Reasons to Care About Bats
Bats are ecologically and economically essential for us. Yet, they are one of the most neglected animals worldwide. This article shows some of the ecosystem services they provide, along with the threats they face, and argues that -rather than demonizing and persecuting them- we should care about and protect them.
The article contains the following sections:
1. Bats are unique and diverse
2. Bats provide ecosystem services:
- 2.1. Bats are pollinators and seed dispersers
- 2.1. Bats are natural pesticides
3. Bats face many threats:
- 3.1. Forest degradation
- 3.2. Overuse of pesticides
- 3.3. Wind energy turbines
- 3.4. Hunting and culling:
-3.4.1. Culling bats to avoid zoonotic diseases
-3.4.2. Culling bats to avoid conflict with farmers
4. Bats need to be protected
1. Bats are unique and diverse
Bats are one of the most marvellous creatures on Earth. Whitout going any further, think for a moment about this: they are mammals that fly in complete darkness using sonar to move around and hunt. But bats are way more interesting than that. They have not only been recognized as the most widespread terrestrial group of mammals out there (1), but one of the most diverse as well (2). Plus, bats are ecologically and economically essential organisms that perform plenty of ecosystem services for us.
Yet, at the same time, they are clearly one of the most disregarded animal groups, typically associated with multiple misconceptions and plenty of disdain worldwide (3). Unfortunately, this becomes especially aggravated by the fact they serve as important disease reservoirs. While it is undoubtedly true they carry a wide range of potential zoonotic viruses, this should not be used as an excuse to persecute and kill them on mass but rather as another powerful reason to protect them further. As a matter of fact, the contrary has not only proven ineffective but also counterproductive to a large extent (4).
During the past 15 years, scientists have described more species of bats than of any other mammal (1). Nowadays, there are around 1400 species known to science, which makes them second -only after rodents- in terms of species number. They represent a group with an incredibly rich variety of behavioral, feeding and roosting habits (3), being the most ecologically diverse order of mammals, according to some accounts (2).
Their diets encompass nectar, pollen, fruit, arthropods (e.g. insects and spiders), fish, amphibians, reptiles, birds, small mammals, and even the blood of birds and large mammals (2). They are cosmopolitan animals which -with the exception of the highest mountains- have reached and live pretty much everywhere around the globe, typically inhabiting caves, trees and man-made structures, such as houses and mines (5).
As everybody knows, the majority have crepuscular or nocturnal habits. However, contrary to what many people believe, bats are not blind. In fact, some species are able to see well, and flying foxes see even better than humans under conditions of low light (5).
It is true though that the majority do not have good sight and need to rely instead on the use of a biological sonar, a phenomenon technically known as echolocation (5). In other words, they locate objects in space through the echo of sound waves. Briefly, this works as follows:
Bats emit active ultrasounds (i.e. high pitched sounds we cannot hear) that, after hitting the surrounding objects, bounce back. The rebounding waves are captured by the well-tuned ears of bats, whose brains immediately process and interpret auditory cues encoded by the echo waves (e.g. about size, direction, distance and motion). As a result, bats are able to form a mental acoustic image in their heads, a map of the physical objects and where they are placed, which allows them to recognise obstacles and prey (2).
From a human perspective, this natural ability to navigate by sound rather than light is almost like a superpower, a kind of sixth sense that bats only share with toothed whales and some bird (e.g. oilbirds). By evolving this ability to fly and echolocate in the dark, bats were able to exploit an abundant food resource (flying insects) and conquer a new whole ecological niche for mammals (the night sky), where they are now top predators with no parallel competitors at all (2, 5).
2. Bats provide ecosystem services
Bats are a key part of many ecosystems around the world. In this sense, in addition to their intrinsic value and importance for forests and global biodiversity, bats also provide highly valuable services for us, with countless benefits to agricultural settings (1). These ecosystem services have mainly to due with the role bats play in the food chain, where they can be primary consumers (herbivores), secondary consumers (carnivores eating primary consumers) and tertiary consumers (carnivores eating secondary consumers). While herbivore bats help in the propagation of hundreds of plants by pollinating and dispersing their seeds, carnivore bats control the populations of organisms that could easily become pests if left unchecked (3).
