Variable detectability in long-term population surveys of small mammals

From species detection to population size indexing: the use of sign surveys for monitoring a rare and otherwise elusive small mammal

Monitoring the occupancy and abundance of wildlife populations is key to evaluate their conservation status and trends. However, estimating these parameters often involves time and resource-intensive techniques, which are logistically challenging or even unfeasible for rare and elusive species that occur patchily and in small numbers. Hence, surveys based on field identification of signs (e.g. faeces, footprints) have long been considered a cost-effective alternative in wildlife monitoring, provided they produce reliable detectability and meaningful indices of population abundance. We tested the use of sign surveys for monitoring rare and otherwise elusive small mammals, focusing on the Cabrera vole (Microtus cabrerae) in Portugal. We asked how sampling intensity affects true positive detection of the species, and whether sign abundance is related to population size. We surveyed Cabrera voles’ latrines in 20 habitat patches known to be occupied, and estimated ‘true’ population size at each patch using DNA-based capture-recapture techniques. We found that a searching rate of ca. 3 min/250m2 of habitat based on adaptive guided transects was sufficient to provide true positive detection probabilities > 0.85. Sign-based abundance indices were at best moderately correlated with estimates of ‘true’ population size, and even so only for searching rates > 12 min/250m2. Our study suggests that surveys based on field identification of signs should provide a reliable option to estimate occupancy of Cabrera voles, and possibly for other rare or elusive small mammals, but cautions should be exercised when using this approach to infer population size. In case of practical constraints to the use of more accurate methods, a considerable sampling intensity is needed to reliably index Cabrera voles’ abundance from sign surveys.

Monitoring small mammal abundance using NEON data: are calibrated indices useful?

Small mammals are important to the functioning of ecological communities with changes to their abundances used to track impacts of environmental change. While capture–recapture estimates of absolute abundance are preferred, indices of abundance continue to be used in cases of limited sampling, rare species with little data, or unmarked individuals. Improvement to indices can be achieved by calibrating them to absolute abundance but their reliability across years, sites, or species is unclear. To evaluate this, we used the US National Ecological Observatory Network capture–recapture data for 63 small mammal species over 46 sites from 2013 to 2019. We generated 17,155 absolute abundance estimates using capture–recapture analyses and compared these to two standard abundance indices, and three types of calibrated indices. We found that neither raw abundance indices nor index calibrations were reliable approximations of absolute abundance, with raw indices less correlated with absolute abundance than index calibrations (raw indices overall R2 < 0.5, index calibration overall R2 > 0.6). Performance of indices and index calibrations varied by species, with those having higher and less variable capture probabilities performing best. We conclude that indices and index calibration methods should be used with caution with a count of individuals being the best index to use, especially if it can be calibrated with capture probability. None of the indices we tested should be used for comparing different species due to high variation in capture probabilities. Hierarchical models that allow for sharing of capture probabilities over species or plots (i.e., joint-likelihood models) may offer a better solution to mitigate the cost and effort of large-scale small mammal sampling while still providing robust estimates of abundance.

Improving detectability of the harvest mouse (Micromys minutus Pallas, 1771) by above ground live-trapping

Harvest mouse detectability represents a real issue for surveys, as usual ground-trap monitoring is known to fail to detect this species, especially in summer. The present study proposes to test ground versus aerial live-trapping efficiency for harvest mouse in summer, with a trapping design using paired ground and aerial traps over a 2-year survey in a reedbed. Over 10,720 trap-nights, the harvest mouse represented 85% of the 1078 small mammals captured, and it was the only species among five which was more often trapped above ground (n = 702) than on the ground (n = 213). Capture probability was significantly higher in aerial than in ground traps, with a slightly positive effect of leaf litter and, surprisingly, a negative effect of dense vegetation cover. Following these results, recommendations are made for harvest mouse monitoring.

A comparison of the effectiveness of camera trapping and live trapping for sampling terrestrial small-mammal communities

Context. There is an increasing reliance on the use of camera-trap technologies for surveys of medium to large terrestrial mammals. Camera trapping may, however, also have significant applications for broad-scale surveys of small mammals. Aims. The present study aims to compare results from camera-trapping surveys to those of the more traditional live-trapping techniques. Specifically, it aims to test the effectiveness of the techniques for detecting species, and the cost effectiveness of both approaches. Methods. Surveys were conducted across 36 sites in the Grampians National Park, Victoria, Australia, between April and July 2009. At each site, independent surveys were conducted for small mammals by using a combination of Elliot and cage trapping, then camera trapping. Results for the two different approaches were compared for both their ability to generate small-mammal presence data and their cost effectiveness. Key results. Camera-trapping surveys of 36 sites in the Grampians National Park compared favourably with those of live-trapping surveys. Similar species were detected across the sites, and camera trapping was a considerably more cost effective than live trapping. Conclusions. Camera-trapping surveys of small terrestrial mammals may provide a new and cost-effective technique for surveying terrestrial small mammals. This is particularly the case when presence data are the main requirement of the survey, with no requirement to capture and tag animals. Implications. Given the cost-effective nature of camera trapping, there is potential to use this approach to increase the level of replication and spatial coverage of small-mammal surveys. Improving the replication and spatial coverage of studies has the potential to significantly increase the scope of research questions that can be asked, thus providing the potential to improve wildlife management.