Wondering about wandering possums: Understanding possum movement will save our livestock

New Zealand is home to many introduced species, some of which pose threats to the native flora and fauna. One such “invader” is the brushtail possum, a cat-sized and nocturnal marsupial introduced into New Zealand during the Nineteenth Century, for the fur industry. Possums have big impacts not only on the native species, that they eat, but also on New Zealand’s livestock raising industry.

Brushtail possums have become the main culprits behind bovine tuberculosis in New Zealand;  transmitting the disease to cattle and farmed deer. Given the importance of farming for New Zealand’s economy, how can we stop those freaking possums? Well, Lincoln University’s Dr. James Ross and his colleagues, Phd student Belinda Whyte and Dr Helen Blackie, may have found an answer for us! Through the study of possum home range and the calculation of possum population densities, James Ross is providing insights into how we can stop the possums from infecting our livestock with diseases.

By finding out

A brushtail possum. Picture credit: Steven D. Miller

the possum’s home range characteristics we can prioritize areas for pest control. Home range means the area in which an animal lives and moves. Keeping this in mind, Dr. Ross and his colleagues from the Centre for Wildlife Management and Conservation, conducted a field study on possums in the Canterbury Plains.

Their goal was to find how possum home range characteristics change between sites with different densities of possums. They chose three sites with known possum’s densities (this is the number of possums per unit area) across Canterbury. Two of these sites had a low density of brushtail possums while the third site was much higher. Each of the sites was located within mixed landscapes of forests and open pasture areas with agriculture and livestock grazing. The low density sites had pine forests, while the high-density site had oak forests.

How do you actually study the possum themselves, since physically following them 24 hours a day is imposible! In order to study an animal like the possum, researchers must find methods to track them constantly without actually being present. What Ross and colleagues did is capture as many possums as possible on each of the sites and fit them with special collars which contained radio units and/or GPS units. The signal from these units can then picked up by an antenna in the hands of the researcher, thus revealing the possum’s location. Through a 5 week period, the researchers followed the collared individuals, recording their locations to find out where they moved (be it in the forest or in pasture). After that, they analyzed the results, and what they found turned out to be very surprising.

It turned out that possums in the two sites with low densities, shared more space with each other than possums in the high density site. This may sound counter intuitive, having more individuals in the same area should result in individuals sharing more space, as individuals are more likely to find each other. The reason why possums exhibited the opposite pattern is not clear. Perhaps it has to do with the fact that the low density sites had fewer resources, therefore possums needed to cover larger areas to obtain food and were more likely to come into contact with each other.

Another hint comes from the possums’ territorial behavior; dominant possums will often prevent weaker individuals from accessing resources. In a situation with fewer possums, perhaps weaker individuals have less pressure from the dominant ones, and can move over a larger area.

These findings are especially relevant for controlling bovine tuberculosis. One might think that pest control should exclusively target areas with a high density of possums. However, Ross and colleagues suggest there may be at least as high a risk in low density sites. This is because at those low-density sites, possums move longer distances and over larger areas.

Lovely New Zealand Cows. Image source: Dr. Gavin Globe

Individual possums who wander through larger areas are more likely to come into contact with cattle. How do we address this issue? A good way to start would be to take into account the home range characteristics of these possums when designing pest control plans. For example, more traps and baits could be needed depending on whether or not the possums in the area have large home ranges. This would also result in higher funding needs for effective possum control programs.

This study has given us great insights into the possum home range characteristics. However, we are barely scratching the surface of this issue. James and his colleagues do admit that this study raises even more questions. For example, we do not know what factors drive possums to move into cattle pastures. This study only suggested that possums at lower density sites move farther, but not necessarily whether this is more into pastures than the possums at high density sites.

More research into the drivers of home range size, such as resource availability or possum population density, could potentially help us predict when possums are most likely to increase their home range and move farther. If we know this, we can step ourselves ahead of them in the game and exterminate them before they infect our precious livestock!

The author Lester Fox Rosales is a postgraduate student in the Master of International Nature Conservation taught at Lincoln University and University of Göttingen. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.

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