Light Pollution Affects Amphibians in the Environment
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Salamanders try to avoid predators by foraging at night, but if it never gets dark enough for them, then their foraging and their reproduction rates go down too. Tree frogs stop calling for mates in brightly-lit areas. The next paper summarizes some research done on these effects.
Studying the Ecological Impacts of Light Pollution on Wildlife: Amphibians as Models
Department of Biology. Utica College of Syracuse University, Utica, NY U.S.A.
As humans expand near and into natural habitats, fragile ecosystems become affected by our artificial night lighting. Amphibians are an important part of forest and aquatic ecosystems, and they are very sensitive to environmental changes. For example, they are effected by chemical pollutants because of their skin. Frogs and toads can hibernate in winter and even inside mud because oxygen can pass right through their skin, unfortunately, so can chemicals that they wouldn't want, if they had any choice. As such, they are important indicator species of the health of ecosystems. Their populations have been declining world-wide as a result of environmental stresses including increases in UV-B radiation (from ozone depletion), climatic change, habitat loss and destruction, acidification caused by acid rain and infection (e.g. fungal disease, chytridiomycosis). Light pollution may also contribute to global decline of amphibians, because many amphibians are nocturnally active or have biological rhythms regulated by light. Wise's paper summarized research looking at the impact of light pollution on amphibians, using some example laboratory experiments, field experiments, and natural (observational) studies.
Laboratory experiments used different lighting conditions to study the effects on cellular physiology, the organism's most fundamental level, for day length strongly effects gonadal maturation timing in many organisms. This level of inquiry includes cell division, cellular function, and periodicity (i.e. melatonin production and the natural rhythms of cells). In other words, growth, specifically tadpole growth, and other subsequent normal functions of the cell. Early experiments simply varied the number of hours a tadpole was exposed to similar levels of light. Either the tadpoles experienced constant light or periods of 12 hours of light followed by 12 hours of dark (12L:12D). Current investigations vary the light levels or spectral characteristics of the light on amphibians. One 1983 paper, studied light's impact on plasma melatonin in neotenic tiger salamanders (Ambystoma tigrinum). Melatonin is a master hormone that is regulated by photoperiods (it is produced only in dark periods and is inhibited by light). Under constant light, the melatonin production was reduced, this affected the salamander's thyroid hormones (involved in metamorphosis), gonadal development, reproductive behavior, skin coloration, thermoregulation, and ability to adapt visually to darkness.
More a recent laboratory investigation, used varied nocturnal illuminations on the African clawed frog, Xenopus laevis, to study its effect on tadpoles growth and metamorphosis. They found that increased levels of darkness directly contributed to a greater number of matured frogs. Metamorphosis was greatly reduced for those in constant light. So even low light levels at night (such as bright moonlight or man made lights) can delay metamorphosis. If this finding applies to other species of frogs which have limitations in the length of the larval stage due to drying (such as those in temporary ponds) or temperature (those in vernal pools), then such a delayed metamorphosis may decrease the chance of escaping a pool before it dries or cools and may increase tadpole mortality from artificial lights at night exposure. The Wikipedia link mentioned the unwelcome spread of this toads. Note that a use of light pollution as a means of control would also impact native species.
Simple field experiments show short-term effects of artificial light at night on amphibian behavior. In these experiments, researchers used artificial lights in natural habitats. A control to the experiments, no artificial light, however is difficult to achieve in light-polluted habitats, particularly with sky-glow. In one experiment, researchers examined the reproductive behavior (calling) and movement activity of male green frogs, Rana clamitans melanota, in Ontario, Canada that were exposed to artificial light by flashlight or a control (no artificial light) on moonlit nights (higher natural ambient illumination) or darker nights (new moon or cloudy nights, lower natural ambient illumination). In the artificial light treatment, frogs were lit by a flashlight for 5 min before observations began (habituation period, so that the eyes of the frogs could partially adapt to the rapid increase in illumination). Observations were made using an infrared (IR) viewer, since frogs cannot see IR light. The researchers found the number of calls reduced and movements by males increased in the artificially lighted treatment compared to the control, regardless of the natural ambient illumination (moonlight or no moonlight). A reduction in the number of calls by males may affect selection of mates (mate choice) by females. A long-term and widespread effect may cause changes in the population dynamics of frogs exposed to artificial night lighting.
Another simple field study examined the effect of artificial night lighting on the foraging activity of the redback salamander, Plethodon cinereus. These salamanders occupy the leaf litter in eastern North American deciduous forests, maintaining territories under cover objects (rocks and logs) that provide protection from predation and desiccation. Their above-ground foraging activity is limited by moisture during rainy or humid nights, these salamanders emerge from under the leaf litter and cover objects to forage on the forest floor. To see the effect of artificial night lighting on the redback salamanders' foraging, transects were defined in forested areas at the Mountain Lake Biological Station in Virginia, U.S.A. Half the transects were lit by strings of white mini-lamps and the control transects were not lit. Light levels were 0.01 lux (comparable to bright moonlight) on the forest floor in the lighted areas and 0.0001 lux in the control areas. The researchers systematically walked each transect in random order after dark and counted the number of salamanders found on the forest surface. Significantly more salamanders actively foraged on the forest floor in the dark transects than in the lighted transects. Chronic exposure to artificial night lighting should limit foraging opportunities, which leads to reduced growth and reproductive output, survival during winter hibernation (when salamanders do not feed and use up stored fat for energy), and population size and distribution.
In natural studies, experimental and environmental factors are not controlled and other factors, besides artificial light, may cause detected differences in lighted and unlighted areas. For example, artificial lighting often occurs in areas where there is habitat destruction or fragmentation. Thus, it is very important to have a control treatment (dark areas) that is similar in as many ways as possible to the habitat in lighted areas. One study examined the effect of artificial night lighting on foraging activity of common (European) toads, Bufo bufo, at Walton Lake, Milton Keynes, U.K. Tadpoles of this species often metamorphose into juvenile frogs simultaneously; thus, there is often a mass emigration of newly metamorphosed toads away from their aquatic environments. During one of these mass emigrations, the researcher counted more young toads in lit areas under street lamps than the darker control areas between these lamps. He hypothesized that toads gathered under street lamps because of the increased insect amount they use for food found there. While the lights may help the toads get a meal, they were also endangered by their locations on the roads by automobile traffic.
Florida Atlantic University
Boca Raton, Florida
E-mail: vandernoot at mail dot sci dot fau dot edu
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