Multiple effects of land‐use changes impede the colonization of open water in fen ponds

Question: Dutch fen areas have become embedded in intensively used landscapes, resulting in biodiversity loss. Hence, plant species that colonize open water inducing the formation of species‐rich floating peat mats have disappeared. Despite many restoration efforts, they have not returned. Is natural succession towards floating mats impeded by site conditions, dispersal limitations or changed biotic interactions? Location: Six Dutch fen reserves: De Deelen, De Weerribben, De Wieden, Westbroek, Molenpolder and Terra Nova. Methods: In 62 fen ponds we determined plant species richness and expansion into open water. We related these to habitat quality (chemical composition of soil and surface water, pond morphology), dispersal potential (distance to remnant populations, likelihood of dispersal) and biotic interactions (presence of muskrats [Ondatra zibethicus L.] and the keystone species Stratiotes aloides). Results: Factor analysis showed that plants expanded further into open water and bank vegetation had higher species richness in areas with older ponds and lower muskrat densities. Locally, high turbidity hampered colonization. Whenever the water was clear, colonization was higher in shallow ponds, and in deep ponds only if Stratiotes was present. Species richness was negatively correlated to nutrient availability in soil and positively correlated to hydrological isolation (decreased sulphate concentrations). We also found that species richness was higher in sheltered banks. Conclusions: Multiple habitat characteristics (turbidity, water depth, nutrient and sulphate concentrations) and the influence of muskrats and Stratiotes all play a role in the lack of restoration success in Dutch fen ponds. Dispersal limitations seem to be overruled by habitat limitations, as colonization often fails even when sufficient propagule sources are present, or when connectivity is high.     

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.     

Spatial variation in mink and muskrat interactions in Canada

We investigated the spatial attributes of mink ( Mustela vison ) and muskrat ( Ondatra zibethicus ) interactions in Canada using 160 geographically paired historic time series of mink ( n =80) and muskrat ( n =80) harvest data obtained from Hudson's Bay Co. Archives. All series were 25 years in length (1925–1949) and were distributed primarily throughout five ecozones. We used autoregressive models and cross-correlation analysis to characterize the interactions between mink and muskrat. Model selection results did not differ among ecozones, and indicated that a predator-prey autoregressive model incorporating a delayed density-dependent term best described both the mink and muskrat harvest time series. Subsequent analysis of autoregressive coefficients and estimated lags indicated that mink and muskrat interactions vary throughout Canada. In western Canada, the trophic interactions appear to be strong, and mink population cycles lag behind muskrats 2–3 years. In central Canada, mink harvests lagged behind muskrats 1 year, and mink and muskrat interactions in central Canada, with the exception of the Hudson Plains ecozone, were intermediate. In eastern Canada, the trophic interactions appeared weakest, and there were no distinct time lags between mink and muskrat. Stronger interactions in western Canada may be a result of decreased prey diversity, forcing mink to specialize more on muskrats, whereas comparatively stronger perturbations stemming from other trophic interactions may alter the estimated interaction between mink and muskrat in eastern Canada.     

Body mass and correlated ecological variables in the North American muskrat lineage: evolutionary rates and the tradeoff of large size and speciation potential

This study tracks evolutionary change in body mass (W) and correlated ecological variables over the 3.75 million year history of the North American muskrat (Ondatra zibethicus). A new model is presented suggesting muskrat body mass has been in equilibrium for most of its history. Four pulses of pronounced size increase are correlated with glacial dynamics and volcanic events. Ranges of evolutionary rates in darwins and a new metric based on percent change in W document episodic size change. Proportional size change is independent of interval length, with a background range attributed to natural selection ≤25–30%. In increasing body mass by a factor of ten to about 1 kg mass-specific metabolism was halved, home range quadrupled, population density decreased fourfold, and average biomass more than doubled. Estimates of species diversity in ancient cotton rats (Sigmodon) and muskrats are calculated from a function derived from the correlation of numbers of North American rodent species and mean W. The phyletic mode of muskrat body size increase is explained as a combination of large body size reducing speciation coupled with an aquatic lifestyle. To the ecological consequences of large size in evolving clades (Cope’s rule) we can now add reduced speciation potential.     

