacements when non-native ones are harvested. Old, dying trees are no longer grubbed out, but allowed to rot slowly and provide for a whole range of insects that recycle the wood and are, in turn, recycled by the bats. Also the bats can continue to use rot-holes for roosting.
Farming practices are being slowly changed to help wildlife, too. In Europe, subsidies are being given to allow some farmland to remain fallow. There is more financial encouragement to plant hedgerows and laws are now in place to protect old hedgerows after decades of hedgerow removal to increase field size for more efficient crop production. Bats use hedgerows as route-ways to their feeding areas and find more insects where native flowers have been allowed to grow around field edges and in fallow ground. Many farmers are also becoming more aware of the benefits of working with nature. The hedgerows provide some shelter from the wind and help prevent soil erosion, and the increased number of insects help with pollination of some crops.
In Europe, biodiversity agreements have resulted in the feeding areas of some of the rarer bats being protected. This means that huge areas of the landscape are managed in a better way for bats and no undesirable influence is allowed to impinge. Some of the horseshoe bats are protected in this way.
This purpose-made bat brick in the roof of a hibernaculum is being used by 2 brown long-eared bats, Plecotus auritis, and a Natterer's bat, Myotis nattereri. (Image: Phil Richardson)
In temperate climates the importance of hibernation sites is well known to bat conservationists. In the UK some bat workers have embarked on the major project of building tunnels to serve as hibernacula for bats - huge constructions of large concrete pipes angling into the ground for 20m or more, and covered with a thick layer of insulating soil. Some of these pioneering conservationists have dug into chalk cliff-faces too, making caverns suitable for the species in these areas.
These sites can often be improved by modifying the air flow, which can alter the humidity as well as the temperatures within them. They can also be greatly enhanced by providing more hanging places for the bats. Many wartime bunkers have been turned into excellent hibernacula by covering them with about 1m of earth to insulate them. Disused railway tunnels make good sites, but have too much air blowing through them, so blocking one end creates a more stable environment. Modifications such as these need to be carried out with official approval from the relevant authorities and with full agreement from statutory nature conservation bodies as it is too easy to mean well but find that your adjustments have made the site excellent for a locally common species and useless for the national rarity that used to occur there!
Perhaps the quickest and greatest improvement that can be made to hibernacula is preventing human access. Some people love to explore underground and they can, by accident or design, inflict damage on roosting bats, which are sensitive to noise and changes in temperature of less than 1°C. Bats in hibernation survive by slowly burning up stored fat, but when disturbed they begin to burn up more of their precious fat in order to wake and move to a safer, quieter place. If they suffer too many disturbances, bats will die of starvation.
Caves that are used by large conglomerates of bats produce enough guano (accumulated faeces) each year to make it worth harvesting. The droppings from microbats are rich in nitrates and phosphates and make a good fertilizer. In Thailand, for example, some local people rely on this resource to make their living. They enter the big bat caves periodically to dig out sackfuls of the guano, which may lie many feet thick on the ground, and sell it to local farmers and growers. They jealously guard the caves, and this prevents disturbance of the bats, helping their conservation.
Much success has been achieved in protecting bats that are roosting in houses in Europe and the United States by talking to the house-owners and asking them to look after their guests in the attic. Sooner or later, buildings used as bat roosts need repair. If the work is carried out with care, both the site and the bats can be saved. The bats tend to roost in such places for only part of the year, so there is likely to be a time when there will be none present to disturb. The basic conditions of the existing roost can then be copied in the renovations, such as a similar size entrance slit in the same place, the same roosting area and no obstructions outside the entrance. Sometimes, parts of the old timbers can be re-used by the roost entrance so the new home even smells familiar to the bats.
In England any remedial work on ancient parish churches around the country first requires a report from a church architect. After lobbying from bat conservationists, bats were included on the official report form, so if signs of bats are present then the correct authorities must be notified before work can commence. Including an extra tick box on the forms has meant that hundreds of bat roosts have been saved each year.
In some countries, sites may be protected to help the local employment situation. As well as the importance of the roosts in large bat caves to guano miners, there is an increasing tourist industry springing up around spectacular bat roosts in places such as the United States, India, Australia, Indonesia and Thailand.
Bats and their roosts have been given protection by law in most parts of the world to some degree. In Britain, for example, all bats and their roosts are protected, as are the feeding areas of some species. In the United States, the rarer species are protected across the continent, and different states have their own laws as well, some protecting species at their roosts.
Generally information about bats is scarce and the little that does exist is often wrong. Bat conservation organisations around the world are now publishing more and more information about bats in books, in newspapers and on the Internet, as well as using television and radio broadcasts, and slowly the public's perception is changing. Bats are losing their reputation as creatures of horror and fear, and becoming known as the harmless, fascinating, endangered creatures that they are.