Changing light conditions due to human activities represents an important emerging environmental concern. Although changes to natural light conditions can be independently detrimental, organisms commonly face multiple stressors simultaneously. Yet, few studies consider these interactive effects. This study aims to identify both the direct effects of changing light conditions on fitness as well as the interactive effects of these changing conditions when they occur concomitantly with additional stressors. To test this, we exposed a model amphibian (wood frog; Lithobates sylvaticus) to three anthropogenic light conditions: control, intensified illuminance, and artificial light at night (ALAN: intensified illuminance + extended photoperiod). We measured (1) metrics of fitness (hatching success, survival to, size at and time to metamorphosis), (2) susceptibility to an anthropogenic stressor, road salt (NaCl), (3) susceptibility to a common parasite (trematode), and (4) two potential mechanisms that may affect these parameters: tadpole behavior (activity levels) and stress response via corticosterone (CORT) release rates.
Results/Conclusions
We found that both the intensified illuminance and ALAN treatments reduced hatching success. In contrast, for amphibians that successfully hatched, neither treatment affected amphibian survival or time to metamorphosis. However, individuals exposed to ALAN were larger at metamorphosis. While amphibian susceptibility to NaCl was not significantly affected by ALAN, amphibians exposed to the ALAN treatment were significantly more susceptible to trematodes. Finally, tadpoles exposed to ALAN moved significantly less than tadpoles in the control and intensified illuminance treatments, while light conditions had no effect on CORT release rate. Overall, we demonstrate that changes in light conditions, in particular ALAN, had significant consequences on an amphibian model in laboratory conditions. This work underscores the importance of considering not only the direct effects of light on fitness metrics but also the interactive effects of light with other abiotic and biotic stressors. Anthropogenic-induced changes to light conditions on natural ecosystems are expected to continue increasing over time and understanding the diverse consequences of shifting light conditions will be paramount to protecting wildlife populations.