White-Footed Mouse Populations Run Higher in Northeastern Pennsylvania, Survey Finds

White-footed mice, the primary reservoir of the bacterium that causes Lyme disease, are significantly more abundant in northeastern Pennsylvania than in the northwestern part of the state, according to a multi-year live-trapping survey that also found noticeably richer small mammal communities in the east. The study, published in Ecology and Evolution, provides the first direct regional comparison of white-footed mouse (Peromyscus leucopus) population sizes and small mammal species richness spanning multiple counties in Pennsylvania, a state with the fifth highest Lyme disease incidence rate of any US state between 2010 and 2023.

Researchers from East Stroudsburg University trapped small mammals at 38 field sites across four counties: Monroe and Pike in the northeast (NEPA) and Crawford and Mercer in the northwest (NWPA). Each county contained nine sites, an even mix of public parks, residential communities, and schools. Trapping followed 825-meter transects along trails and forest edges, with Sherman traps placed every 10 meters along a parallel trap line. Trap numbers varied between 73 and 91 per site, averaging 83.9. Field work spanned seven trapping seasons from Fall 2022 through Summer 2025, with three consecutive trapping nights per site per season. Two sites with extremely low capture rates were discontinued and replaced, and were excluded from population size analyses.

Across all seven seasons, the team recorded 8,298 small mammal captures from 5,250 unique individuals, including 3,040 recaptures, out of 56,168 capture opportunities, an overall capture rate of 14.78 percent. White-footed mice dominated: 6,873 captures from 4,337 unique individuals, a capture rate of 12.24 percent. Thirteen species were positively identified overall. After P. leucopus, which accounted for 89.55 percent of total captures, the most frequently caught species were the northern short-tailed shrew (3.40 percent), the southern red-backed vole (2.62 percent), and the eastern deer mouse (1.83 percent), followed by the eastern chipmunk, meadow vole, southern flying squirrel, and several rarer species including the masked shrew, woodland jumping mouse, smoky shrew, American red squirrel, meadow jumping mouse, and hairy-tailed mole.

Species richness differed sharply between regions. NEPA sites averaged 5.25 species while NWPA sites averaged 2.83, a statistically significant difference. Within regions, however, richness did not differ significantly between constituent counties: Monroe and Pike were statistically indistinguishable, as were Crawford and Mercer. The authors suggest the regional gap may reflect physiography. Most sites fell within the Appalachian Plateau, but parts of NEPA lie in the Ridge and Valley province, and the regions belong to different ecological provinces: NWPA sites sit in the Eastern Broadleaf Forest (Oceanic) province while NEPA sites are mostly in the Laurentian Mixed Forest. NEPA sites also showed far greater topographic variation, with average elevation of 407.6 meters and a standard deviation of 143.1 meters, compared with 372.1 meters and 58.2 meters in NWPA. Notably, the eastern deer mouse appeared at 15 of 20 NEPA sites and none of the NWPA sites.

To estimate population sizes, the researchers used robust design mark-recapture modeling in program MARK, treating each trapping day as a capture event, 21 events in total across the seven seasons. The best-fitting model estimated survival as a function of time and abundance of unobserved individuals as a function of both time and region, while estimating initial capture and recapture rates separately.

Site-level population estimates, averaged across capture seasons, ranged from 7.93 to 45.97 mice. Populations were significantly higher in NEPA, averaging 28.63 mice with a standard deviation of 10.10, than in NWPA, averaging 19.68 with a standard deviation of 7.81. As with richness, counties within the same region did not differ significantly. Seasonal estimates varied qualitatively across years: statewide summer averages peaked in 2024 at 41.2 mice, after 28.7 in summer 2023 and before 22.6 in summer 2025. Fall estimates also peaked in 2024 at 37.8, following 20.7 in fall 2022 and 17.0 in fall 2023. The lowest statewide average, 9.2, occurred in Spring 2023. The regions also differed in seasonal shape, with NEPA showing a single peak in summer 2024 while NWPA showed two peaks, in fall 2023 and summer 2025.

The authors connect these fluctuations to the cyclical population dynamics documented in rodents, which in white-footed mice have been tied to acorn crops in eastern deciduous forests. They caution, however, that three years of surveying is shorter than the roughly four-year average period of a full population cycle reported in earlier work, so the observed patterns should be read as qualitative fluctuations rather than a complete picture of the cycle. They note that such fluctuations can ripple through ecosystems, influencing pathogen prevalence in blacklegged ticks and the abundance of prey arthropods that affect oak health, and they call for surveys of annual acorn crops across both regions to clarify the drivers of regional differences.

When the researchers tested whether species richness and white-footed mouse population size were related, the answer was no, at least not significantly. Across both regions combined, the linear model yielded a positive but non-significant estimate of 1.67 (standard error 0.83, p = 0.052), and neither region alone showed a significant relationship. The authors point out that a previous study in southeastern Canada did report a significant positive correlation between the two, a result they describe as counterintuitive given the mouse’s status as a generalist that thrives in disturbed habitats. They suggest that extending the study to more sites and additional seasons, which remain ongoing, may clarify whether the borderline trend reflects an underlying biological process.

The findings carry potential implications for tick-borne disease risk assessment. White-footed mice are a highly competent reservoir for Borrelia burgdorferi, and their abundance has been directly linked to greater prevalence of the bacterium in blacklegged ticks and to general Lyme disease risk. Because the mice were observed at every site, their mere presence cannot predict tick host availability; instead, the authors argue, community diversity and the mouse’s relative abundance within communities offer better insight. The lack of a significant association between richness and mouse abundance suggests the mouse’s population size is independent of host community composition in this system, but the authors emphasize that assessing tick density and pathogen prevalence is necessary to determine whether a dilution effect operates here. They also note that higher species richness can increase the chance of disease emergence, so the human health implications of changing community structure are not straightforward.

The study has limitations the authors acknowledge openly. Trapping was restricted to trails and forest edges over roughly 750 meters, likely underestimating true species richness, particularly for species associated with contiguous forest. Trap disturbance by mesocarnivores, mainly raccoons in NWPA and black bears in NEPA, reduced effective sampling effort, prompting the deployment of raccoon exclusion devices at some sites. Because the communities are open populations, the spatial bounds of animal movement are difficult to determine, so the estimates are population sizes rather than densities and are best used for comparisons across time and among sites rather than against other systems. Even so, the authors conclude, the data provide novel and detailed information on the structure of a tick-borne pathogen reservoir community, and the regional baseline they establish may help inform integrated pest management planning and regional assessments of tick-borne pathogen infection risk as Lyme disease continues to expand westward in Pennsylvania.

Subject of Research: Regional variation in white-footed mouse population size and small mammal species richness in northern Pennsylvania

Article Title: Geographic Variation in Peromyscus leucopus Population Size and Small Mammal Community Richness in Northern Pennsylvania

Article References: Rowley, M. G., Davies, G., Langlais, P., Williams, M., & Chinnici, N. (2026). Geographic Variation in Peromyscus leucopus Population Size and Small Mammal Community Richness in Northern Pennsylvania. Ecology and Evolution, 16(10), Article e74535. https://doi.org/10.1002/ece3.74535

Image Credits: AI Generated

DOI: 10.1002/ece3.74535

Keywords: white-footed mouse, Peromyscus leucopus, small mammals, species richness, population size, Pennsylvania, Lyme disease, Borrelia burgdorferi, mark-recapture, tick-borne disease, ecological provinces, rodent population cycles