Bobcat and Fisher Habitat Use and Interactions

Intraguild predation is when different carnivore species that use similar resources in the same areas (said to belong in the same guild) compete with one another and even kill each other. This is common in many carnivore communities around the world, including bobcats and fishers here in Minnesota. This type of competition can have strong effects on the populations of the intraguild predators and on the food web interactions in the greater forest community. Competition for shared prey resources is thought to be the most common cause of intraguild predation, or in other words, conflict arises between predators when trying to eat the same food item. 

Intraguild predation can have important implications for species conservation because intraguild predation can lead to reductions in the abundance of the smaller, non-dominant species. As the smaller species, fishers in Minnesota are occasionally killed by bobcats, which may be contributing to declining fisher numbers in their core range area in northern Minnesota.

Habitat is defined as the conditions and resources that an animal needs to survive and reproduce. For example, shelter and concealment, food, and mating opportunities are all part of habitat. Bobcat and fisher habitat in Minnesota is generally forested land, with bobcats using younger, more disturbed forest and fishers requiring older, mature forest. Older forest stands have larger trees with greater amounts of decay, leading to cavity formation that female fishers depend on for denning sites. Timber harvest provides humans with important materials but also alters habitat, temporarily removes trees from the landscape, and shifts the landscape to a younger forest regime. 

Past and current harvest practices have been predicted to have reduced the amount of suitable fisher habitat while simultaneously increasing bobcat habitat. If true, this could be contributing to declining fisher populations and increasing bobcat populations.

We are analyzing location data collected from bobcats and fishers with GPS collars to estimate the extents of their home ranges, which are the areas they use to access the resources they need. When estimating bobcat and fisher home ranges, we get an idea of the area they use frequently as well as how often they spend time in different parts of that area. Like humans, other animals use and are familiar with some areas that they use very frequently and other areas that they use less often. By looking at the areas they spend a lot of time and comparing those areas to what is available in their home range, we can learn about why they are choosing to go some places more often.

Additionally, we are very interested in learning about what features on the landscape are used frequently by both species. Areas that are used by both species represent areas where fishers are more likely to encounter bobcats and, therefore, have a greater risk of being attacked or killed. These same risky areas may have high prey densities, and so fishers may need to balance a tradeoff between prey availability and predation risk. This information is important for informing future land management activities so that the habitat needs of both carnivores are met, and, if possible, so that we can manage for habitats that provide fishers with the resources they need but that have lower predation risk.

We are analyzing bobcat and fisher diets using both scats and hair samples collected during field work. Prey remains in scat samples are identified to determine what species were consumed. We start by washing scat samples through a series of sieves to remove waste material. After separating the remaining contents and allowing them to dry, we can identify what species of prey were consumed by examining the undigested hair, bones, and teeth (see images below of some items we have found in bobcat and fisher scats). 

Hair samples from bobcats and fishers are analyzed using a process called stable isotope analysis. Stable isotope analysis can be generally explained with the old saying “you are what you eat.” As a consumer eats food, the chemical components of the food item are broken down, absorbed and then incorporated within the consumer’s tissues, including hair. We send hair samples from bobcats and fishers along with tissue samples from potential food items to a laboratory that will determine the chemical signatures of predators and their prey. We then compare the chemical signatures of consumer and food sources to determine the relative contribution of each food item to the consumer’s diet. 

After we have our scat and stable isotope results, we can measure the amount of dietary overlap between bobcats and fishers to understand how similar their diets are. Greater overlap in diet would suggest that competition for prey resources may be a factor in bobcat predation of fishers (i.e., bobcats killing fishers to reduce such competition for shared prey resources). 

The GPS collars we are using have a tri-axial accelerometer to measure collar movement and acceleration. Using the data collected by the accelerometer, we get a very detailed picture of when each study animal is active and when they are inactive (see graphs below). These data can help us determine what factors influence the activity patterns of both species, as well as identify times of the day when they may be most likely to both be active. 

Our collars use the data from the accelerometer (3 values, one for each axis) to calculate a single value called Overall Dynamic Body Acceleration. In general, higher numbers mean more dynamic activity, while low numbers indicate inactivity. A measurement is taken every 90 seconds, providing us with a really rich dataset to learn about bobcat and fisher activity patterns.