Photo by Robert Linder, Unsplash.
The Role of Landscape Scale in Determining Mesopredator Abundances
Mesopredators live in a dynamic and evolving landscape, where large scale changes by humans require species to adapt to be successful. Human modification of the natural landscape has been significant across the globe in the past century, with humans intentionally transforming wild landscapes into landscapes better fit for people in the form of cities and agricultural fields. The transitional areas between agriculture and natural landscapes, the so-called ‘brown-to-green gradient,’ often creates challenges for wildlife, as their habitat becomes increasingly fragmented and prey and refugia become scarcer.
Not all species are negatively affected by these human-induced changes, however, with some animals (called synanthropic species) being not only resilient but successfully adapting and thriving in close association with humans. These animals, including mesopredators such as Virginia opossums, striped skunks and northern raccoons, have learned to capitalize on the resources available in human-dominated landscapes while also navigating these complex environments safely.
There are also unintentional effects to landscape change, such as the introduction and spread of invasive species. Common invasive species in Illinois include the shrubs buckthorn, bush honeysuckle, Russian and autumn olive, and multiflora rose. Invasive species can disrupt natural ecosystems, leading to decline in natural populations and reduced biodiversity. Most of these invasive species in Illinois have been introduced as ornamentals in gardens and public areas and have now escaped into the wild.
When assessing ecological processes, it is important to consider the different spatial scales at which organisms can operate. For instance, the local scale may consider the immediate habitat that an animal may use (such as a small, wooded patch) while the landscape scale considers the broader environmental context to which the local habitat is situated (for example, a small, wooded patch surrounded by heavy agriculture). As such, local habitat conditions can be important to animals in one setting but not in another. Therefore, it is also important to consider scale when examining responses of wildlife to human-induced landscape change. An important element to changing landscapes is edge habitat, the boundary between one habitat and another. Animals often thrive at these interfaces, capitalizing on the increased diversity of food resources available, or using different habitats for different purposes (for example, hunting in one habitat and resting/denning in another). Other species utilize the habitat edges themselves, such as skunks who often den on the edge of agriculture and wetland, and bobcats who use the cover of woodlands to spot moving rodents among grasslands. However, the intrusion of some habitats (such as those created by humans) onto others can also negatively affect wildlife, limiting the spaces used by animals to hunt or seek refuge.
In recently published research in the journal Wildlife Research led by Dr. Robbie Emmet, researchers at the Illinois Natural History Survey studied the responses of mesopredators to landscape change at multiple spatial scales. Dr. Emmet and coauthors analyzed 17-years of spotlight surveys conducted by the Illinois Department of Natural Resources (IDNR) between 2001 and 2017. Spotlight surveys are a common tool for wildlife biologists to estimate trends in wildlife over time, generally conducted from vehicles using spotlights at night to detect animals along specified routes. In this long-running study, IDNR biologists drive along 25-mile routes at low speeds after dusk and count mesopredators and other wildlife they spot along the routes.
Researchers also used the data collected by the Critical Trends Assessment Program (an IDNR supported research project that has been quantifying plants, birds and bugs across 600 forest, grassland and wetland sites throughout the state since 1997), to predict the probability that the invasive species were present along spotlight survey routes. They also quantified land cover (such as forest and agriculture) along survey routes and were sure to include these effects at both local and landscape scales.
The researchers found that the mesopredators reacted differently at different scales, sometimes having opposite effects between the local and landscape scales. For example, a greater proportion of agriculture at the local scale decreased the abundance of raccoons, whereas a greater proportion of agriculture at the landscape scale increased the abundance. This is likely because raccoons may occur in higher numbers in northern Illinois where there is a lot of agriculture, but have a preference for forested habitat within these regions at smaller spatial scales. Similarly, skunks responded negatively to greater forest cover at the local scale, but positively to predicted presence of autumn olive at the landscape scale. Whereas, opossums responded positively to multiflora rose and autumn olive at the local and landscape scales, respectively.
These results highlight that the context in which a particular habitat type is found matters greatly to wildlife. As Dr. Emmet told me over email, “a patch of forest means something different to these species in heavily forested parts of southern Illinois, parts of northern Illinois with an abundance of corn and soybeans and in downtown Chicago.”
The positive effect invasive species had on mesopredator abundances is likely because conditions that make habitat suitable for mesopredators, such as disturbance and more forest, are shared by the invasive shrubs. We should be careful to draw positive conclusions from the presence of invasives, however. Whilst the presence of invasive species may provide some benefit to some species, it often detriments others, destabilizing ecosystems. In addition, just because a species utilizes these invasives does not mean that they are preferable, and animals may be trying to best use what they have available to them. The short-term gain for some of these mesopredators may come at the long term-cost of disrupting the natural balance of the environment and eroding the diversity of Illinois’ native flora. As Dr. Emmet put it, “any good done to mesopredator population by keeping these invasive species would probably be outweighed by the harm done to native plant species diversity.”
Altogether, this study shows that “humans change the landscape and its ecosystems in ways that can be surprising,” as noted by Dr. Emmet. Therefore, understanding wildlife means appreciating the nuances of effects at different spatial scales.
Nathan Proudman is a Postdoctoral Research Associate at the Illinois Natural History Survey. His research has primarily focused on the ecology of mammals. Currently, he is working on a statewide monitoring program for mammals in Illinois.