Seasonal changes in the oxygen storage capacity and aerobic dive limits of the muskrat (Ondatra zibethicus)

Summary The oxygen storage capacity and partitioning of body oxygen reserves were compared in summer-and winter-acclimatized muskrats (Ondatra zibethicus). Blood volume, blood oxygen capacity, and skeletal muscle myoglobin content were higher in December than in July (P<0.02). Total lung capacity increased only slightly in winter (P>0.05). The oxygen storage capacity of a diving muskrat was calculated at 25.2 ml O₂ STPD · kg^-1 in July, compared to 35.7 ml O₂ STPD · kg^-1 in December. Blood comprised the major storage compartment in both seasons, accounting for 57% and 65% of the total oxygen stores in summer and winter, respectively. Based on available oxygen stores and previous estimates of the cost of diving, the aerobic dive limit (ADL) increased from 40.9 s in July to 57.9 s in December. Concurrent behavioral studies suggested that most voluntary diving by muskrats is aerobic. However, the proportion of dives exceeding the calculated ADL of these animals was shown to vary with the context of the dive. Only 3.5% of all dives initiated by muskrats floating in the water exceeded their estimated ADL. Provision of a dry resting site and access to a submerged food source increased this proportion to 18–61%, depending on the underwater distance that foraging muskrats were required to swim. Serial dives exceeding the estimated ADL were not accompanied by extended postdive recovery periods.Abbreviations ADL acrobic dive limit - Hb hemoglobin - Hct hematocrit - Mb myoglobin - PaO₂ arterial O₂ tension - STPD standard temperature and pressure, dry     

Muskrat predation on a diverse unionid mussel community: impacts of prey species composition,size and shape

1. The muskrat, Ondatra zibethicus, is a predator known to affect species composition, population size and age structure of freshwater unionid mussel communities. Muskrats leave large piles of dead shells (middens) on the edges of rivers, lakes and streams. We compared the species composition and size structure of shells collected from muskrat middens to the nearby live unionid community in the lower Licking River (Kentucky, USA). 2. Like previous studies, our results suggest that muskrats are both size‐selective and species‐specific predators; however, our results indicate that mussel shape is also an important factor. 3. We generated a shape metric (‘cubocity’) sensitive to the overall shape of the mussel. Species with relatively lower cubocity values (around 0.85) are plate‐like or spike‐like, while mussels with more cuboidal shells have higher cubocity values (near 1.0). 4. Our results suggest muskrats prefer cuboidal mussels and generally avoid spike‐shaped mussels. The endangered fanshell, Cyprogenia stegaria, was the most favoured prey; the fanshell’s relative size and shape appear to make it particularly vulnerable to muskrats. 5. We believe the predictive capabilities of this shape metric will be of benefit to those who monitor and manage threatened mussel populations.     

Summer Precipitation Predicts Spatial Distributions of Semiaquatic Mammals

Climate change is predicted to increase the frequency of droughts and intensity of seasonal precipitation in many regions. Semiaquatic mammals should be vulnerable to this increased variability in precipitation, especially in human-modified landscapes where dispersal to suitable habitat or temporary refugia may be limited. Using six years of presence-absence data (2007–2012) spanning years of record-breaking drought and flood conditions, we evaluated regional occupancy dynamics of American mink (Neovison vison) and muskrats (Ondatra zibethicus) in a highly altered agroecosystem in Illinois, USA. We used noninvasive sign surveys and a multiseason occupancy modeling approach to estimate annual occupancy rates for both species and related these rates to summer precipitation. We also tracked radiomarked individuals to assess mortality risk for both species when moving in terrestrial areas. Annual model-averaged estimates of occupancy for mink and muskrat were correlated positively to summer precipitation. Mink and muskrats were widespread during a year (2008) with above-average precipitation. However, estimates of site occupancy declined substantially for mink (0.56) and especially muskrats (0.09) during the severe drought of 2012. Mink are generalist predators that probably use terrestrial habitat during droughts. However, mink had substantially greater risk of mortality away from streams. In comparison, muskrats are more restricted to aquatic habitats and likely suffered high mortality during the drought. Our patterns are striking, but a more mechanistic understanding is needed of how semiaquatic species in human-modified ecosystems will respond ecologically in situ to extreme weather events predicted by climate-change models.     

