Climate and land use have changed dramatically in the last century, but rarely have we been able to evaluate the relative importance of these on habitat suitability for wildlife. Here we extend recent advances in habitat reconstruction and species distribution models to characterize suitable habitat for three weasel species that are declining in parts of their range and show how these have changed since 1938. Our analyses highlighted the widespread loss and fragmentation of suitable conditions for least weasels during the last century. We noted diverging regional trends for American ermine and long-tailed weasels where patterns of decline and fragmentation of suitable conditions dominate the West while increases in suitable conditions were notable in the eastern United States.
In particular, temperature appears to be a critical determinant of habitat suitability for North American weasels. For least weasels, suitability declined with lower values of minimum temperature. While the northernmost areas of the least weasel’s geographic range were not a focus of this study, it may be that as the smallest member of the weasel family examined here, tradeoffs between body size and energetic requirements of thermoregulation in cooler climates play a larger role in limiting the suitability of colder habitats for least weasels than for other weasel species (Blackburn and Hawkins 2004; Zub et al. 2008). In contrast, warm temperatures appeared to limit the southern and lower elevation ranges of all weasel species. Declines in the distributions of suitable conditions for American ermine and long-tailed weasels occurred primarily within the warmer southern and lower elevation regions, with declines more severe in the western United States. For least weasels, declines in predicted suitable conditions were similarly most severe along their southern range boundary. The observed inverse relationship between maximum temperature and habitat suitability for American ermine and long-tailed weasels, alongside the least weasel's intolerance to high minimum temperatures, posits climate change as a potential driver of observed losses and indicates the potential for further range contractions in these areas as the climate warms. These findings are consistent with the southern and low elevation range contractions observed in many climate-impacted species in the Northern hemisphere (Chen et al. 2011; Zimova et al. 2014; Ferreira et al. 2023). Further, decreased winter survival associated with camouflage mismatch due to increasingly inconsistent snow cover resulting from climate change has been widely proposed to explain low latitude and low elevation range contractions in species with distinctive seasonal molts, including weasels (Atmeh et al. 2018). This camouflage mismatch hypothesis is consistent with the mid-latitude and low-elevation losses presented herein. However, populations of long-tailed weasels and American ermine that occur in areas of inconsistent snowpack, such as the southern United States and Pacific Northwest do not undergo distinctive seasonal molts (Hall 1951; Mills et al. 2018; Davis et al. 2019; Miranda et al. 2021; Harty and Stains 2022). In these regions, camouflage mismatch is unable to explain observed range contractions.
Large scale land use change has also altered the distribution and connectivity of suitable conditions for North American Weasels. The trends in forest cover observed here were consistent with wider findings of land use change reported for the United States. In particular, large-scale agricultural abandonment during the mid the 20th century and subsequent forest regeneration resulted in large increases in forest cover in the Northeastern United States through the late 1900s (Litvaitis 1993). However, declines in Northeastern forest cover in the past few decades are beginning to reverse this trend (Adams et al. 2019). In forested areas of the western US, climate, disease, pests, and forest fire resulted in more recent (post 1970) declines in forest cover, consistent with our findings here (Sleeter et al. 2013; Sohl et al. 2016). While few studies have examined the habitat needs of North American weasels, American ermine have a noted association with forests, though use of grassland, shrubland, meadows has also been recorded (Lisgo 1999; Linnell et al. 2017; Evans and Mortelliti 2022). We also noted forest cover positively influenced suitability for this species, and regions (e.g., Northeast) that gained forest cover showed corresponding increased suitability while areas that lost forest cover (e.g., Rocky Mountain and Pacific Northwest; Sleeter et al. 2013, Sohl et al. 2016) showed similar declines in the area of predicted suitability.
While American ermine appears to be associated at broad scales with forested landscapes, least weasels tend to avoid forest cover at landscape scales (Zub et al. 2008). Instead, this species is more commonly associated with more open habitat such as grasslands, meadows, and residual edge features within an agricultural matrix (Polder 1968; Macdonald et al. 2004; Zub et al. 2008; Magrini et al. 2009). Our study similarly observed suitability for least weasels decreased across the range of forest cover typically observed across their distribution but may have slightly increased with the amount of land in agriculture. We also noted an increase in suitability with the number of land cover patches. Midwestern landscapes are the result of a complex history of local cropland expansion during favorable periods and subsequent contraction (Sohl et al. 2016). Ultimately, native grasslands have been lost throughout much of the region while agricultural intensification is associated with the replacement of small, diversely planted croplands with large single-crop fields and a reduction in the amount of favorable edge habitat for many species (Samson et al. 2004; Askins et al. 2007; Raven and Wagner 2021). This transition in farming techniques has already been implicated in the decline of a sympatric mesocarnivore, the plains spotted skunk (Gompper and Hackett 2005; Cheeseman et al. 2021b). Together, the positive association with patch number and agriculture for least weasels suggests that similar to plains spotted skunk, in agriculturally dominated landscapes, the interspersion of other land cover classes and associated edges may be needed to support least weasels.
