Effects of artificial light at night on obligatory nocturnal mammals

Background/Question/Methods

Artificial light at night (LAN) has been shown to affect various organisms. Obligatory nocturnal animals can be expected to be particularly affected by LAN, since LAN is present in urban and also in many rural habitats during their entire activity period. Although certain predator species may benefit from LAN by hunting insects that are attracted to outdoor lamps, LAN mostly comes along with negative consequences for species and whole communities. For example, some bat species avoid commuting and foraging in artificially illuminated areas probably due to the improved visibility to their predators. Consequently, urban bat species richness is usually quite low. LAN at commuting routes or foraging grounds might force bats to use less profitable sites, which could impair body condition. Similarly, young bats that had grown up in roosts situated in artificially illuminated buildings did not reach as much body mass before hibernation as conspecifics in dark maternity roosts, potentially resulting in reduced winter survival. Food scarcity and thermoregulatory demands may cause chronic stress which can in turn compromise immune functions. Additionally, LAN can alter immune responses via a reduction in melatonin levels. Dim LAN was sufficient to alter the immune competence of hamsters for example. Supposedly, the dual effect of LAN on the immune system via resource limitation and reduction of pineal melatonin is detrimental for survival under natural conditions.

As part of the transdisciplinary research platform “Loss of the Night” (www.VerlustDerNacht.de), we investigate the influence of the spectral composition of street lights on urban bat assemblages and how light-induced changes in foraging behaviour may translate into altered food web dynamics. Further, we ask whether LAN modulates stress hormone physiology and immune system of bats. These traits can be directly associated with survival and therefore are of specific conservation concern. Lastly, we work to understand how LAN may affect the ecosystem services provided by bats.

Results/Conclusions

We expect LAN to repel light-sensitive bat species and with them their potential to control pest insects and – in the tropics – to pollinate flowers and disperse seeds. Those bats that stay in spite of LAN and forage on insects around artificial lights are predicted to cause a shift of the entire food web, since they apply top-down control only of those insect groups that accumulate at lights. Further we assume that LAN constitutes a stressor, which causes an increase in glucocorticoid levels and a suppression of the immune competence.