- 2.1. Bats are pollinators and seed dispersers
Nectar-feeding and fruit-eating bats pollinate and disperse seeds for more than a thousand plants, many of which are ecologically and commercially valuable. For instance, bats are the primary pollinators of the majestic African baobabs (Adansonia digitata) (1), as well as of the blue agave (Agave tequilana), the plant from which tequila is obtained, responsible for a multimillion-dollar industry in Mexico (3).
Some of those plants are indeed highly dependant on these flying mammals, to such an extent that the loss of bat pollinators could severely compromise the reproductive success of many of them, particularly in the tropics. In fact, a recent meta-analysis has shown that the dependence of bat-pollinated plants towards bats is higher than the dependence that other vertebrate-pollinated plants have towards their respective rodent or bird pollinators (6).
There are two main families of bats providing pollination and seed dispersion services: the pteropodids (i.e. Pteropodidae), in the Old World (Africa, Asia and Europe), and the phyllostomids (i.e. Phyllostomidae), in the New World (North and South America) (3). Pteropodids are of much larger size than phyllostomids and tend to pollinate mostly canopy trees and shrubs, whereas phyllostomids are also able to pollinate lianas and epiphytes (i.e. plants that grow upon other plants).
Pteropodids, commonly known as fruit bats, are distributed throughout tropical and subtropical regions of the world and feed primarily on flowers (e.g. nectar, pollen), fruit and leaves. In the process, they propagate more than 250 species of plants, of which almost 200 give humans economically important resources and products, such as drinks, foods, timbers, fibres, dyes or medicines (7).
Among these fruit bats, there is a group of great ecological and conservation importance: the flying foxes. In some places like Mauritius, a tiny island off Madagascar, these fruit bats are the only effective seed dispersers of plants, thus the structure and function of entire ecosystems depend on them. Through the dissemination of native plants, flying foxes are essential promoters of ecosystem health, contributing to the long-term regeneration and maintenance of whole tropical forests and the biodiversity they contain (8).
In turn, the healthy ecosystems these bats promote bring us extra benefits through processes such as climate regulation, water and air purification, nutrient cycling and soil stabilization. Not to mention the educational, spiritual, aesthetic and recreational cultural values, all of which contribute greatly to human wellbeing overall (3).
Directly or indirectly, we have been arguably benefiting from bat services since the very beginning of humankind, even if we did not realize. Some societies might have actually been already aware of the goodness of bats long ago. Indeed, one of the most ancient paintings ever recorded, from 17,000–60,000 years ago, depict bats in a cave of Kimberley (Australia), in simple drawings that resemble some stripe-faced fruit bats that today pollinate boabs (Adansonia gregorii), a species of baobabs (7).
Interestingly, genetic evidence indicates the supposed creators of those paintings -the Bradshaw people- must have brought boab’s seeds from Ethiopia (where they come from) to the conquered land of Australia (where they ended), something that might have responded to the utility of this tree as both a food and a building material source. Although within the realm of speculation, this has led some scientists to propose they could have also brought the bat pollinators, and this simple rock art could perhaps represent a recognition of the early ecosystem services provided by those bats (7, 7.1).
- 2.2. Bats are natural pesticides
Perhaps even more importantly, many bats are excellent pest enemies, as they tend to be voracious predators of herbivore insects. In total, around two-thirds of bats are obligate or facultative insectivores, with a preference for small prey items, such as moths, beetles, flies, cicadas, leafhoppers and true bugs, some of which can be harmful crop pests (e.g. June beetles, click beetles, leafhoppers, planthoppers, etc) (3).