Historical surveys reveal a long‐term decline in muskrat populations

The muskrat (Ondatra zibethicus) is an iconic species in Canada, valued for both its fur and its integral role in wetland ecosystems, and widely regarded for its perseverance. However, the resilience of this semiaquatic mammal seems to be in question now as increasing evidence points to widespread population declines. Recent analyses of harvest data across North America suggest a reduction in their numbers, but this has not been widely corroborated by population surveys. In this study we replicated historic muskrat house count surveys at two large Great Lakes coastal wetlands and present confirmation that declines in muskrat harvest correspond to actual declines in muskrat abundance. At the Point Pelee National Park marsh and the Matchedash Bay‐Gray Marsh wetland we found that mean muskrat house counts declined by 93% and 91% respectively between historic surveys 40–50 yrs ago and contemporary surveys over the past 7 yrs. The factors responsible for these dramatic declines remain unclear but there may be a relationship with changes in the habitat quality of these wetlands that have occurred over the same time frame. Not only is the loss of muskrats an issue for the resulting loss of the wetland ecosystem services they provide, but it may be an indication of broader marsh ecosystem degradation. As such, a scarcity of muskrats should be considered a red flag for the state of biodiversity in our wetlands. Continued surveys and ongoing research are needed to shed more light on the current status of muskrat populations and their marsh habitats across their native range.     

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.     

Daily and seasonal activity patterns of the muskrat Ondatra zibethicus as revealed by radiotelemetry

Activity patterns of the muskrat Ondatra zibethicus L. were examined during summer and winter in Delta Marsh, Manitoba, Canada (50°11'N, 98°23'W). Animals exhibited intermittent activity throughout the day with a mean periodicity close to 6 h in both seasons. The 24-h activity pattern in summer was typically bimodal with major peaks occurring between sunset and sunrise. Muskrats were more diurnal in winter when maximal levels of activity occurred in late afternoon and early evening (1500–2000). Pronounced inter- and intra-individual variability in daily activity ensured continuous, or nearly continuous occupation of winter shelters by a variable number of muskrats, and thus enchanced microclimate stability. Daily variation in activity and weather were weakly correlated both summer and winter. Of the environmental variables tested, only photoperiod and wind speed in summer, and photoperiod and air temperature in winter correlated significantly with total time spent in lodge or burrow per day.     

Density-dependent habitat selection in muskrats: a test of the ideal free distribution model

Summary Two predictions of the ideal free distribution model, a null hypothesis of habitat selection, were examined using free-ranging muskrats. We rejected the prediction that the proportion of the animals found in each of five habitats was independent of population size. Data on over-winter occupancy of muskrat dwellings tend also to refute the prediction of equal fitness reward among habitats. Habitat type and water-level had a profound effect on the suitability of a site for settlement. We concluded that the observed pattern of muskrat distribution followed more closely an ideal despotic distribution where some individuals benefited from a higher fitness because of resource monopolization. Current theories of density-dependent habitat selection, which assume an ideal free distribution, would not apply to muskrats and possibly to many other mammal species.     

Population life-cycle and stand structure in dense and open stands of the introduced tall herb <Emphasis Type="Italic">Heracleum mantegazzianum</Emphasis>

Populations of the introduced Heracleum mantegazzianum consist of dense central stands, which gradually give way to open stands towards the margins. To analyse whether open stands are due to unsuitable conditions or represent the invading front for further spread, we studied life-cycle, population dynamics, stand structure and soil conditions of open and dense stands over two transition periods. Populations decreased during the first interval but increased after the extremely dry and warm summer of 2003 during the second interval. Open stands had shorter generation times, lower height, smaller proportions of small individuals and were less in equilibrium with the environment than dense stands. In open stands, growth to higher stages was most important, while in dense stands delayed development (self-loops) had a strong effect on population growth; stasis and fecundity contributed most to the difference in λ between stand types. By petiole extension H. mantegazzianum may raise its leaves just above the resident vegetation. Therefore, younger stages develop faster in open stands, whereas strong competition by conspecific adults leads to longer generation times and a higher proportion of small individuals in dense stands. Disturbance due to extreme climatic conditions in summer 2003 equalised population dynamics of both stand types. Life-cycle variation between stand types makes it difficult to infer simple management rules. However, our data suggest that small and/or open stands of H. mantegazzianum may eventually serve as initials for further spread after land-use changes, whereas dense stands are stable and may represent sources of propagules.     

