Muskrat Population Research Articles

Impact of invasive Typha and wetland interspersion on muskrat declines in North America

Muskrat (Ondatra zibethicus) populations are declining in North America. The exact cause of these declines is largely unknown. Along a similar timeframe, wetlands have been experiencing an invasion of cattail (Typha) throughout the continent. Specifically, T. x glauca, a hybrid of native T. latifolia and non-native T. angustifolia, has been increasing in range and abundance. This hybrid is associated with many negative impacts on wetland ecosystems, including reductions in biodiversity, open water habitat, and interspersion of water and emergent vegetation, the latter of which is an important habitat feature for muskrats. We sought to determine the impact of invasive T. x glauca on muskrat populations. We sampled 39 Typha-dominated marshes in southern Ontario, Canada to test the hypotheses that muskrats are declining in North America due to: (1) the increased relative abundance of T. x glauca in marshes, and (2) reduced wetland interspersion, which is associated with T. x glauca invasions. We estimated muskrat population density using house counts, sampled Typha communities to determine the relative abundance of T. x glauca, and measured interspersion using remote sensing techniques. We found that muskrat population density was positively associated with interspersion, but not associated with the relative abundance of T. x glauca. However, most sites were highly dominated by T. x glauca, limiting our inference. Our findings suggest that changing wetland structure may be contributing to muskrat population declines in North America, but more research is needed to determine the full impact of T. x glauca invasions on muskrat population declines.

Men of the Marshes: Paul L. Errington and H. Albert Hochbaum

Paul L. Errington (1902-1962) and H. Albert Hochbaum (1911-1988) were pioneering wildlife biologists whose research focused on muskrats and waterfowl, respectively. Their publications, especially their books, stressed the importance of wetlands as wildlife habitats. Errington spent his entire professional career at Iowa State University. Much of it studying muskrat population dynamics in prairie potholes. His work on the predation of muskrats and other species changed how predators were perceived from negative to positive for ecologists, hunters, and the general public. Hochbaum spent his entire professional career as the scientific director of the Delta Water Research Station in Canada. Because of his influential publications and those of the many graduate students at Delta whose research he watched over, Hochbaum built Delta into one of the premier waterfowl research institutions in the world. Errington’s and Hochbaum’s books influenced ecologists and the general public, especially those interested in wildlife conservation. They played a significant role in the development of wetland science by demonstrating the importance of wetlands as wildlife habitats and highlighting the urgent need for wetland conservation. Their advocacy contributed to the gradual shift in North American attitudes toward wetlands from negative to positive.

Men of the Marshes: Paul L. Errington and H. Albert Hochbaum

Paul L. Errington (1902-1962) and H. Albert Hochbaum (1911-1988) were pioneering wildlife biologists whose research focused on muskrats and waterfowl, respectively. Their publications, especially their books, stressed the importance of wetlands as wildlife habitats. Errington spent his entire professional career at Iowa State University. Much of it studying muskrat population dynamics in prairie potholes. His work on the predation of muskrats and other species changed how predators were perceived from negative to positive for ecologists, hunters, and the general public. Hochbaum spent his entire professional career as the scientific director of the Delta Water Research Station in Canada. Because of his influential publications and those of the many graduate students at Delta whose research he watched over, Hochbaum built Delta into one of the premier waterfowl research institutions in the world. Errington’s and Hochbaum’s books influenced ecologists and the general public, especially those interested in wildlife conservation. They played a significant role in the development of wetland science by demonstrating the importance of wetlands as wildlife habitats and highlighting the urgent need for wetland conservation. Their advocacy contributed to the gradual shift in North American attitudes toward wetlands from negative to positive.

Surveillance for diseases, pathogens, and toxicants of muskrat (Ondatra zibethicus) in Pennsylvania and surrounding regions.