Although quantitative data on pest damage is limited, it has been estimated that weeds, pathogens and animals probably account for losses of 20–40% of the total crop production worldwide. Herbivorous arthropods (mostly insects), in particular, are responsible for substantial damage to agricultural crops. More specifically, these can destroy around 18–20% of crop production every year (with heaviest losses happening in developing countries). In total, this represents more than US$ 470 billion in total (9).
Globally, up to 99% of potential crop pests can be controlled by natural systems, which include bats (3). In this sense, experimental studies have confirmed insectivorous bats play a very important part, both in temperate and tropical areas (1). As a matter of fact, scientists have estimated that every year, a single colony of 150 big brown bats (Eptesicus fuscus) could devour nearly 1.3 million pest insects in Indiana, while one million little brown bats (Myotis lucifugus) could consume hundreds of metric tons of insects during the active season in the US (10).
Thus, the decline of bat populations can have substantial implications, with quantifiable value to agriculture. Translated into economic terms, the services provided by bats are valued in the billions just to the US agriculture (1). A paradigmatic example is represented by the Brazilian or Mexican free-tailed bat (Tadarida brasiliensis), which feeds on insects that are important agricultural pests when in their larvae stage. Among those, there are devastating moth species, such as the corn or cotton earworm (Helicoverpa zea), one of the most destructive agricultural pests in the world. In a region of south-central Texas, the annual pest control value by the Mexican free-tailed bats has been estimated in the hundreds of thousands per year for cotton production alone (11).
These pest suppression services of bats have been experimentally confirmed and quantified. Using cornfield exclosures (i.e. corn field plots where bats were not allowed to act), scientists empirically showed bats exert sufficient predation on adult corn earworms so as to affect larval density and consequently reduce the damage of corn crop, a suppression of herbivory that is valued at more than $1 billion in corn. Besides, researchers found that bats also suppressed fungal growth and toxic compounds on corn, meaning the top-down predatory effects of bats do not only affect their insect preys and the plants on which they feed, but can even reach pathogens and mycotoxins as well, leading to further benefits (12).
More broadly, it has been estimated that bats could afford US farmers an average of $23 billion per year. This calculation includes direct factors, such as the achievement of less crop damage and lower expenditure in pesticides. However, it does not take into account other indirect factors, meaning this number could actually represent an underestimation of the real economic value of bat services. Indeed, the vital agroecological service of insectivorous bats is a consequence of both direct actions and other more difficult to quantify indirect effects (e.g. potential “downstream” effects derived out of lower pesticide use or bat predation) (10).
The indirect effects of pesticide use are particularly important, as the extensive application of broad-spectrum pesticides facilitates the evolution of pesticide resistance. At the same time, the widespread and indiscriminate use of chemical pesticides can negatively affect insect predators in such a way that insects that were once-controlled by those predators could potentially thrive and become new pests (3). In addition, synergistic effects can arise and make matters worse. For instance, the overuse of insecticides could lead to the emergence of a few resistant insects which -if not immediately checked by their dwindling natural enemies- would reproduce, giving rise to ever more difficult to control pests (13).
Although pesticides are undoubtedly needed to achieve efficient agricultural production worldwide, the use of insecticides could arguably be severely reduced by promoting biological controls like bats. Surprisingly, despite the steady increase in pesticide use during the past few decades, the proportion of crops destroyed by insect pests has actually gone up, being now two times higher than in the 1940s in the US. In total, according to the World Resources Institute, over 400 pest species have already become resistant to one or more pesticides (3).
What is more, since insects tend to eat and breed more as temperatures rise, the potential loss of food crops due to insect pests is projected to seriously aggravate in the face of climate change. More specifically, the global yield losses of the three major grains (wheat, corn and rice) are expected to increase by 10–25% per degree of global mean surface warming (14). This means that, even under a scenario of “only” two degrees increase in global warming, insects are expected to boost crop losses by 20–50% with respect to pre-industrial levels (destroying almost 50% more wheat, 30% more corn and 20% more rice).
