Human-caused degradation of the natural environment is increasing, and a major driver of such degradation is artificial light at night (ALAN)1,2. Over the last 100 years, since lighting technologies were developed and urbanization has progressed3, ALAN has disrupted the natural nocturnal environment, worldwide. Presently, 23% of the earth’s land mass experiences ALAN4, and the level of light pollution is growing approximately 2% a year5. Because most terrestrial biota have been exposed to regular cycles of sunlight and darkness throughout evolutionary history, disruption of the nocturnal environment by ALAN has ecological impacts on the biota6.
In just a few decades, there has been a broad range of studies on the impacts of ALAN. For example, ALAN can alter foraging behaviour7,8,9, migratory behaviour10,11, physiology12,13 and mortality rates14. More recently, some studies have found that population dynamics15 and species interactions16 can also be altered by ALAN. Whereas it is clear that ALAN can have significant effects on wildlife, its impacts may be complex. For example, many kinds of nocturnal invertebrates, such as beetles, flies, and moths, are attracted to artificial light 17,18, which may have significant implications for populations and communities of these groups19,20. In addition, however, attracted invertebrates are a food source for nocturnal predators of invertebrates, such as geckos, anurans, bats, and birds21,22,23. Although such phenomena are well documented21,23, few studies quantify the effect of increased food availablity on predators, although they may be among the strongest impacts of ALAN. If, for example, mesopredators are attracted to areas with ALAN, and then consume other groups, this could be an important impact on community dynamics, given the impact of mesopredator release in other systems24.
Furthermore, the effect of increased food availability on predators of invertebrates caused by ALAN may be influenced by various environmental factors, because the amount of invertebrates attracted to ALAN probably varies with weather and environment. For example, increasing temperature or rainfall may increase the number of invertebrates attracted to ALAN1,6. On the other hand, increasing wind speed and ambient light, such as moon light, may decrease the effect of artificial light on invertebrate activity1,6. It is likely that these effects on invertebrate activity flow on to influence predation success. This chain of reasoning has not been examined, but to better predict the impacts of ALAN on predators of nocturnal invertebrates, it appears we may need to understand the influence of a variety of environmental variables on predation success.
Although many previous studies have examined the effects of ALAN on native species, few have examined interactions between ALAN and invasive species25. While both ALAN and invasive species are global issues causing biodiversity loss and degradation of ecosystem function, these issues have been considered separately25. They may interact, however, because invasive species often proliferate in urban areas, which are a major source of ALAN. There are many reasons why invasive species inhabit disturbed environments26, and ALAN may be a contributing factor. For example, the abundance of invasive house geckos was higher in artificially lit environments27, and they may be more willing to use artificially lit environments to obtain food resources than are native geckos, suggesting ALAN might contribute to their invasion success, globally28. Revealing the potential impact on invasive species of ALAN could provide important insights for the management of invasive species.
Cane toads (Rhinella marina) are a tropical invasive species, originally native to south America29,30. Originally introduced to control agricultural pests31, they have been introduced to the Carribean, most Pacific Islands, several Japanese Islands, and to Papua New Guinea and Australia. Famously, cane toads produce highly toxic secretions stored in their parotoid glands, which are used as anti-predator defences30. It has been well documented in Australia that when native predators, including snakes, lizards, crocodiles, and marsupials attempt to consume toads, they are often poisoned31,32,33,34,35,36. Thus toads typically have negative impacts on the native biota, and require management globally. Toads feed on nocturnal invertebrates, and ALAN provides an artificially large food resource for toads37,38. Although ALAN may have a positive effect on the invasion success and proliferation of toads, environmental factors influencing the food intake of toads have not been investigated. Revealing environmental factors associated with the indirect effects of ALAN on toads could contribute to efficient management of invasive cane toads.
Here, we experimentally quantified the influence of ALAN on food intake in toads. In addition, we determined the effect of four environmental factors: ambient light, temperature, rainfall and wind speed on food intake by toads in the field. We constructed field enclosures supplied with artificial light for toads in northeast Australia. The toads were allowed to feed freely overnight, after which they were euthanised, and we measured the mass and taxonomic composition of their gut contents, and categorized them as ‘flying’ or ‘non-flying’. We hypothesized that the mass of the gut contents of toads would increase with increasing temperature and rainfall, and with decreasing wind speed. We based these predictions on the likely impact of these environmental variables on invertebrate activity1,6. Similarly, we hypothesized that the mass of toad gut contents would increase with decreasing lunar phase, and with ambient light pollution levels. We based these predictions on the likely impact of light sources (i.e., the moon and ALAN) on invertebrate activity in the vicinity of our artificial light, and assumed that toads would eat more if more invertebrates were available.
Source: Ecology - nature.com
