Limitations of relative abundance indices calculated from camera-trapping data

Abstract

Abundance is an important parameter used to estimate the population size of various wildlife species. With the growing application of camera-traps (movement or heat activated) to monitoring wildlife, the relative abundance index (RAI) has become one of the most popular indicators of population abundance for inventories and assessment. Despite a simple and convenient indicator of population size, RAI obtained from camera-trapping data can be greatly affected by many factors such as species traits, detection rates and environmental factors. Therefore, we need verify the correlation between RAI and population density prior to its general application. So far, several types of RAIs have been developed based on camera-trapping data, and it is critical to compare these RAI indices and their applications. In this paper, we summarized the methods calculating RAI with camera-trapping data and reviewed their applications in wildlife monitoring and inventories in China. Four main types of RAIs were identified including (1) the number of days when one animal is photographed, (2) the number of photographs of focal species per 100 trap days, (3) the number of photographs of focal species per trap day, and (4) the proportion of photos from the focal species compared to all photos of all animals. Among them, the second RAI type is the most widely used (72.5%) in wildlife monitoring and inventories in China, and the fourth RAI type is used to compare species components in communities. Consequently, we recommend the second RAI type for estimating population abundance in particular when camera-trapping data are used for broad-scale comparisons over different spatial and temporal scales.