Using diagnostic data and contemporary sampling efforts, we conducted surveillance for a diversity of pathogens, toxicants, and diseases of muskrats (Ondatra zibethicus). Between 1977 and 2019, 26 diagnostic cases were examined from Kansas and throughout the Southeast and Mid-Atlantic, USA. We identified multiple causes of mortality in muskrats, but trauma (8/26), Tyzzer’s disease (5/6), and cysticercosis (5/26) were the most common. We also conducted necropsies, during November 2018—January 2019 Pennsylvania muskrat trapping season, on 380 trapper-harvested muskrat carcasses after the pelt was removed. Tissue samples and exudate were tested for presence of or exposure to a suite of pathogens and contaminants. Gastrointestinal tracts were examined for helminths. Intestinal helminths were present in 39.2% of necropsied muskrats, with Hymenolepis spp. (62%) and echinostome spp. (44%) being the most common Molecular testing identified a low prevalence of infection with Clostridium piliforme in the feces and Sarcocystis spp. in the heart. We detected a low seroprevalence to Toxoplasma gondii (1/380). No muskrats were positive for Francisella tularensis or Babesia spp. Cysticercosis was detected in 20% (5/26) of diagnostic cases and 15% (57/380) of our trapper-harvested muskrats. Toxic concentrations of arsenic, cadmium, lead, or mercury were not detected in tested liver samples. Copper, molybdenum, and zinc concentrations were detected at acceptable levels comparative to previous studies. Parasite intensity and abundance were typical of historic reports; however, younger muskrats had higher intensity of infection than older muskrats which is contradictory to what has been previously reported. A diversity of pathogens and contaminants have been reported from muskrats, but the associated disease impacts are poorly understood. Our data are consistent with historic reports and highlight the wide range of parasites, pathogens and contaminants harbored by muskrats in Pennsylvania. The data collected are a critical component in assessing overall muskrat health and serve as a basis for understanding the impacts of disease on recent muskrat population declines.

Muskrats as a bellwether of a drying delta

Wetlands worldwide are under threat from anthropogenic impacts. In large protected North American areas such as Yellowstone and Wood Buffalo National Parks, aquatic habitats are disappearing and wetland-dependent fauna are in decline1–3. Here we investigate population dynamics of an indicator species in Canada’s Peace-Athabasca Delta (“the delta”), a World Heritage Site. Based on population surveys, habitat mapping and genetic data from 288 muskrats, we use agent-based modeling and genetic analyses to explain population expansion and decline of the semi-aquatic muskrat (Ondatra zibethicus). Simulations quantify a large population (~500,000 individuals) following flood-induced habitat gains, with decreased size (~10,000 individuals) during drying. Genetic analyses show extremely low long-term effective population size (Ne: 60–127), supporting a legacy of population bottlenecks. Our simulations indicate that the muskrat population in the delta is a metapopulation with individuals migrating preferentially along riparian pathways. Related individuals found over 40 km apart imply dispersal distances far greater than their typical home range (130 m). Rapid metapopulation recovery is achieved via riparian corridor migration and passive flood-transport of individuals. Source-sink dynamics show wetland loss impacts on the muskrat metapopulation’s spatial extent. Dramatic landscape change is underway, devastating local fauna, including this generalist species even in a protected ecosystem.

Population dynamics of muskrats (Ondatra zibethicus) and American mink (Neovison vison): investigating contemporary patterns in a classic predator–prey system

A well-studied predator–prey relationship between American mink (Neovison vison (Schreber, 1777); formerly known as Mustela vison Schreber, 1777) and muskrats (Ondatra zibethicus (Linnaeus, 1766)) in Canada has advanced our understanding of population cycles including the influence of density dependence and lagged responses of predators to prey abundances. However, it is unclear if patterns observed in Canada extend across the southern half of their native range. We used data from the United States to create a 41-year time series of mink and muskrat harvest reports (1970–2011) for 36 states. After controlling for pelt-price effects, we used second-order autoregressive and Lomb–Scargle spectral density models to identify the presence and periodicity of muskrat population cycles. Additionally, we tested for evidence of delayed or direct density dependence and for predator-driven population dynamics. Our results suggest muskrat populations may cycle in parts of the United States; however, results varied by modeling approaches with Lomb–Scargle analyses providing more precise parameter estimates. Observed cycle lengths were longer than expected with weak amplitudes and we urge caution when interpreting these results. We did not detect evidence of a predator–prey relationship driven by a lagged numerical response of American mink. American mink and muskrat fur returns were largely correlated across the region suggesting extraneous factors likely synchronize both populations.

