Aim Artificial light at night (ALAN) is an unprecedented stressor recently introduced in the abiotic milieu of natural landscapes. As such, understanding how ALAN and other natural stressors act in concert to shape the spatial distribution of biodiversity is a core goal in conservation ecology. Here, we aim at understanding how ALAN and climate interact with life-history traits and courtship signalling systems to dictate the composition of firefly communities in a global biodiversity hotspot. Location An extensive elevational gradient in the Atlantic Rainforest (Brazil) currently known as the hottest hotspot of fireflies on Earth. Methods We used multivariate species distribution models to understand how species traits and courtship signalling systems interact with climate and ALAN to determine species abundances within firefly communities. We also investigated how species-specific responses to climate and ALAN scale up to determine compositional changes in firefly communities along the elevational gradient. Results We found that climate shapes communities by filtering species according to their body size and trophic position. ALAN dictates the dominant courtship signalling system within communities by affecting the abundance of species that use bioluminescence or a combination of bioluminescence and pheromones in courtship. We also found that associations between beta-diversity and ALAN were non-stationary, being higher in regions under low levels of light pollution. This suggests that even incipient increases in ALAN within protected areas can yield fast changes in the composition of firefly communities. Main Conclusions Firefly responses to climate and ALAN are modulated by traits associated with different facets of their life histories. Given the alarming changes in both stressors predicted for the foreseeable future, our findings indicate that firefly communities are vulnerable to compositional changes even within protected areas. --

Sampling was carried out in the Serra dos Órgãos National Park (22°28ʹ S, 43°02ʹ W WGS 84), one of the oldest conservation units in Brazil that spans 20,024 ha. with elevations ranging from 80 to 2263 meters above sea level. Along an elevational transect that ranges from 100 to 2100 meters above sea level, two flight-interception Malaise traps were placed at elevational intervals of approximately 150 meters, summing up to 15 sampled elevations (see Supp. Material Fig. I). The collecting bottle of each trap was replaced in monthly intervals (30 days) from December 2014 to November 2015, summing up to approximately 360 samples in total (2 traps × 15 elevations × 12 months). To control for potential false absences associated with intra-annual dynamics in population demographics, we considered all species sampled in a given elevation over the year as the operational definition of community used in further analyses. This allowed for more accurate characterization of the spatial structure of firefly communities because it maximized the detection of: (i) species that coexist as juveniles in a focal site, but that emerge as (detectable) flying adults in different moments of the year, and; (ii) species that emerge as flying adults in different locations and during previous moments in time but that immigrate to the focal location and become part of the existing community.

A data logger was placed close to each pair of traps to record hourly variation in temperature and air relative humidity over the entire sampling period. We then calculated mean daily temperature, mean daily thermal amplitude, and mean air relative humidity in each site over the whole sampling period.

To obtain data on ALAN, we used our samples' geographic coordinates to extract radiance values (i.e., amount of light emitted or reflected by a surface that is detected by an optical system, units in nW/cm2×sr) estimated from the satellite imagery fostered by the United States National Oceanic and Atmospheric Administration Earth Observation Group (https://ngdc.noaa.gov/eog/download.html) and processed by the Radiance Light Trends web application (Stare & Kyba, 2019). This web application uses data from visible infrared imaging radiometer day-night-band (VIIRS DNB) satellite system to display information about radiance at the resolution of 0.5 square km. The VIIRS DNB satellite system measures ALAN in the broad spectral range of 500 to 900 nm. This spectral range overlaps with the spectral sensitivity of photoreceptors in firefly species, being an appropriate measure of the large-scale effects of ALAN on the sensory environment of fireflies.