Reasons to Care About Bats
By: Álex Tuñas Corzón
Tags: Conservation, Ecology, Nature, Covid 19, Animals

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).

A corn earworm eating a corn ear (Photo by Scot Nelson, CC0 1.0)

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).

Two bat boxes hanging from a tree (Pixabay license)

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).

A pile of tree trunks ready to be picked up (Photo by Sharad Bhat, Unsplash license )

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).

A farmer spraying pesticide on a lettuce crop in Yuma, Arizona (Photo by Jeff Vanuga, Public Domain Image)

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).

Wind turbines in Zevenbergen, Netherlands (Photo by Peter Franken, Unsplash license)

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).

Bats are primarily eaten in Southeast Asia and West and Central Africa (Photo by Thierry Leclerc, CC BY-ND 2.0)

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).

A common vampire bat in at Sangayan Island, Peru (Photo by Acatenazzi, CC BY-SA 3.0)

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.