Demographic parameters of sexes in an elusive insect: implications for monitoring methods

Abstract

AbstractEstimating demographic parameters in rare species is challenging because of the low number of individuals and their cryptic behaviour. One way to address this challenge is to gather data from several regions or years through mark‐release‐recapture (MRR) and radio‐tracking monitoring. However, the comparison of demographic estimates obtained using these methods has rarely been investigated. Using 5 years of intensive MRR and radio‐tracking surveys of an elusive and endangered saproxylic insect, the hermit beetle (Osmoderma eremita), in two regions of France, we aimed to estimate population size at the adult stage for each sex separately and to assess differences in demographic parameter estimates between survey methods. We found that males were approximately three times more likely to be recaptured than females. Taking this into account, we determined that the sex ratio was male‐biased in almost all populations, except in Malus trees, where it was female‐biased. Temporal fluctuations of sex ratios were also detected in one region. The radio‐tracking transmitter (450 mg) allowed only the largest individuals (>2 g) to be targeted. However, we found that, among non‐equipped individuals, the larger males survived better than the smaller males. We also confirmed that transmitter‐equipped individuals survived approximately 25 % better than non‐equipped individuals. Extrapolating the estimates from radio‐tracking surveys to the population scale may result in overly optimistic population projections. Our results revealed large temporal and spatial variations in population size and sex ratios. This knowledge is crucial for predicting the persistence of small populations in fragmented landscapes. This study also questioned the representativeness of radio‐tracking surveys for insect species in estimating demographic parameters at the population scale.