By protecting bats, the fight against these pests could be performed with lower use of insecticides, while promoting global ecosystem health at the same time. As an illustrative example of a mutualistic relationship between humans and bats, a few bat boxes (artificial roosts for bats) were installed next to rice paddies in a region of Spain back in 1999, allowing the accommodation of ~3,500 soprano pipistrelle bats (Pipistrellus pygmaeus). These in turn now act as effective natural pesticides of a devastating moth that ruins rice crops. Indeed, the value of this agro-ecological service has been estimated to be equivalent to the cost of the avoided chemical pesticide that would otherwise be required (15).
3. Bats face many threats
During the past few decades, many anthropogenic activities have contributed to bat mortality overall, which has consequently led to severe bat population declines worldwide. Some of the most important threats include -though are not limited to- forest degradation, overuse of pesticides, wind energy turbines and both hunting and culling (3). As a result, around 80% of the bats assessed by the International Union for Conservation of Nature are now in need of conservation or research attention (1).
- 3.1. Forest degradation
The loss of forests, especially in the tropics, is one of the main threats bats face. This is because, although subterranean habitats -like caves- serve as roosting refuges for around 40% of bat species, many of them still depend on forests for foraging. Also, for many bats, forests can also provide crucial roosting structures, be it inside tree hollows, under bark, or in foliage. In fact, on a global scale, forests represent the most important habitats when it comes to supporting species diversity and local abundance of bats. This is in agreement with the fact the highest bat biodiversity is found within tropical forests. Therefore, investing in the protection of forests would greatly benefit bat biodiversity as well (1).
- 3.2. Overuse of pesticides
Agriculture has significant negative impacts on biodiversity and this includes bats. In general, the land-use change linked to agricultural practices causes direct habitat loss and limits both roosting and foraging grounds for bats. In addition, the overuse of insecticides restricts even further what these animals can eat. Although there are not many studies on the effects of agricultural interventions on bats, there is some evidence suggesting that low-intensity agricultural systems can be more beneficial for bat populations than high-intensity ones, in terms of supporting more activity, species richness and diversity (16).
- 3.3. Wind energy turbines
One of the main causes of observed bat mortality nowadays comes from collisions with renewable wind energy turbines, both in Europe and North America. In Germany alone, over 300,000 bats are estimated to be killed every year due to impacts with wind energy facilities, while over 500,000 bats are estimated to die annually for the same reason across Canada and the US. The most affected species tend to be those that rely on tree-roosting and migrate long distances (17), with fatality rates of some of these migratory species high enough so as to increase their risk of extinction (1). Although the exact reasons why so many bats die at turbines remain unclear (17), research has shown that altering operations of wind turbines during high-risk periods could significantly reduce bat mortality (18).
- 3.4. Hunting and culling
Surprisingly as it may sound, bushmeat hunting is becoming another major threat for bats, with more than 150 species of bats being hunted. This is especially true for fruit bats in Africa, where around half of the total species are hunted, mainly for their flesh (1).
Apart from this hunting exploitation for food, bats are exploited for traditional medicine, and even sport sometimes. But bats are also being massively killed for other motives, including their condition as potential vectors of zoonotic diseases and the increasing conflict with fruit farmers (1).
-3.4.1. Culling bats to avoid zoonotic diseases
Culling animals to fight infectious diseases can only be potentially effective in the short term and in very specific cases, such as when disease outbreaks are highly localized or when disease happens in domestic and nonnative animals that can be easily eradicated (19). Instead, in the wild, culling has rarely demonstrated to be an effective practice at controlling different animal infections (20).
For bats, this has been seen multiple times in different parts. For instance, in North America, with culling campaigns to control the transmission of the so-called white-nose-syndrome, a disease that has devastated bat populations in the US; in Latin America, with the indiscriminate killings of bats for rabies control, where entire caves have even been reported to be destroyed or gassed (1); or in Africa, with bat depopulation attempts to control the spread of Marburg virus through the trapping of bats in fishing nets and the sealing of cave entrances (20).