Muskrat life history: a comparison of a northern and southern population

Muskrats Ondatra zibethicus were studied at Old Crow flats Yukon Territory (68°05'N, 140°05W) and Tiny Marsh, Ontario (44°35'N, 79°52'W) during the summers of 1985 and 1986 Life–history traits of these two populations were compared Litter size was not significantly different between the two sites However average annual productivity was significantly greater at Tiny Marsh where individuals produced more litters per season Growth rates of juvenile and adult muskrats were significantly greater at Tiny Marsh, as was juvenile and adult weight However overwinter survival of juvenile muskrats was significantly greater at Old Crow than at Tiny Marsh We suggest that the shorter growing season and proximate energetic limitations on growth and reproduction favour a conservative reproductive strategy by the northern muskrat population For the southern muskrat population a long growing season and high quality food resources allow for a more productive reproductive strategy     

Wide-Area Estimates of Stand Structure and Water Use of Tamarix spp. on the Lower Colorado River: Implications for Restoration and Water Management Projects

Tamarix spp. removal has been proposed to salvage water and allow native vegetation to recolonize western U.S. riparian corridors. We conducted wide‐area studies on the Lower Colorado River to answer some of the scientific questions about Tamarix water use and the consequences of removal, combining ground surveys with remote sensing methods. Tamarix stands had moderate rates of evapotranspiration (ET), based on remote sensing estimates, averaging 1.1 m/yr, similar to rates determined for other locations on the river and other rivers. Leaf area index values were also moderate, and stands were relatively open, with areas of bare soil interspersed within stands. At three Tamarix sites in the Cibola National Wildlife Refuge, groundwater salinity at the site nearest to the river (200 m) was relatively low (circa 2,250 mg/L) and was within 3 m of the surface. However, 750 and 1,500 m from the river, the groundwater salinity was 5,000–10,000 mg/L due to removal of water by the Tamarix stands. Despite the high groundwater salinity, the sites away from the river did not have saline surface soils. Only 1% of the mean annual river flow is lost to Tamarix ET on the Lower Colorado River in the United States, and the opportunities for water salvage through Tamarix removal are constrained by its modest ET rates. A possible alternative to Tamarix removal is to intersperse native plants among the stands to improve the habitat value of the riparian zone.     

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 significant 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.     

A synergistic trio of invasive mammals? Facilitative interactions among beavers,muskrats, and mink at the southern end of the Americas

With ecosystems increasingly having co-occurring invasive species, it is becoming more important to understand invasive species interactions. At the southern end of the Americas, American beavers (Castor canadensis), muskrats (Ondatra zibethicus), and American mink (Neovison vison), were independently introduced. We used generalized linear models to investigate how muskrat presence related to beaver-modified habitats on Navarino Island, Chile. We also investigated the trophic interactions of the mink with muskrats and beavers by studying mink diet. Additionally, we proposed a conceptual species interaction framework involving these invasive species on the new terrestrial community. Our results indicated a positive association between muskrat presence and beaver-modified habitats. Model average coefficients indicated that muskrats preferred beaver-modified freshwater ecosystems, compared to not dammed naturally flowing streams. In addition, mammals and fish represented the main prey items for mink. Although fish were mink’s dominant prey in marine coastal habitats, muskrats represented >50 % of the biomass of mink diet in inland environments. We propose that beavers affect river flow and native vegetation, changing forests into wetlands with abundant grasses and rush vegetation. Thus, beavers facilitate the existence of muskrats, which in turn sustain inland mink populations. The latter have major impacts on the native biota, especially on native birds and small rodents. The facilitative interactions among beavers, muskrats, and mink that we explored in this study, together with other non-native species, suggest that an invasive meltdown process may exist; however further research is needed to confirm this hypothesis. Finally, we propose a community-level management to conserve the biological integrity of native ecosystems.     

