Snow tracking reveals different foraging patterns of red foxes and pine martens

Long-term snow-track indices of a Finnish native mesopredator declined while those of an invasive one increased

Monitoring both native and invasive species is crucial for understanding their ecological impacts. However, obtaining reliable data can be challenging, especially for elusive species like mesopredators. This study utilized snow-track surveys in Finland (1989-2022) to examine population trends of the invasive raccoon dog and the native red fox. While raccoon dogs are known to reduce activity during cold weather, we demonstrated that accounting for temperature and snowfall allows for effective population trend estimation using snow-track data. Track accumulation decreased in cold and snowy weather more clearly for raccoon dogs than for red foxes. We also found that the track accumulation of the raccoon dog had significantly increased, while those of the red fox population had declined, particularly in southern parts of the country. Notably, raccoon dog snow-track numbers increased in northern regions, suggesting a potential for further range expansion under a warming climate. These findings reveal a concerning shift in Finland's mesopredators abundance due to the invasive species' success and the decline of the native species. Thus, the invasive raccoon dog is likely to have an increasing role in those northern ecosystems where it interacts with the native fauna.

How do microtine rodent abundance, snow and landscape parameters influence pine marten Martes martes population dynamics?

The pine marten (Martes martes) occupies the northernmost extent of its distribution in Norway, where microtine rodents are an important food item. The relationship between microtine rodent abundance and pine marten population dynamics is not well understood. In this paper, we examined this relationship and tested if environmental factors (e.g. snow depth, elevation, mature spruce forest density and agricultural land density) modulate pine marten population dynamics. We calculated pine marten abundance indices using data collected from 593 unique snow transects surveyed between 2003 and 2014 in Hedmark, Norway. We employed a Partial Rate Correlation Function to identify potential cyclicity in pine marten populations. We did not observe any cyclical patterns in pine marten populations within our short time series. Instead, their population appeared to be directly density-dependent. Although the population growth rate of pine marten tended to increase with increasing elevation, it was not affected by individual variables including a microtine rodent abundance index and snow depth. However, the annual growth rate of pine marten populations was positively affected by the interaction between the microtine rodent abundance index and increasing elevation. Pine marten abundance increased with microtine rodent abundance, elevation, snow depth and density of mature spruce forest, but decreased with increasing agricultural land density. Pine martens are opportunistic diet generalists that can switch between prey and cache food for later consumption. They are also strongly territorial with delayed implantation and are slow to respond to environmental changes due to their relatively low reproductive potential. These life-history traits may mitigate the effects of fluctuating microtine rodent abundance on pine marten reproduction and survival. Nevertheless, our findings suggest that microtine rodents still serve as important prey which can influence the population dynamics of pine martens in higher elevation habitats where alternative prey may be less available.

Snow cover-related camouflage mismatch increases detection by predators

Camouflage expressed by animals is an adaptation to local environments that certain animals express to maximize survival and fitness. Animals at higher latitudes change their coat color according to a seasonally changing environment, expressing a white coat in winter and a darker coat in summer. The timing of molting is tightly linked to the appearance and disappearance of snow and is mainly regulated by photoperiod. However, due to climate change, an increasing mismatch is observed between the coat color of these species and their environment. Here, we conducted an experiment in northern Sweden, with white and brown decoys to study how camouflage (mis)-match influenced (1) predator attraction to decoys, and (2) predation events. Using camera trap data, we showed that mismatching decoys attracted more predators and experienced a higher likelihood of predation events in comparison to matching decoys, suggesting that camouflage mismatched animals experience increased detection by predators. These results provide insight into the function of a seasonal color coat and the need for this adaptation to maximize fitness in an environment that is exposed to high seasonality. Thus, our results suggest that, with increasing climate change and reduced snow cover, animals expressing a seasonal color coat will experience a decrease in survival.

