Optimization of capture–recapture monitoring of elusive species illustrated with a threatened grasshopper

Abstract

Information on population sizes and trends of threatened species is essential for their conservation, but obtaining reliable estimates can be challenging. We devised a method to improve the precision of estimates of population size obtained from capture-recapture studies for species with low capture and recapture probabilities and short seasonal activity, illustrated with population data of an elusive grasshopper (Prionotropis rhodanica). We used data from 5 capture-recapture studies to identify methodological and environmental factors affecting capture and recapture probabilities and estimates of population size. In a simulation, we used the population size and capture and recapture probability estimates obtained from the field studies to identify the minimum number of sampling occasions needed to obtain unbiased and robust estimates of population size. Based on these results we optimized the capture-recapture design, implemented it in 2 additional studies, and compared their precision with those of the nonoptimized studies. Additionally, we simulated scenarios based on thresholds of population size in criteria C and D of the International Union for Conservation of Nature (IUCN) Red List to investigate whether estimates of population size for elusive species can reliably inform red-list assessments. Identifying parameters that affect capture and recapture probabilities (for the grasshopper time since emergence of first adults) and optimizing field protocols based on this information reduced study effort (−6% to −27% sampling occasions) and provided more precise estimates of population size (reduced coefficient of variation) compared with nonoptimized studies. Estimates of population size from the scenarios based on the IUCN thresholds were mostly unbiased and robust (only the combination of very small populations and little study effort produced unreliable estimates), suggesting capture-recapture can be considered reliable for informing red-list assessments. Although capture-recapture remains difficult and costly for elusive species, our optimization procedure can help determine efficient protocols to increase data quality and minimize monitoring effort.

A roadmap for ladybird conservation and recovery

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Ecology and abundance of a relict population of the bush cricket Saga pedo in the Northern Apennines, Italy

“…While we carried out a relatively large number of (repeated) surveys at each sampling sites ( N = 20), we see the possibility to optimize the sampling effort to five‐ten surveys, similar to the approach of Bröder et al. ( 2020 ), which aims to identify the minimum number of sampling occasions needed to obtain unbiased and robust estimates of population size of elusive insect species. Surveys of S. pedo should be scheduled from the middle of May to the middle of June for juveniles and from the middle of June to the middle of July for adults, according to the peak of observations during our sampling sessions.…”

Avoiding bias in estimates of population size for translocation management

“…In contrast, when we fitted our translocation JS model, the AIC supported simpler models, and the top‐ranked model had fewer parameters (). This was achieved through being able to use covariates to explain patterns in capture, allowing optimization of future surveys (Broder et al, 2020). The reduced uncertainty and lack of boundary estimates produce realistic estimates of population size from this model.…”