Ecology of an Isolated Muskrat Population during Regional Population Declines

Evidence indicating a decline in Ondatra zibethicus (Muskrat) populations in the United States during the past 40 years has led to speculation regarding factors influencing Muskrat survival. In order to understand population dynamics and survival, it is important to first define the ecology of local populations. We investigated the dwelling structure use, movements, home range, and survival of radio-tagged Muskrats (n = 14) in an urban wetland complex in central Pennsylvania. We used locations collected from intensive radio-telemetry monitoring to determine number of lodging structures used, hourly movement, and size and percent area overlap of home ranges. Muskrats shared an average of 9 lodging structures, and on average, 68% of a Muskrat's home range overlapped home ranges of other Muskrats. We used 4 home-range estimators (kernel density estimator [KDE]href, KDEad hoc, KDEplug-in, and local convex hull estimator) to assess the ability of each estimator to represent Muskrat home ranges. The KDEplug-in that constrained the estimate of home range to habitat boundaries provided the most appropriate home-range size for Muskrats in a linear–non-linear habitat matrix. We also calculated overwinter survival estimates using known-fate models. Our top model indicated a positive effect of the average weekly precipitation on survival, with an overwinter survival estimate of 0.59 (SE = 0.16). The main cause of Muskrat mortality was predation by Neovison vison (American Mink; n = 6). The small sample size and uncertainty surrounding our model selection led to weak estimates of survival; however, our model suggests that snowfall may be an important factor in Muskrat survival. Our study provides novel data on Muskrat ecology in Pennsylvania as well as preliminary evidence for future investigations of factors affecting Muskrat survival during the winter months.

The occurrence of muskrat Ondatra zibethicus in central and eastern Poland in the period 1996–2007

Muskrat, Ondatra zibethicus (L. 1766), is an introduced species to European fauna. It is widespread in Poland, however, signs of its decline were observed in the last decades. While most of the information on muskrat is based on questionnaires and bag record, in this paper results of field census for muskrat signs (tracks and faeces) at a total of 1554 sites are presented. Signs of muskrats were present at 19.5% of 1111 sites surveyed from 1996-1998, and at 5.3% of 413 sites surveyed in 2007. Muskrats inhabited mostly small and medium rivers in the western part of the study area. The decrease in the occurrence of the species over a study period is the best document by a significant reduction in the frequency of muskrat records at the 249 sites surveyed in 1996-1998 (present at 44% sites) and 2007 (7%). The decline of the muskrat population is most probably the effect of increasing predation by American mink Neovison vison.

Drying landscape and interannual herbivory‐driven habitat degradation control semiaquatic mammal population dynamics

AbstractSemiaquatic muskrat (Ondatra zibethicus) are in decline throughout their native range. We report on the environmental and behavioural mechanisms responsible for a half‐century of muskrat decline in a model system floodplain region of the Peace–Athabasca Delta (“Delta”). Using the Landsat satellite record, trapping and population survey records, and ecological field studies of muskrat, we develop conceptual and quantitative models to investigate mechanisms driving muskrat population decline at Egg Lake, an archetypal Delta region. Simulation results show processes occurring over nested timescales control population decline: Multidecadal drying has diminished shoreline habitat, and inter‐annual shoreline habitat deterioration resulted from excessive herbivory by muskrat thereby exacerbating their die‐off. Simulations without herbivory feedback show peak population values up to 73% higher than simulations with herbivory. Simulation results also suggest that trapping is not the primary cause of declines in muskrat abundance. The process model highlights factors responsible for long‐term decline and rapid die‐off, providing insights into mechanisms for observed muskrat population decline.

Drying drives decline in muskrat population in the Peace-Athabasca Delta, Canada

Empirical and anecdotal reports suggest that muskrat are in decline across North America, including in the Peace-Athabasca Delta (‘Delta’), Canada, one of the largest inland deltas in the world and part of a World Heritage Site with ‘in Danger’ status pending. Muskrat are a key ecological indicator in the Delta. We investigate whether the large-scale loss of critical habitat over the past half-century could be driving a decline in muskrat abundance in the Delta. To do this, we use the Landsat record (1972–2017) to construct a 46 year record of inundation, and compare changes in the extent of critical habitat to the survey record for muskrat (1970–2016) over this 5500 km2 region. Results show that the declines in critical habitat and muskrat numbers in the Delta are synchronous: ∼1450 km2 of temporarily inundated regions that support critical habitat have diminished by ∼10 km2 yr−1 over the past 46 years, while the muskrat population density (houses/km2) has also declined and is significantly related to critical habitat area (km2) (R2 = 0.60, P = 0.0001). These findings have implications for the Delta, a Ramsar Wetland of International Importance in part for its role as a habitat for nearly 200 species of birds, many of which rely on the aquatic habitat considered here. Our results further suggest that the loss of wetland habitat is a primary driver of the decline of muskrat across the species’ native range.

