In the Florida Keys, invasive Burmese pythons are threatening the survival of the Key Largo woodrat and Key Largo cotton mouse

In North Key Largo, Florida, a quiet ecological crisis is reshaping the fate of the island's small mammals. Two endangered rodent subspecies live only on Key Largo: the Key Largo woodrat (Neotoma floridana smalli) and the Key Largo cotton mouse (Peromyscus gossypinus allapaticola). Both belong to the family Cricetidae and the subfamily Neotominae. The Key Largo woodrat is the larger of the two, reaching about 23 to 46 cm in body length, whereas the Key Largo cotton mouse is smaller, about 14 to 21 cm long. These animals inhabit tropical hardwood hammocks, but their populations were already restricted by urbanization, agricultural development, habitat loss, and habitat fragmentation. Now they face an even harder threat to control: the invasive Burmese python (Python bivittatus). This large constrictor, native to Southeast Asia, has established populations in South Florida and has expanded into the Florida Keys, where it has become a growing source of pressure on native mammals.

Key Largo woodrat（Image source：U.S. Fish and Wildlife Service Southeast Region， CC BY 2.0 ）

Key Largo cotton mouse（Image source：Mike Cove，CC0 1.0 ）

Burmese python（Image source：Everglades NPS，CC0 1.0 ）

In September 2017, Hurricane Irma made landfall in the Florida Keys as a Category 4 storm, bringing flooding, high winds, extensive canopy damage, fallen trees, and heavy debris deposition. For an island ecosystem as spatially limited as North Key Largo, the hurricane altered available habitat, cover, nest sites, and predation risk. It may also have facilitated the spread of Burmese pythons across the island.

A recent study used live-trapping data collected from 2017 to 2024 to examine how the two endangered rodents changed after the hurricane. The study took place in Crocodile Lake National Wildlife Refuge and Dagny Johnson Botanical State Park. Researchers placed Sherman live traps across 10 survey grids and sampled the area during four sessions: spring 2017, December 2017, fall 2022, and spring 2024. They recorded captures, marked individuals, identified recaptures, and used spatially explicit capture-recapture models to estimate population density. This approach is more reliable than simply counting the number of animals caught because it incorporates capture locations, recapture probability, and the movement scale of individual animals, allowing a stronger estimate of true density.

North Key Largo is located in southern Florida. The map shows the areas of Crocodile Lake National Wildlife Refuge and Dagny Johnson Botanical State Park; black squares represent the 10 survey grids（Image source：Sayers SM et al. (2026)， CC BY 4.0 ）

The survey data showed that these two endangered local rodents followed very different trajectories. The estimated density of the Key Largo woodrat was about 3.59 individuals per hectare in spring 2017. By December 2017, after Hurricane Irma, it had fallen to 1.84 individuals per hectare. In 2022, it declined further to 0.60 individuals per hectare, and in 2024 it remained at only 0.61 individuals per hectare, meaning that a hectare of habitat might not even contain a single woodrat. In other words, the population dropped sharply after the hurricane and failed to recover in the following years. Its spatial distribution also contracted. In spring 2017, woodrats were detected in all 10 survey grids. By December 2017, they were detected in only six grids, and by 2022 and 2024 they remained detectable in only four.

The Key Largo cotton mouse showed a more complex pattern. Its estimated density was about 1.57 individuals per hectare in spring 2017, increased to 3.45 in December 2017, reached 5.35 in 2022, and then declined again to about 2.8 in 2024. Its distribution also expanded from eight survey grids to all 10 grids, without the clear spatial contraction seen in the Key Largo woodrat. The researchers suggested that the post-hurricane increase in cotton mice may have been related to a temporary release from competition as woodrats declined. It may also have reflected short-term changes in vegetation regrowth, food availability, or microhabitat conditions after the hurricane. In addition, the cotton mouse likely has a higher reproductive capacity than the woodrat, which may help buffer the effects of predation.

Estimated density trends of Key Largo woodrats (orange) and Key Largo cotton mice (blue), expressed as individuals per hectare（Image source：Sayers SM et al. (2026)， CC BY 4.0 ）

However, the decline of the Key Largo cotton mouse in 2024 shows that it is not truly safe. As larger prey such as Key Largo woodrats and black rats (Rattus rattus) become less available, Burmese pythons may increasingly shift toward smaller prey. This means that the Key Largo cotton mouse may also face intensifying predation pressure, suggesting that its response to the python invasion may simply be delayed compared with that of the woodrat.

Future conservation work should continue tracking the density, spatial distribution, and nest use of both rodent subspecies. Efforts to remove Burmese pythons should also be strengthened, especially in areas where Key Largo woodrats still persist and where Key Largo cotton mice have begun to decline. Without sustained monitoring and more intensive invasive predator management, these endangered island subspecies may move closer to extinction.

Author: Shui-Ye You

Reference:

Sayers SM et al. (2026). Density Estimates of Endangered Endemic Rodents Suggest Broader Impacts of Invasive Burmese Pythons Following a Category 4 Hurricane in the Florida Keys. Biological Diversity.