Date of Award

2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Specialization

Ecology and Ecosystem Sciences

Department

Natural Resources Science

First Advisor

Jason Parent

Abstract

Semi-aquatic mammals (i.e., animals that depend on both terrestrial and aquatic environments for their daily needs) are an ecologically and economically important group, but they are often difficult to monitor because of their restricted habitat which makes them unlikely to appear in monitoring schemes designed for all mammals (e.g., camera trapping). Rhode Island is home to several semi-aquatic mammal species, including North American beavers (Castor canadensis), North American river otters (Lontra candensis), and muskrats (Ondatra zibethicus). Historically, these species were all exploited for their furs, which led to drastic declines in otter and (especially) beaver populations by the turn of the 20th century. In recent decades, beavers and otters have expanded their range in the continental United States and their populations have increased with improving water quality and land use transitioning from agriculture to forests. This dissertation evaluates the distribution and habitat use of these three species across Rhode Island, with the aim of understanding current distributions and drivers of those distributions for each. The first chapter focuses on muskrats, with the aim of adding to our understanding of ongoing apparent declines in muskrat populations throughout their native range. Several hypotheses have been proposed to explain these declines, and this chapter focuses on evaluating whether habitat loss could cause declines in muskrat populations in Rhode Island. Muskrat distributions were estimated using a time to detection occupancy model, which found that occupancy was largely driven by the availability of wetlands and open water, although muskrats were also more likely to occupy areas with more urban landcover. Muskrat occupancy was predicted based on current and past landcover to estimate changes in wetland area over the past two decades, which showed that muskrat habitat has not declined during this time period, suggesting that other drivers are causing muskrat population declines. The second chapter focuses on otter distributions, using an integrated species distribution model fit to detection-nondetection data from 2021 to 2023 and records of otter latrines and roadkilled otters from 1999 to 2020 to understand current patterns of otter distribution and to evaluate trends in otter intensity of use over these years. Otter intensity of use was strongly correlated with several landcover variables, such as the presence of second order or larger streams and the amount of water in an area, as well as measures of habitat quality such as the presence of benthic invertebrates. Otter intensity of use declined over the study period, despite being protected from harvest and in contrast to range expansions and population recovery in other parts of their range.

The third chapter focuses on beavers, which have been expanding their range in Rhode Island over the past two decades. This chapter investigates whether beaver management, especially through public harvest, is effective at reducing resident complaints about beavers. As a species uniquely able to alter their environment, beavers have a high potential for human-wildlife conflict, often flooding buildings or roadways with their dams. This could lead to negative human responses and reduced support for conservation, prompting management to attempt to reduce the potential for these conflicts, in part by reducing beaver populations. This chapter fit a multistate occupancy model to time to detection data from 2021-2023 to evaluate patterns in current beaver occupancy and the probability of building dams, given occupancy, and found that beaver occupancy was driven by landscape factors such as the availability of open water and softwood forests, while the probability of dam-building was driven by the availability of third order or smaller streams and forested wetlands. These estimated probabilities were included in a zero-inflated Poisson model fit to records of public harvests, complaints, and nuisance removals of beavers from 1999 to 2021. The probability of beavers colonizing a town that had not previously had beaver detections was higher in towns with more habitat and which were adjacent to towns occupied by beavers in the previous year. The number of beavers harvested in a town and year was influenced by pelt prices, the number of trapper’s licenses issued, and access to public lands on which to trap. Nuisance removals were largely a function of the number of complaints in the town and year, while the number of complaints was higher in towns with higher human population and more landcover associated with beaver dams, and lower with increased harvest in the previous year and time since beavers were first detected in the town. These results suggest that beaver management in Rhode Island is effective at reducing human-beaver conflict.

Each chapter of this dissertation involves a different semi-aquatic mammal, and focuses on different hypotheses about their distribution, but they share commonalities, including making use of field data collected simultaneously and using hierarchical modeling to understand relationships between species and landcover. Taken together, the dissertation shows that semi-aquatic mammals in Rhode Island have experienced significant changes over the past two decades, with beavers expanding throughout the state while muskrat populations and otter intensity of use decline. Understanding the causes of otter and muskrat declines will aid conservation of these species, while beaver management appears to be successful in Rhode Island at meeting the twin goals of maintaining beaver populations and reducing human-beaver conflicts. Semi-aquatic mammals are very valuable to the continued functioning of Rhode Island’s wetland ecosystems, and this work aids in our understanding of their current distributions and trends.

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