Credit: Christian Giese

Light pollution is a commonly occurring issue in urban areas and other areas of high human activity. An excess of artificial lighting, while it may be useful to us, has the potentially to negatively impact nocturnal or other light sensitive species. Bats make up the second largest order of mammals on earth but roughly a quarter of these species are threatened which may in no small part be due to the disruptive effects of light pollution (Stone et al 2015). However, some bat species have readily colonized city habitats so it is important to understand how artificial light may negatively impact these species

Exposure to artificial lights can significantly alter foraging and travel patterns in many bat species. Bats have been shown to alter their travel routes in order to avoid sources of artificial light (Kuijper et al 2008, Stone et al 2008). The avoidance of light sources by many bats effectively fragments their habitat. Additionally, alternate routes that bats may take in response to light pollution may be longer or impose a greater risk, such as predation, on bat species. Bats will also avoid foraging areas that are too well lit which can limit food availability and force bats to travel to farther or lower quality foraging sites (Polak et al 2011). However, not all bats avoid artificial light sources. Some species of insectivorous bat are actually drawn to artificial light sources as these sites often contain a higher density of insects (Schoeman 2015). These illuminated foraging sites are not without risk though as attraction to light sources put bats at a greater risk of being struck by vehicles and light levels have also been shown to interfere with a bat’s ability to avoid obstacles (McGuire and Fenton 2010, Stone et al 2015).

Bats can also experience light pollution at the roost. The presence of artificial light near bat roosts can artificially extend daylight conditions delaying the emergence of bats in the evening and reducing the time available for them to forage (Stone et al 2009). This may explain in part why light pollution around the roost sites has been linked to reduced growth rates in young bats (Stone et al 2015). Light pollution around roosts may also deter returning bats from re-entering the roost and in extreme cases may result in abandonment of the roost (Stone et al 2015).

While light pollution can have a number of impacts on bats the negative effects do not just stop there. Many bats provide essential ecosystem services which may be disrupted by artificial lighting. Fruit-eating bats in the tropics are seed dispersers and play an important role in the recolonization of abandoned farmland. However, the presence of artificial light near these abandon fields can cause fruit-eating bats to avoid the area which can delay the introduction of native plant life back into the area (Lewanzik and Voigt 2014). This is just one example of the cascading effects that light pollution can cause by disturbing bat species, but bats provide a range of important ecological services such as pollinating plants and regulating insect populations. In order to prevent the loss of these services and species that provide them measures need to be taken in order to reduce the impact of light pollution on bats. Some recent work has found that tree cover can help to mitigate the negative effects of light pollution so the expansion of green spaces within urban centers may be one viable method of conservation (Straka et al 2019). Other strategies such as limiting the number of streetlights near bat habitats or using lower intensity bulbs may also be viable, but more work still needs to be done to fully understand how light pollution impacts bat species and how it can be managed.

References

Kuijper DPJ, Schut J, van Dullemen D, Toorman H, Goossens N, Ouwehand J, Limpens HJGA (2008) Experimental evidence of light disturbance along the commuting routes of ponds bats (Myotis dasycneme). Lutra 51:37-49.

Lewanzik D, Voigt CC (2014) Artificial light puts ecosystem services of frugivorous bats at risk. Journal of Applied Ecology 51:388-394.

McGuire LP, Fenton MB (2010) Hitting the Wall: Light Affects the Obstacle Avoidance Ability of Free-Flying Little Brown Bats (Myotis lucifugus). Acta Chiropterologica 12:247-250.

Polak T, Korine C, Yair S, Holderied MW (2011) Differential effects of artificial lighting on flight and foraging behaviour of sympatric bat species. Journal of Zoology 285:21-27.

Schoeman MC (2015) Light pollution at stadiums favors urban exploiter bats. Animal Conservation 19:120-130.

Stone EL, Jones G, Harris S (2009) Street Lighting Disturbs Commuting Bats. Current Biology 19:1123-1127.

Stone EL, Harris S, Jones G (2015) Impacts of artificial lighting on bats: a review of challenges and solutions. Mammalian Biology 80:213-219.

Straka TM, Wolf M, Gras P, Buchholz S, Voigt CC (2019) Tree Cover Mediates the Effect of Artificial Light on Urban Bats. Frontiers in Ecology and Evolution https://doi.org/10.3389/fevo.2019.00091

Credit: Heather Paul, CC by 2.0

Human activity in an area whether it be urban development, farming, or tourism can have significant impacts on wild species. Among the species most effected by human activity are large carnivores. Large carnivores struggle to coexist with humans because they often face persecution for the perceived threat they present to livestock and residence. Additionally, large carnivores can often occupy large territories and human activity in an area can limit the abundance of acceptable territory. However, not all carnivores suffer equally in the face of human encroachment some species like the spotted hyena (Crocuta crocuta) have been shown to adapt to and even thrive in areas of human activity.