Empirical evidence for declines in muskrat populations across the United States

ABSTRACTMuskrats (Ondatra zibethicus) are native to North America and widely distributed across the continent. Recent evidence and anecdotal reports suggest that muskrat populations may be declining; however, this assumption has not been rigorously evaluated. We used 42 years of muskrat harvest data (1970–2012) from 37 states to examine trends in muskrat populations across the United States. Annual harvest data are highly correlated with annual pelt prices, which must be controlled for prior to analysis. After adjusting harvest data for the effects of current‐year and 1‐year‐lagged pelt prices, we found strong support that muskrat populations have declined during this period. The slope of decline appeared stronger in the southeastern states and less pronounced in the midwestern states. Our results suggest that wildlife managers should consider active management programs for muskrat populations in regions where declines are observed. Along with recording annual muskrat harvest data, managers should also use rigorous surveys to identify changes in muskrat population abundances. Additionally, our study highlights the need for future research directed at revealing mechanistic explanations for synchronous muskrat declines across this large spatiotemporal scale. © 2017 The Wildlife Society.

Introduction of muskrat in Omsk oblast

The muskrat population was formed in Omsk oblast in four stages. The first stage began in 1936 and lasted for 15 years; it was then interrupted by tularemia epizootic. The muskrat number recovered in the next 20 years and was again interrupted by another tularemia epizootic. The recovery of the muskrat population and its introduction throughout Omsk oblast began in 1971. Since 1996, the area has been fully populated by muskrat and changes in the number of the species have assumed the nature of fluctuations with a periodicity of 5 to 6 years. Under conditions of changing moisture content from 1936 to 2015, changes in the muskrat population had a weak direct correlation with the phases of the Bruckner cycle and with the water level in water bodies and a very weak inverse correlation with the solar activity indices (W, Wolf numbers). At the current stage of the formation of the muskrat population in Omsk oblast, the distribution of the indices of its average annual abundance and the average annual density of the population have a medium direct correlation with the area of the water bodies that are available in the area under study.

Spatial analysis of Leptospira infection in muskrats in Lower Saxony, Germany, and the association with human leptospirosis

Leptospirosis has been reported as an emerging zoonosis and the most widespread zoonotic disease in the world. Muskrats (Ondatra zibethicus) have been known to carry Leptospira serovars, which have been linked to Weil's disease – a serious and life threatening disease in humans. In this study, muskrats were sampled across 43 regions of Lower Saxony, Germany, during 2007 to 2009 and were tested for leptospirosis using Polymerase Chain Reaction (PCR). Circular and flexibly shaped spatial scan tests were applied and detected two leptospirosis clusters in Lower Saxony, Germany. Several predictor variables were investigated using spatial Poisson rate regression analysis; however, only maximum regional temperature was associated with the incidence rate of leptospirosis in muskrats. For every 5°C increase in temperature, the incidence rate of leptospirosis in muskrats decreased 0.97 times. The association between the smoothed muskrat leptospirosis prevalence and the rate of human leptospirosis was also investigated using a Poisson rate regression model and revealed no relationship. Although no relationship was found at the level investigated in this study, recent human leptospirosis outbreaks have been linked to muskrats, so muskrats as a risk factor for human leptospirosis should not be ruled out. Overall, leptospirosis clusters were found within the muskrat population in Lower Saxony, Germany and temperature was confirmed as a risk factor.

