Few reports have been published on detection distances of bat calls because the evaluation of detection distance is complicated. Several of the approaches used to measure detection distances are based on the researcher’s experience and judgment. More recently, multiple microphones have been used to model flight path. In this study, the validity of a low-cost and simple detectability metric was tested. We hypothesize that the duration of an echolocating-bat-pass within the area of an ultrasonic bat detector is correlated with the distance of detection. Two independent datasets from a large-scale acoustic bat survey—a total of 25,786 bat-passes from 20 taxa (18 species and two genera)—were measured. We found a strong relationship between these measures of bat-pass duration and published detection distances. The advantages of bat-pass duration measures are that, for each study, experimenters easily produce their own proxy for the distance of detection. This indirect measure of the distance of detection could be mobilized to monitor the loss in microphone sensitivity used to monitor long-term population trends. Finally, the possibility of producing an index for distance of detection provides a weight for each bat species’ activity when they are aggregated to produce a bat community metric, such as the widely used “total activity”.

The ongoing environmental crisis, driven by human activities, has resulted in significant biodiversity losses across various taxa, affecting ecosystem functioning. To deal with this crisis, policymakers have notably established the Kunming-Montreal Global Biodiversity Framework, which includes targets to mitigate biodiversity loss by 2050. To achieve this goal, reliable and ecologically relevant indicators are essential to quantify and qualify biodiversity changes. Temporal trends in species abundance or occurrence have been proposed as useful indicators. In France, the Vigie-Nature program engages volunteers in biodiversity monitoring through various schemes, thereby producing relevant data to estimate country-wide temporal trends for various taxonomic groups. Some indicators of population trends are already produced for some taxa, but the analysis pipelines remain unpublished and need extensions to accommodate monitoring schemes collecting presence/absence instead of abundance data, such as the Vigie-flore plant monitoring scheme. Here, we present a newly developed analysis pipeline to estimate population trends, which handles different data types and protocol specificities, and goes beyond linear population trends by considering multiple time periods and visualizing non-linear dynamics. In addition to introducing the methodology and making it available, we ran this pipeline to produce population trends for 148 bird and 181 plant species in France, based on abundance data from STOC (French Breeding Bird Survey) and occurrence data from Vigie-flore schemes. Results show as many increasing as decreasing bird population trends over the past 23 years, and a tendency for more decreasing than increasing plant population trends over the past 15 years, thereby revealing significant changes in community composition. Specifically, for birds, most habitat generalist species showed stable or increasing population trends, while most habitat specialist species showed stable or decreasing population trends, suggesting biotic homogenization. This pipeline and first analyses provide an unprecedented overview of bird and plant population trends, and contribute to the production of biodiversity indicators based on open science and reproducible research.

Roads impact bat populations through habitat loss and collisions. High quality habitats particularly increase bat mortalities on roads, yet many questions remain concerning how local landscape features may influence bat behaviour and lead to high collision risks (e.g. influence of distance to trees, or of vegetation density). When comparing the potential danger of different road sections, the most popular method today is the use of simple bat detectors to assess the local densities of current populations at road sites. Yet, it is not known to which extent bat behaviour influences collisions (i.e. bats flying at vehicle height or on the side or above, co-occurrence of bats and vehicles). Behaviour is very rarely taken into account in practice, and this might lead to hazardous site selections for mitigation. Our goals were thus (i) to estimate how local landscape characteristics affect each of the conditional events leading to collisions (i.e. bat presence, flight in the zone at collision risk and bat-vehicle co-occurrence), and (ii) to determine which of the conditional events most contributed to collisions risks. In this study, we recorded bat activity and characterised flight behaviour with three variables: position at collision risk, bat-vehicle co-occurrence, and flight path orientation, using acoustic flight path tracking at 66 study sites in the Mediterranean region for two to five full nights. We modelled the effect of the local landscape, i.e. in a radius of 30 m around the road (vegetation height, distance, density and orientation), road features (road width, traffic volume) and the time of year on eleven species or species groups. We built models for each conditional probability of the road collision risk (i.e. species density, presence in the zone at risk, bat-vehicle co-occurrence) and multiplied their estimates to calculate the overall collision risk. Our results show that the local landscape had different effects on bat density and presence in the zone at collision risk. Increasing distance to trees and decreasing tree height were associated with a decrease in bat density at roads. Forests were the local landscapes where bats flew more often in the zone at collision risk. The overall collision risk was higher either in forests or at tree rows perpendicular to the road depending on species. Contrary to common preconceptions, mid-range echolocators seemed to be generally more at risk of collision than short-range or long-range echolocators. In addition, collision risk was greatest in summer or autumn for most species. Finally, bats mainly followed the road axis regardless of the type of landscape. Our results contribute to a better understanding of bat movements in different local environments at the scale where they directly sense their surroundings with echolocation calls. Disentangling bat density from flight behaviour allowed us to better understand the temporal and spatial contributors of roadkills, and to provide guidance for road impact assessment studies.