The spotted hyena is often portrayed in popular media as a villainous scavenger while in actuality it is a top predator and keystone species in many of the environments it inhabits. Spotted hyenas are also highly social forming matriarchal clans of up to 80 individuals. So how then does a large predator living in such large social groups manage to persist in human disturbed areas? It may in part be due to the negative effects that human activity has on other carnivore species. The African lion (Panthera leo) is another top predator and the main competitor of the spotted hyena. Lions and hyenas will often steal each other’s kills as well as killing isolated or juvenile individuals that they come across. However, the African lion has shown striking declines in population size in response to human disturbance (Green et al 2017). The decline of their main competitor and predator has allowed hyena populations to grow rapidly in human disturbed areas (Green et al 2017). However, this fact alone is not sufficient to explain the success of the spotted hyena. Why does it not suffer the same fate as its rival? Well the answer may be in part due to the hyena’s iron stomach. Although hyenas will hunt and kill the majority of the food, they eat they have shown a remarkable ability to consume almost any organic material they can scavenge (Yirga et al 2012). In the wild hyenas consume even the bones, hair, and hooves of their prey, and will consume putrefied or diseased carcasses they encounter. While near human settlements hyenas will eat a wide variety of garbage from offal, to scraps, to even feces (Yirga et al 2012). The incredible flexibility of their diet may allow hyenas to utilize anthropogenic food sources that other carnivore species are unable to consume.

However, not all aspects of human disturbance are so beneficial to spotted hyenas. Although hyenas can consume putrefied food with seemingly few consequences, they are still vulnerable to disease introduced as a result of human activity. The introduction of canine distemper virus to Africa led to an outbreak in the 90s that decimated lion and hyena populations in the Serengeti (Marescot et al 2018). However, while lion populations recovered in a matter of years spotted hyena populations took nearly five times as long to recover (Marescot et al 2018). This is due to the low reproductive rate of hyenas. Hyena reproduction is a tricky business as females are very masculinized and possess a pseudo-penis through which copulation and birthing must occur. This makes reproduction stressful and painful for hyenas and human disturbances have been shown to increase levels of stress hormones in hyenas which may make reproduction even more taxing (Van Meter et al 2009). Finally, hyenas only tend to have 1 or 2 cubs in a litter and mothers will nurse their cubs for up to a year and a half, much longer than other carnivore species. All these factors taken together mean that hyenas are slow to recover from populations declines.  In addition to their slow rate of recovery, human disturbance may also make hyenas more vulnerable to future threats. A study by Belton et al (2017) found that hyena clans inhabiting areas of higher human activity had lower connectivity within their social groups than clans in less disturbed areas. This lower connectivity may make clans more vulnerable to future disturbances (Belton et al 2017).

While hyenas may benefit from human activity they do not do so without risk. Although populations may be thriving locally the global population of spotted hyenas is on the decline due to habitat loss and persecution by local peoples. While spotted hyenas are currently listed as least concern by the IUCN their low reproductive rate could potentially make recovery difficult if their population were to decline to much. Therefore, it is important to understand how human activities may affect the long-term trends of this population and to change public perception of spotted hyenas in order to garner support for conservation efforts.

Credit: Oliver Honer

References

Belton LE, Cameron EZ, Dalerum F (2017) Social networks of spotted hyaenas in areas of contrasting human activity and infrastructure. Animal Behaviour 135:13-23.

Green DS, Johnson-Ulrich L, Couraud HE, Holekamp KE (2018) Anthropogenic disturbance induces opposing population trends in spotted hyenas and African lions. Biodiversity and Conservation 27:871-889.

Marescot L, Benhaiem S, Gimenez O, Hofer H, Lebreton JD, Olarte-Castillo XA, Kramer-Schadt S, East ML (2018) Social status mediates the fitness costs of infection with canine distemper virus in Serengeti spotted hyenas. Functional Ecology 32:1237.

Van Meter PE, French JA, Dloniak SM, Watts HE, Kolowski JM, Holekamp KE (2009) Fecal glucocorticoids reflect socio-ecological and anthropogenic stressors in the lives of wild spotted hyenas. Hormones and Behavior 55:329-337.

Yirga G, De Iongh HH, Leirs H, Gebrihiwot K, Deckers J, Bauer H (2012) Adaptability of large carnivores to changing anthropogenic food sources: diet change of spotted hyena (Crocuta crocuta) during Christian fasting period in northern Ethiopia. Journal of Animal Ecology 81:1052-1055.