Influence of heterospecifics on mesocarnivore behaviour at shared scavenging opportunities in the Canadian Rocky Mountains

In seasonal environments, the ability of mustelid species to acquire carrion-a dietary resource heavily depended upon-is driven by a collection local habitat characteristics and competition dynamics. In resource-scarce winter, sympatric mesocarnivores must balance energetic rewards of carrion with avoiding antagonistic interactions with conspecifics. We examined scavenging interactions among three mustelid species in the northern Canadian Rocky Mountains. Camera traps (n = 59) were baited with carrion during winter between 2006 to 2008. Spatial and temporal dimensions of scavenger behaviour (i.e., carcass use) were evaluated using a multi-model approach, which enabled us to recognize potentially adaptive behavioural mechanisms for mitigating competition at carcass sites. Best performing models indicated that carrion site use is governed by a combination of competition threats and environmental factors. A decrease in scavenging with increasing snow depth was observed across all species. Mustelids adopted a host of adaptive behavioural strategies to access shared scavenging opportunities. We found evidence that wolverine (Gulo gulo) and American marten (Martes americana) segregate in space but temporally tracked one another. Short-tailed weasel (Mustela erminea) scavenging decreased with greater site use by marten. Carcass availability across a spatially complex environment, as well as spatial-temporal avoidance strategies, can facilitate carrion resource partitioning.

Eurasian beaver - A semi-aquatic ecosystem engineer rearranges the assemblage of terrestrial mammals in winter

There is increasing awareness of the ecosystem engineering services provided by recovering populations of Eurasian beaver. By modifying aquatic environments, this species has a significant, positive influence on biodiversity. Beaver activity affects not only aquatic ecosystems but also terrestrial habitats and organisms. Our study compares and evaluates the species richness and activity of terrestrial mammals in winter at beaver ponds (N = 65) and randomly-selected reference sites along nearby watercourses unmodified by beavers (N = 65) in Poland (central Europe). Mammal assemblages were investigated near pond/watercourse edges, and also at some distance from them. The species richness of mammal and numbers of their tracks were respectively 25% and 33% greater on the beaver than on the reference sites. The higher species richness on beaver sites extended to areas 40-60 m distant from ponds, devoid any signs of beaver activity. Twenty-three mammal species were recorded on beaver sites (mean species richness 3.8 ± 1.6 SD), and 20 on reference ones (3.0 ± 1.5 SD). The numbers of tracks of grey wolf, least weasel and European polecat were higher on beaver than reference sites. Mammal species richness and activity were related to the existence of beaver ponds, but were also correlated with the numbers of snags and coverage of grass, bramble and coniferous saplings in neighbouring terrestrial habitats. Large and small carnivores occurred more frequently and were more active on beaver sites. The frequencies of occurrence of mesocarnivores, mesoherbivores and small herbivores were correlated with habitat characteristics, regardless of whether beavers were present or not. Our results highlight the fact that both pond creation and the habitat changes resulting from the presence of beavers rearrange the occurrence and activity of the terrestrial mammal assemblage.

Small rodent cycles influence interactions among predators in a boreal forest ecosystem

Cyclic fluctuations of prey have profound effects on the functioning of ecosystems, for example, by changing the dynamics, behavior, and intraguild interactions of predators. The aim of this study was to assess the effect of rodent cyclic fluctuations in the interspecific interactions of a guild of small- and medium-sized predators: red fox (Vulpes vulpes), pine marten (Martes martes), and weasels (Mustela erminea and Mustela nivalis) in the boreal ecosystem. We analyzed eight years (2007–2014) of snow tracking data from southeastern Norway using structural equation models to assess hypothesized networks of causal relationships. Our results show that fluctuations in rodent abundance alter the strength of predator’s interactions, as well as the effect of determinant environmental variables. Pine marten and weasel abundances were positively associated with rodent population growth rate, but not red fox abundance. All predators were positively associated with each other; however, the association between red fox and the other predators weakened when rodents increased. Rodent fluctuations had variable effects on the habitat use of the predators. The presence of agricultural land was important for all predators, but this importance weakened for the mustelids as rodent abundance increased. We discuss the shifting role of interference and exploitative competition as possible mechanisms behind these patterns. Overall, we highlight the importance of accounting for the dynamics of prey resources when studying interspecific interactions among predators. Additionally, we demonstrate the importance of monitoring the predator populations in order to anticipate undesirable outcomes such as increased generalist predator abundances to the detriment of specialists.