In all of these cases, research has shown that bat culling did not reduce the transmission of the disease. In fact, in some cases, the response of wildlife can exacerbate the problem (19), as it has been observed in different wildlife disease systems (e.g. European fox rabies and bovine tuberculosis of badgers in the UK). Arguably, prevention programs focused on minimizing disease exposure, along with vaccination of susceptible populations, would probably represent a more efficient strategy, in general (4).
The following is a paradigmatic example of the ineffectiveness of bat culling to avoid zoonotic diseases:
-The case of the Amazonian vampire bats
The common vampire bats (Desmodus rotundus) from South America are characterized by the unique feeding habit of drinking mammalian blood. Due to the increasing clearing and occupation of the Amazon rainforest, the cases of human rabies caused by these bats have already surpassed the cases caused by dogs, and vampire bats are now the major vectors of rabies. As such, many countries have opted to cull them in order to prevent rabies in both livestock and humans (4).
The culling of these bats is based on different methods, such as the application of an anticoagulant agent that bats spread to the rest of the colony by grooming, the destruction of roosts by logging trees or lighting fires in caves, and the direct killing of captured individuals. All of this is done under the assumption that culling reduces host population sizes and therefore pathogen transmission would weaken. Nonetheless, even though the large-scale sacrifice of vampire bats started back in the 1970s, and have been extensively carried out ever since, lethal outbreaks have been occurring with frequency (4).
Evidence has shown that these culling campaigns are unable to effectively eliminate rabies and might instead be counterproductive. Indeed, scientists found a significant positive association between the periodic culling of colonies and the prevalence of the rabies virus in Peru. Namely, bats living in colonies subjected to periodic cullings showed higher viral prevalence than bats living in colonies that were never culled (4).
This result might be explained in part by the fact culling campaigns target mostly adult bats, when in reality the ones that have a higher prevalence are sub-adults and juveniles. On the other hand, although adults have a lower viral prevalence than immature bats, their immunity is higher due to their repeated exposure history to the pathogen. This further supports that the removal of adult individuals leads to an increase in the population-level prevalence of the virus(4).
In addition to that, there might be other important demographic and behavioral mechanisms behind. For instance, the spaces left by culled colonies could be colonized by neighbouring bats, in a sort of demographic “vacuum effect”; and the culling of adult bats might perhaps give juveniles a higher chance of survival by alleviating density-dependent constraints, leading to a parallel increase in the transmission of rabies in the population (4).
-3.4.2. Culling bats to avoid conflict with farmers
Due to human expansion and the consequent encroachment into natural habitats, human-wildlife conflicts are on the rise. This inevitably affects many wild animals, including bats, which on many occasions have no chance but to become more dependant on human-dominated infrastructures (1). As a result, this does not only increase the probability of zoonotic disease outbreaks but can also generate competition for shared food resources with farmers (21).
As a matter of fact, as forests are cleared, the natural food sources of many bats become scarce and, in their subsequent search for new foraging grounds, bats can inadvertently cause damage to a wide range of fruit crops, causing economic losses. Here too, lethal control is ineffective; and since shotguns are typically used, the cullings represent a violation of animal welfare, leaving behind more animals injured or orphaned than actually killed, according to some accounts (7).
This especially affects fruit bats (flying foxes, in particular), which are considered agricultural pests in many countries and thus are persecuted and shot, sometimes legally and with government support (7, 21). This is a lot of the times done out of sheer prejudices instead of factual knowledge. For instance, it has been reported that farmers in Thailand use nets to kill dawn bats (Eonycteris spelaea), as they see them as responsible for the falling of durian flowers. In a way, farmers are right to think flowers fall because of these bats, though what happens in reality is that flowers fall as a natural response mechanism after they are pollinated by bats (7). That is, the bats killed by farmers were actually doing a favor.