Long-tailed weasels, with the widest distribution of any weasel in the western hemisphere, are the least specialized of the three species examined (Ruiz-Campos et al 2009). While some studies have suggested a positive association with forested environments, grasslands, or edge habitats (Polder 1968; Sheffield and Thomas 1997), Gehring et al. (2021) noted habitat selection of long-tailed weasels does not differ from the assumption of random use at landscape scales. Likewise, Evans and Mortelliti (2022) found no relationship between forest cover and long-tailed weasel occupancy, suggesting reported variability in habitat associations for this species may be due to their large geographic distribution. We also found that neither forest cover, nor any other land cover class was a strong predictor of suitable conditions for long-tailed weasels.
Landscape diversity, however, is commonly associated with positive outcomes for generalist carnivores (Oehler and Litvaitis 1996; Pita et al. 2009; Güthlin et al. 2013; Heim et al. 2019), such as American ermine and long tailed weasels. Consistent with this generalization we also found positive associations between landscape diversity and American ermine and long-tailed weasel habitat suitability and marginal evidence for this association in least weasels. Edge features, landscape disturbance, and landscape mosaics have all been identified as important habitat features for weasels (Polder 1968; Magrini et al. 2009; Evans and Mortelliti 2022). For example, while forest does not appear to be a strong predictor at landscape scales for long-tailed weasels, at smaller scales Gehring et al. (2021) noted selection for edge features such as fencerows and drainage ditches, as well as forest cover in agricultural dominated landscapes. In forest dominated environments, Evans and Mortelliti (2022) found the probability of occupancy increased with the amount of forest disturbance on the landscape for both long-tailed weasels and American ermine. In both studies, weasel habitat use was thought to be driven by access to abundant prey associated with edge and early successional features. We suggest the u-shaped relationship between suitability and landscape diversity reflects a balance between higher suitability in areas with large intact tracts of habitat where land cover diversity is low, and an increasing benefit as more land cover types increase the amount of edge habitat, provide disturbances in intact landscapes, and otherwise create landscape mosaics that provide access to diverse prey sources.
Carnivores, which often have large area requirements and occur at low densities, may be particularly sensitive to the effects of habitat fragmentation (Purvis et al. 2000). Weasels, as smaller bodied carnivores may additionally perceive their landscape at a smaller scale intensifying the negative effects of landscape fragmentation (Gehring and Swihart 2003). Consistent with this, fragmentation of suitable habitats has been linked with higher predation rates, lower population viability, and lower vagility in weasels (Crooks 2002; Gehring and Swihart 2003, 2004). We note climate and land use change have contributed to a general trend toward higher niche fragmentation today than historically for all weasel species. Such trends may make populations more vulnerable to decline as large tracts of suitable habitat continue to be fragmented into smaller, isolated patches containing fewer individuals (Willi et al. 2006; Oliver and Morecroft 2014). Moreover, niche fragmentation can limit species’ ability to shift or adapt to changing environmental conditions as population size is reduced and movement is limited by the intervening matrix (Oliver and Morecroft 2014). Preserving population connectivity may be of particular concern for weasels as adaptation to climate change may hinge on preserving gene flow between populations with and without seasonal coat change.
Our models, producing hindcasted predictions of environmental niche for North American weasels may help to elucidate trends in these otherwise cryptic and poorly understood species. These models predict the area of environmental suitability based on input variables across a defined extent, yet do not explicitly consider external factors such as dispersal abilities or interspecific interactions that may limit the realized niche of a species (Sinclair et al. 2010). For example, these models predicted areas external to the recognized range of modeled weasel species (grayed out Fig. 6, not considered in modeled calculations), as well as substantial loss and recovery of niche space in regions such as the Northeast. Whether these regions are or are not occupied depends on the unknown colonization abilities of individual species. As we modeled and considered change within their currently defined distribution, our results explicitly represent predicted suitability and change in suitable conditions across this area. Environmental niche models also assume species response to their environment is static through time. Adaptation to changing environmental conditions can alter species response to modeled covariates and may result in inaccurate predictions; however, studies of evolutionary change have emphasized high rates of niche conservatism, particularly over geologically short time spans, such as examined here (Peterson 2011). Further work regarding movements, space use, and landscape dynamics of weasels could greatly enhance our understanding of weasel response to landscape change and enable their consideration in future ENMs. As we demonstrate North American weasels may be highly susceptible to both climate and land use change, future ENMs should consider the movement abilities of weasels and forecast responses to future environmental change could greatly benefit species managers.
Currently, weasels are most commonly managed as harvestable furbearers in the United States, which could contribute to their decline. While listed as uncommon (S3) at the southern boundary of their western range, American ermine are largely unprotected from harvest in western states where we note suitability has declined. Least weasels, which lost suitable habitat rangewide, are generally recognized as species of conservation concern along their eastern range boundary, but not in the western half of their range, where we found habitat suitability has also greatly decreased. Conversely, while our models suggest western declines of long-tailed weasels, the consideration of long-tailed weasels as a species of concern demonstrates no clear pattern in all regions but the South, where they are listed as a species of concern in most states, but where our models did not identify many areas of high suitability or declines. One possibility is that the South does not contain much high-quality habitat and as a result, long-tailed weasels have always occurred at low density; however, this is not supported by high historic harvest from this region (Jachowski et al. 2021). Another possibility is North American weasels face multiple threats. Secondary poisoning from rodenticides, increase in predator populations, overharvest, and disease have been implicated alongside land use and climate to describe declines of North American weasels, and may present particular concern where population declines are recognized but no substantial change in environmental suitability is recognized (Jachowski et al. 2021).