Economic influences on trapper participation and per capita harvest of muskrat

ABSTRACTMuskrat (Ondatra zibethicus) populations in North America appear in decline in parts of their geographic range, prompting in‐depth investigations into population trends. However, long‐term data on muskrat population abundance are rare and managers often rely on harvest data to track relative changes in population sizes. These data are likely biased by factors that influence trapper participation and effort and, if left unidentified and uncontrolled for, may provide misleading indices of muskrat abundance. Using 36 years (1976–2011) of detailed muskrat‐harvest information from Illinois, USA, we estimated the relative importance of economic factors to trapper participation and annual per capita muskrat harvest. As expected, annual muskrat harvest was positively correlated with the number of individuals participating in muskrat trapping. Participation in muskrat trapping was associated positively with pelt and gas prices and negatively with the unemployment rate during the trapping season (R2 = 0.76). Variance partitioning within our most‐supported model revealed that pelt prices, independent of gasoline prices and unemployment rates, was the most important predictor of muskrat trapper participation. Our only supported model of per capita muskrat harvest indicated that gasoline prices negatively influenced the number of muskrats harvested per trapper each year (r2 = 0.17). Remaining unexplained variation in this model may be due to factors affecting muskrat availability (e.g., muskrat abundance, access to good‐quality trapping areas) to trappers and may be useful for managers investigating population trends. We recommend that future investigations using muskrat harvest data, at minimum, statistically adjust for the strong positive effect of pelt prices. © 2016 The Wildlife Society.

Genetics reveal the origin and timing of a cryptic insular introduction of muskrats in North America.

The muskrat, Ondatra zibethicus, is a semiaquatic rodent native to North America that has become a highly successful invader across Europe, Asia, and South America. It can inflict ecological and economic damage on wetland systems outside of its native range. Anecdotal evidence suggests that, in the early 1900s, a population of muskrats was introduced to the Isles of Shoals archipelago, located within the Gulf of Maine, for the purposes of fur harvest. However, because muskrats are native to the northeastern coast of North America, their presence on the Isles of Shoals could be interpreted as part of the native range of the species, potentially obscuring management planning and biogeographic inferences. To investigate their introduced status and identify a historic source population, muskrats from Appledore Island of the Isles of Shoals, and from the adjacent mainland of Maine and New Hampshire, were compared for mitochondrial cytochrome b sequences and allele frequencies at eight microsatellite loci. Appledore Island muskrats consistently exhibited reduced genetic diversity compared with mainland populations, and displayed signatures of a historic bottleneck. The distribution of mitochondrial haplotypes is suggestive of a New Hampshire source population. The data presented here are consistent with a human-mediated introduction that took place in the early 1900s. This scenario is further supported by the zooarchaeological record and island biogeographic patterns. This is the first genetic study of an introduced muskrat population within US borders and of any island muskrat population, and provides an important contrast with other studies of introduced muskrat populations worldwide.

Indirect evidence of prey-switching in minks: empirical evidence, theoretical modeling, and spatial drivers

A spatial gradient in the interactions between American minks (Neovison vison) and muskrats (Ondatra zibethicus) occurs in the Hudson’s Bay Company fur harvest returns of Canada. Evidence for strong dependence of minks on muskrats exists in northwestern Canada, whereas evidence for weaker interactions exists in central and eastern Canada. We tested the hypothesis that minks consume fewer muskrats and more alternative prey in some areas, using fur records from 56 Hudson’s Bay posts. Both muskrats and small microtines were found to explain mink dynamics, with small microtines and other alternative prey gaining importance in the eastern portion of our study area. Mink fur returns exhibited a range of cycle lengths from 2.6 to 13 years encompassing typical small microtine periodicities of 3–5 years and typical muskrat periodicities of 8–13 years. A time lag of 0 years occurred between mink and muskrat harvest data frequently in the eastern portion of our study area, hypothesized to be a result of minks consuming alternative prey. To biologically verify small microtines as a potential prey source, we modeled mink and muskrat population dynamics assuming small microtines were an alternative prey by modifying the Turchin and Hanski (1997) model. Simulated mink and muskrat time series replicated observed periodicity and time-lag range, suggesting that minks can be generalist predators and consume alternative prey. Finally, we examined species richness and land cover as potential drivers of mink prey-switching, but were unable to find support for either hypothesis, suggesting that additional environmentalor competition-related interactions influence mink population dynamics.

