Differentiating Footprints of Sympatric Rodents in Coastal Dune Communities: Implications for Imperiled Beach Mice

Identifying techniques for conducting frequent, effective, and inexpensive monitoring of small mammals can be challenging. Traditional approaches such as livetrapping can be laborious, expensive, detrimental to animal health, and ineffective. Passive approaches such as tracking (e.g., from tracks on the ground or footprints collected at a tracking station) have been shown to lessen those burdens, but a problem with tracking, particularly for rodents, is the uncertainty in identifying species from footprints. To address the need for a more accurate method of identifying small mammal tracks, we measured footprints from live-captured rodents and developed a classification tree for distinguishing between subspecies and species using footprint widths treated as having known or unknown identification. We captured rodents within or near the coastal dunes of Florida and Alabama with a focus on areas occupied by threatened and endangered beach mice Peromyscus polionotus subspp., whose populations warrant regular monitoring but whose tracks are not easily distinguished from those of some sympatric species. We measured 6,996 front and hind footprints from 540 individuals across eight species. The overall accuracy of our classification tree was 82.6% and we achieved this using only the front footprint width. Footprint width cutoffs for species identification were < 5.5 mm for house mice Mus musculus, 5.5–6.7 mm for beach mice, and 6.7–8.3 mm for cotton mice Peromyscus gossypinus. We were most successful in confirming the identity of beach mice: we correctly classified approximately 94% of beach mice, while we misclassified fewer than 6% as house mice and fewer than 1% as cotton mice. When we input a beach mouse individual into the classification tree as of an unknown species, we correctly identified 78.1% of individuals as beach mice from their tracks, and most incorrect identifications were of house mouse tracks. Our study demonstrates that researchers can identify sympatric rodent species in coastal dune communities from tracks using quantitative classification based on footprint width. Accurate identification of beach mice or other imperiled species from tracks has important management implications. Not only can wildlife managers determine the presence of a species accurately, but they can monitor populations with considerably less effort than livetrapping requires. Although our study was specific to coastal dune communities, our methods could be adapted for the creation of a classification tree for identifying tracks from suites of species in other areas.

Increasing Detection by Reducing Disturbance and Excluding Nontarget Small Mammal Species: An Occupancy Study Approach

Track tubes are a noninvasive, efficient method to monitor populations of small mammals that can be implemented on a large landscape scale and are a cost-effective approach for certain sampling situations. As with all field sampling tools, modifications are made depending on research objectives, habitat being sampled, and target species. We conducted two experiments with the objective to increase efficiency and decrease labor while retaining high detection probabilities as part of an Annual Multi-Agency Regional Southeastern Beach Mouse Peromyscus polionotus niveiventris Habitat Occupancy Survey. We conducted studies along a contiguous 72-km coastline of the Cape Canaveral Barrier Island Complex in east-central Florida, USA, that includes the Kennedy Space Center, Cape Canaveral Air Force Station, and Canaveral National Seashore. We conducted the experiments to address several issues that had regularly compromised our survey data. One experiment assessed the ideal length of deployment times for track tubes in various habitat types with multiple species where southeastern beach mice were previously detected. Another experiment, determined how to restrict certain meso-mammal species (raccoons Procyon lotor, and eastern spotted skunks Spilogale putorius) from knocking over the tubes or reaching into the tubes, thus reducing disturbance that results in missing data; or from entering the tubes (cotton rats Sigmodon hispidus), thereby obscuring any potential or actual footprints of the southeastern beach mouse or other targeted species. We observed an increase in the detection rate of southeastern beach mice in track tubes with increased nights of deployment (likelihood ratio test χ2 = 18.71, df = 3, P < 0.001) with the greatest increase between 3 nights and 6 nights and apparent leveling off between nights 9 and 12 and a large decrease in the detection rate of southeastern beach mice in track tubes with 0.5-in. (1.3-cm) excluders compared with the other-size excluders (likelihood ratio test χ2 = 167.89, df = 5, P < 0.001). We also found that 1-in. (2.5-cm) excluders prevented access by meso-mammals or entry by cotton rats and did not adversely affect the detection of either beach mice or cotton mice Peromyscus gossypinus, but did reduce disturbance and resulted in fewer missing data. These statistically significant process improvements have application for others involved with small mammal monitoring and species management.