In Malaysia and Indonesia, bats have been massively killed even when farmers reported several other animals as more important causes for damage. In some cases, employing large fishing hooks and monofilament lines, which not only is a non-tested method but totally inhumane. Other methods to eliminate fruit bats include walling up roost entrances or fumigating entire caves.
Similarly, in Australia, flying foxes have long been known to cause many economic losses and are also considered agricultural pests. However, there is now a general acknowledgement -even among fruit farmers- that bats are increasingly feeding on commercial crops because their natural habitats are being destroyed. In this context, some authors have proposed farmers should be economically compensated for the bat-caused losses so that the whole society bears the necessary cost of conservation (7).
Unless this vision is extended to other regions, and effective management measures are implemented, many bats might go extinct by the end of this century; and with them, the structure of entire forest might go as well. Actions as simple as using bags or nets to protect fruits like dates, or picking fruits like bananas, mangos and papayas before they ripe could make a significant difference (7).
The following is a paradigmatic example of the ineffectiveness of bat culling to avoid conflict with farmers:
-The case of the Mauritian flying foxes
In Mauritius, there is an endemic species of flying fox bat known as the Mauritian flying fox (Pteropus niger). Though not supported by scientific evidence, these bats are now widely claimed as the main responsible cause for fruit production loss (8, 21). As a matter of fact, flying foxes account for 9–11% of damage to the two most important trees of the island, a much lower amount than the damage attributed to physical causes (e.g. high winds) and the improper management of fruits by farmers (e.g. let the fruits overripe on the tree) (21).
In response to the fruit farmer concerns, and against all scientific evidence and advice (21), the Mauritian government has been implementing several campaigns to kill the endemic bats of the island. This has led to severe declines of this threatened species, whose populations have plummeted by more than half since the beginning of the massive attacks in 2015 (8). What is more, the culling of flying foxes has proven to be an ineffective strategy to boost profits, with fruit yields actually going down after the mass cullings (21), in some cases by up to 70%.
Given the keystone ecological role that flying foxes play at maintaining the structure of the very few rich native habitats Mauritius retains, the indiscriminate elimination of these animals poses a great threat not only to this unique bat species but to the entire ecosystem of the island, which contains many other unique, threatened species as well (8).
In addition to that, the implementation of the mass-cull events has also been criticised for its cruelty manners, involving the use of rifles during the breeding season. Such a strategy leads to distressing injuries of many adult bats, along with a inhumane situation for many dependant pups, which are left alone when their mothers are culled and inevitably starve to death (21).
4. Bats need to be protected
Apart from the aforementioned threats that devastate bat populations worldwide, bats also face other issues. On the one hand, as it happens with thousands of other animals, bats are -and will be- subject to the inclemencies of climate change, with shifts in climate patterns disrupting the behaviors of migratory bats, changing the food availability of pollinating species or reducing their survival and reproduction. On the other hand, the destruction of roosting structures, such as inactive mines, due to either renewed mining activities or intentional closure, is another major cause of habitat loss for bats. Even cave tourism can add some stress and is now known to affect over a third of all threatened species of bats (1).
With all, instead of protecting them, bats have been unfairly stereotyped as “creepy” disease-carrying animals by many cultures, making some people believe they are dangerous creatures that should be better avoided or even exterminated. In actuality, if bats are not disturbed in their natural habitats, they are pretty much harmless to anyone (1). In fact, bats are not bad, but good for us. As such, they should not be seen as mere ugly vectors of deadly viruses, as some media have recently portrayed, but rather as the key pieces of the puzzle of life they truly are.
Certainly, bats are irreplaceable creatures, authentic biological treasures that were already here long before any hominoid roamed the Earth. Now, we are fortunate enough to obtain invaluable services they freely provide. The intrinsic ecological and economic importance of these animals is such that conserving them would be good not only for nature but also society. For all that and more, these fragile flying mammals deserve much better attention and care worldwide. If we protect them, we protect ourselves.
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- To see the full list of scientific paper references used, click here.