Population studies of three aquatic gastropods in an intermittent backwater

Summary Three snail populations of an intermittent backwater were studied over a 20-month period, and estimates were made of rate of population change, mean biomass, annual and summer net production, and survival under conditions of little standing water. Lymnaea palustris and Physa integra populations were essentially univoltine, while G. parvus appeared to produce several generations during the year. L. palustris formed a protective epiphragm and was apparently better suited to recurrent dry periods than either G. parvus or P. integra, the latter population being maintained through immigration from the stream population. The population density of G. parvus was usually well below, and more aggregated, than the other two species, and there was considerable sampling error associated with its estimated population parameters.The estimated annual production rates for L. palustris and P. integra were 2.18 g/m² and 1.59 g/m², respectively, and these two species accounted for about 98% of the total gastropod production. About 75.9% and 66.5% of the L. palustris and P. integra production, respectively, occurred between June and November. Summer turnover ratios (ratio of production to mean standing crop) in 1969 were 4.69 for L. palustris and 2.94 for P. integra. An extended dry period early in the summer of 1970 reduced total snail production to about 18–24% that of the previous season, although summer turnover ratios were fairly consistent.     

Escape in space from enemies: a comparison between stands with and without enhanced densities of the spruce bark beetle

1 Populations of the spruce bark beetle, Ips typographus (L.), are known to grow rapidly in storm‐disturbed stands as a result of relaxation from intraspecific competition. In the present study, it was tested whether a second mechanism, escape in space from natural enemies, also contributes to the rapid population increase. 2 The experiment was conducted during the initiation phase of five local outbreaks of I. typographus triggered by a storm‐disturbance in November 1995 in southern Sweden. 3 The impact of natural enemies on the ratio of increase (number of daughters per mother) of I. typographus was compared pairwise between disturbed stands with high numbers of storm‐felled trees and undisturbed stands without wind‐felled trees. 4 Enemy impact was assessed by comparing the ratio of increase in uncaged (exposed to enemies) and caged (protected from enemies) bolts colonized by I. typographus prior to being placed in the stands. The experiment was conducted in the second and third summers after the storm‐felling. 5 Enemy impact was about twice as high in stands without wind‐felled trees compared with in stands with wind‐felled trees in the second summer whereas there was no significant difference between the stand types in the third summer. 6 The result demonstrates that spatial escape from enemies contributes to the rapid population growth of I. typographus after storm‐disturbances.     

Yersinia philomiragia sp. n., a New Member of the Pasteurella Group of Bacteria, Naturally Pathogenic for the Muskrat (Ondatra zibethica)

A bacterium experimentally pathogenic for muskrats (Ondatra zibethica), white mice, mountain voles (Microtus montanus), and deer mice (Peromyscus maniculatus) was isolated from the tissues of a sick muskrat captured on the Bear River Migratory Bird Refuge (Brigham City, Utah) and from four surface water samples collected within 15 miles of that point. In culture, the cells are chiefly coccoid, but in the tissues of muskrats and voles they resemble the bizarre forms of Yersinia pestis, except for their smaller size. The characteristics of the organism are described and the name Yersinia philomiragia sp. n. is proposed.     

Effects of structural connectivity on fine scale population genetic structure of muskrat,Ondatra zibethicus

In heterogeneous landscapes, physical barriers and loss of structural connectivity have been shown to reduce gene flow and therefore lead to population structuring. In this study, we assessed the influence of landscape features on population genetic structure and gene flow of a semiaquatic species, the muskrat. A total of 97 muskrats were sampled from three watersheds near Sudbury, Ontario, Canada. We estimated population genetic structure using 11 microsatellite loci and identified a single genetic cluster and no genetic differences were found among the watersheds as a result of high levels of gene flow. At finer scales, we assessed the correlation between individual pairwise genetic distances and Euclidean distance as well as different models of least cost path (LCP). We used a range of cost values for the landscape types in order to build our LCP models. We found a positive relationship between genetic distance and least cost distance when we considered roads as corridors for movements. Open landscapes and urban areas seemed to restrict but not prevent gene flow within the study area. Our study underlines the high‐dispersal ability of generalist species in their use of landscape and highlights how landscape features often considered barriers to animal movements are corridors for other species.