DENSITY ESTIMATES, INTERACTIONS, AND HABITAT USE OF TWO ENDEMIC ENDANGERED RODENTS IN RESPONSE TO INVASIVE SPECIES IN THE FLORIDA KEYS, USA

Author

Shauna Marie Sayers, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2025

Degree Name

Master of Science

Department

Forestry

First Advisor

Pease, Brent

Abstract

In the Florida Keys, where invasive species continue to expand, understanding population trends for endemic species is vital for determining their status and ensuring effective conservation strategies. The Burmese python (Python bivittatus), a large constrictor snake native to Southeast Asia, is one of Florida's most notable invasive predators. Since 2016, an established population of Burmese pythons has increased in prevalence across Key Largo, Florida, USA, posing a significant threat to native wildlife. In 2017, Hurricane Irma made landfall as a Category 4 storm, inducing flooding and extensive canopy damage. The Key Largo woodrat (Neotoma floridana smalli) and Key Largo cotton mouse (Neotoma floridana smalli) are two endangered rodent subspecies, endemic to Key Largo that have struggled to maintain stable populations in the face of these obstacles. Here, I investigated how Hurricane Irma and invasive species impacted the Key Largo woodrat and Key Largo cotton mouse populations and their potential interactions. I used spatially explicit capture-recapture (SECR) to analyze live-trapping data from across North Key Largo, 2017 to 2024, with data collected from 2017 prior to Hurricane Irma and during the pythons' increasing prevalence. Woodrat density declined following Hurricane Irma, coinciding with increased detections of pythons, decreasing from 3.48 (SD = 0.50) to 0.61 (SD = 0.20) individuals/ha in 2017 to 2024, respectively. Furthermore, cotton mouse density increased from 1.57 (SD = 0.28) individuals/ha in 2017 to 5.35 (SD = 0.57) in 2024. My findings illustrate the contrasting responses of two native species to large-scale natural disturbance and emphasize the Key Largo woodrat’s potential risk of extinction as global change continues to impact South Florida ecosystems. Following the density trends of cotton mice and woodrats between 2017 to 2024, I sought to examine a possible interaction between the local abundance of woodrats and cotton mice and differences in habitat use. Using hierarchical N-mixture co-abundance models, I developed three competing models testing no, constant, and varying effects of woodrats on cotton mice as well as examining environmental drivers that may influence population variation and the effect of invasive black rats (Rattus rattus). I found no interactive effect of woodrats on cotton mice with regression coefficients that had 95% credible intervals spanning zero, suggesting coexistence with no competitive suppression. In contrast, invasive black rats had a significant negative effect on cotton mice, but did not impact woodrats. Habitat association differed between species as woodrats were positively associated with young forests, while cotton mice responded positively to young, medium, and legacy forest age classes, indicating a broader habitat preference than previously noted. Both cotton mice and woodrats exhibited a positive response with increasing distance to major development, likely reflecting the negative impact of habitat degradation and increasing pressure from invasive species near urban borders. Together, these results further our knowledge on the interactions and habitat use of woodrats and cotton mice, and provide density estimates that enable managers to assess both species’ current population status. This research can be used to inform future management strategies that promote species persistence as invasive species continue to shape Key Largo’s landscape.