DOI: 10.5061/dryad.s7h44j195
title: Multi-surveyor capture-mark-recapture as a powerful tool for butterfly population monitoring in the pre-imaginal stage

description: For many elusive insect species, which are difficult to cover by standard
 monitoring schemes, innovative survey methods are needed to gain robust
 data on abundance and population trends. The recording of pre-imaginal
 butterfly life stages provides great potential for ecological studies and
 conservation monitoring. However, using counts of pre-imaginal stages for
 quantitative research requires detection probability to be determined. We
 tested different removal and capture-mark-recapture (CMR) approaches to
 determine the detection probability for overwintering larvae of the
 endangered nymphalid butterfly Limenitis reducta. Classical removal and
 CMR studies require movement of the organisms under study but in our
 approach, we replaced movement of the study organisms by random movement
 of multiple different surveyors. The study was conducted in three plots
 within a spruce clear-cut in the 'Alb-Donau' region, Germany.
 Our dataset provides detection data of nine/ten different surveyors per
 study plot. The surveyors differed in their experience
 ('experts' vs. 'novices'). In the R-scripts we present
 the analysis of the data using i) the removal method, ii) CMR approaches
 with varying personnel expenditure. In addition, we test the validity of
 our method by comparing observed and simulated detection frequencies. The
 results of our study indicate that multi-surveyor removal/CMR techniques
 are highly suitable for estimating abundance of overwintering L. reducta
 larvae and that the proposed methodology has several strengths: long
 survey period, estimates of the absolute population size accompanied by
 uncertainty measures, estimates of overwinter mortality. The methods from
 our study can be adapted and used for several different butterfly species,
 other insect taxa with specific immobile life-stages, and some sessile
 organisms, e.g. elusive plants, fungi, or corals.
description: The data was collected by 13 surveyors of different experience on
 a spruce clear-cut near Merklingen (48.494° N, 9.788° E, 677 m a.s.l.).
 Within a time period of six weeks (5 February 2021 - 22 March 2021), the
 surveyors independently searched for hibernating larvae of <em>L.
 reducta</em>. Seven surveyors searched in all three plots, two
 surveyors searched in two plots, and four surveyors searched in only one
 plot. Consequently, the three plots were surveyed by ten, nine, and ten
 different surveyors, respectively. The surveyors were allowed to search
 for a maximum of two hours per plot. For most surveyors, this time budget
 allowed a very thorough search, aiming at finding all hibernacula in a
 plot. The surveyors conducted their searches, one after the other, without
 prior information and without seeing each other. The detected hibernacula
 were recorded with GPS coordinates and a photograph. After all surveyors
 had finished their searches, we counted the hibernacula detected by each
 individual surveyor and matched those with the records of the
 others. The raw dataset contains the detection
 histories for all hibernacula detected by at least one surveyor. The
 different columns represent the different surveyors/occasions. No further
 processing of the data was necessary prior to the analysis.
description: The tabular data (.xlsx) can be read with standard software
 programs. The analysis scripts (.R) can be read with the open-source
 software package R. We recommend the use of the interface
 RStudio.