Zmiany w występowaniu piżmaka Ondatra zibethicus w centralnej i wschodniej Polsce w latach 1996 – 2007

Muskrat, Ondatra zibethicus (L. 1766), is an introduced species to European fauna. It is widespread in Poland, however signs of it decline were observed in last decades. While most of information on muskrat is based on questionnaires and bag record, in this paper results of field census for muskrat signs (tracks and feaces) at total of 1554 sites are presented. Signs of muskrats were present at 19,5% of 1111 sites surveyed from 1996-1998, and at 5,3% of 413 sites surveyed in 2007, Muskrats inhabited mostly small and medium rivers in western part of the study area. Decrease in occurrence of the species over a study period is best document by significant reduction in frequency of muskrat records at the 249 sites surveyed in 1996-1998 (present at 44% sites) and 2007 (7%). The decline of the muskrat population is most probably the effect of increasing predation by American mink Neovison vison.

Muskrat (Ondatra zibethicus) decline after the expansion of American mink (Neovison vison) in Poland

Field survey data in Central Poland revealed that the proportion of sites inhabited by muskrats decreased from 44% to 7% over one decade. This corresponded to the decline in hunting bags of muskrat over the whole of Poland. The largest hunting harvest of muskrat was recorded in 1987/1988 (66,416 individuals), the smallest in 2007/2008 (4,567 individuals). The decline in hunting bags occurred in all regions analysed; however, it was most rapid in the north and north-east. Before the expansion of mink, which started in northern Poland at the beginning of the 1980s, muskrat densities in particular regions depended on the availability of aquatic habitats. A comparison of hunting bags of muskrat and American mink in years 2002-2008 indicated a signifi- cant negative correlation between the numbers of these two species harvested in seven regions of Poland. The negative correlation between numbers of muskrat and mink suggests that mink predation is one of the most important factors in the decline of the muskrat population in Poland.

THE CARRIAGE OF LARVAL ECHINOCOCCUS MULTILOCULARIS AND OTHER CESTODES BY THE MUSK RAT (ONDATRA ZIBETHICUS) ALONG THE OURTHE RIVER AND ITS TRIBUTARIES (BELGIUM)

In Belgium, the carriage of Echinococcus multilocularis by the red fox (Vulpes vulpes) can be very high in some areas. This study was designed to evaluate the carriage of the larval form of E. multilocularis and other cestodes in a musk rat (Ondatra zibethicus) population trapped along the Ourthe River (southeastern Belgium). Six hundred fifty-seven musk rats were necropsied, and the larval cestodes of the abdominal and pleural cavities were identified. For E. multilocularis, the fertility of the cysts was verified in 58 liver samples. The following species were found: Taenia taeniaeformis (65.8%), Taenia martis (22.2%), E. multilocularis (22.1%), Taenia polyacantha (2.6%), and Taenia crassiceps (0.9%). Results were analyzed according to the site of capture (upper, middle, and lower Ourthe). There was a highly significant relationship between the carriage of E. multilocularis and the site of capture (the prevalence being higher in the upper part of the river). This difference could be due to different geoclimatic conditions. All but one hepatic lesion were found to contain protoscoleces of E. multilocularis (98.8%). The musk rat is probably infected through the consumption of plant material contaminated by the fox's feces. The red fox can occasionally prey on musk rats, but the musk rat cadavers that are left on the river banks by the trappers are probably also consumed. This could favor the maintenance of E. multilocularis life cycle. In conclusion, the musk rat seems to be highly susceptible to E. multilocularis and in Belgium could play the role of reservoir; when present this species could represent an inexpensive and sensitive bioindicator for the study and monitoring of the zoonosis.

Alien Grazing: Indirect Effects of Muskrats on Invertebrates

Effects of alien herbivores on plants are quite well known, especially on oceanic islands. Much less is known about the indirect effects of alien herbivores. Via indirect effects indigenous organisms can become more vulnerable to other ecological factors. We studied the effect of dense muskrat population on aquatic invertebrates in patches, which were in different succession phases after grazing. The succession was divided into three phases:(i) open-water area, (ii) mixed floating and submerged vegetation, and (iii) undisturbed Equisetum stands. Intensive muskrat grazing affected especially the size distribution of invertebrates. The proportion of small invertebrates was greatest in clear-cut areas, whereas that of large invertebrates was greatest in the Equisetum-stands. According to activity traps, the clear-cut areas harboured good populations of small perch, which are known as invertebrate predators. The study showed that the indirect effects of alien herbivore can be considerable. In this case, they seemed to be mediated to invertebrates through the change in vegetative habitat structure. This change rendered invertebrates more vulnerable to fish predation.