Spatiotemporal patterns of red fox scavenging in forest and tundra: the influence of prey fluctuations and winter conditions

Concern has been raised regarding red fox (Vulpes Vulpes) population increase and range expansion into alpine tundra, directly and indirectly enhanced by human activities, including carrion supply, and its negative impact on native fauna. In this study, we used cameras on bait stations and hunting remains to investigate how spatiotemporal patterns of red fox scavenging were influenced by abundance and accessibility of live prey, i.e., small rodent population cycles, snow depth, and primary productivity. We found contrasting patterns of scavenging between habitats during winter. In alpine areas, use of baits was highest post rodent peaks and when snow depth was low. This probably reflected relatively higher red fox abundance due to increased reproduction or migration of individuals from neighboring areas, possibly also enhanced by a diet shift. Contrastingly, red fox use of baits in the forest was highest during rodent low phase, and when snow was deep, indicating a higher dependency of carrion under these conditions. Scavenging patterns by red fox on the pulsed but predictable food resource from hunting remains in the autumn revealed no patterns throughout the rodent cycle. In this study, we showed that small rodent dynamics influenced red fox scavenging, at least in winter, but with contrasting patterns depending on environmental conditions. In marginal alpine areas, a numerical response to higher availability of rodents possible lead to the increase in bait visitation the proceeding winter, while in more productive forest areas, low availability of rodents induced a functional diet shift towards scavenging.

Scavenger community structure along an environmental gradient from boreal forest to alpine tundra in Scandinavia

Scavengers can have strong impacts on food webs, and awareness of their role in ecosystems has increased during the last decades. In our study, we used baited camera traps to quantify the structure of the winter scavenger community in central Scandinavia across a forest-alpine continuum and assess how climatic conditions affected spatial patterns of species occurrences at baits. Canonical correspondence analysis revealed that the main habitat type (forest or alpine tundra) and snow depth was main determinants of the community structure. According to a joint species distribution model within the HMSC framework, species richness tended to be higher in forest than in alpine tundra habitat, but was only weakly associated with temperature and snow depth. However, we observed stronger and more diverse impacts of these covariates on individual species. Occurrence at baits by habitat generalists (red fox, golden eagle, and common raven) typically increased at low temperatures and high snow depth, probably due to increased energetic demands and lower abundance of natural prey in harsh winter conditions. On the contrary, occurrence at baits by forest specialists (e.g., Eurasian jay) tended to decrease in deep snow, which is possibly a consequence of reduced bait detectability and accessibility. In general, the influence of environmental covariates on species richness and occurrence at baits was lower in alpine tundra than in forests, and habitat generalists dominated the scavenger communities in both forest and alpine tundra. Following forecasted climate change, altered environmental conditions are likely to cause range expansion of boreal species and range contraction of typical alpine species such as the arctic fox. Our results suggest that altered snow conditions will possibly be a main driver of changes in species community structure.

The impact of human land use and landscape productivity on population dynamics of red fox in southeastern Norway

In the boreal forest, the red fox (Vulpes vulpes) is a key species due to its many strong food web linkages and its exploitation of niches that form in the wake of human activities. Recent altitudinal range expansion and a perceived population increase have become topics of concern in Scandinavia, primarily due to the potential impacts of red foxes on both prey and competitor species. However, despite it being a common species, there is still surprisingly little knowledge about the temporal and spatial characteristics of its population dynamics. In this study, we synthesized 12 years of snow-track transect data covering a 27,000-km2 study area to identify factors associated with red fox distribution and population dynamics. Using Bayesian hierarchical regression models, we evaluated the relationships of landscape productivity and climate gradients as well as anthropogenic subsidies with an index of red fox population size and growth rates. We found that landscapes with high human settlement density and large amounts of gut piles from moose (Alces alces) hunting were associated with higher red fox abundances. Population dynamics were characterized by direct density-dependent growth, and the structure of density dependence was best explained by the amount of agricultural land in the landscape. Population equilibrium levels increased, and populations were more stable, in areas with a higher presence of agricultural lands, whereas density-dependent population growth was more prominent in areas of low agricultural presence. We conclude that human land use is a dominant driver of red fox population dynamics in the boreal forest. We encourage further research focusing on contrasting effects of anthropogenic subsidization on predator population carrying capacities and temporal stability, and potential impacts on prey dynamics.