Getting out and about, it depends on how dense you are

In which we look at how possum home range is influenced by density.

I have just been home for lunch and found an infestation of 16 year old boys at my house. Most had just completed their last school exam for the year and come to see my son and hang out. All 12 of them! Usually my sons are either on their own or have a friend of two with them when they are at home. When Dad lectures animal behaviour it is always an opportunity to see some of the actions that I teach and research in, well, action. When my sons are on their own they tend to do things like x-boxing, shooting hoops in the driveway, raiding the fridge, play music, watching telly, facebook, annoying brothers/mother/father/cats as they roam around various parts of the house. Sometimes they will leave the house to visit a friend or go to the gym. Add a friend at home and you get similar behaviours but with slightly less movement around the house and less of a likelihood for visiting other friends. When you have an infestation level of 16 year olds you get quite a change in behaviour. The boys tend to congregate in the games room playing pool, xbox or out on the driveway shooting hoops. The boys seldom move into the rest of the house and they seldom leave the property in a big group to visit friends (presumably because most of the friends are here and because not many houses can cope with so many large teens). With my animal behaviour glasses on it looks like the density of the population affects the movement behaviour of the boys.


In the wildlife management business, we almost always are concerned with changing the densities of animal populations: reducing density of pests and increasing the densities of threatened species. One of the difficulties involved in altering densities is when behaviours change when numbers in an area drop. One pest species that we spend a lot of time in trying to reduce in numbers is the brush-tailed possum. This Australian species was introduced into New Zealand over one hundred years ago and has grown to a population in the tens of millions which does enormous damage to native diversity. Possums also carry bovine tuberculosis and can spread the disease through cattle. Hence, possums are public enemy number 1 in New Zealand and are controlled (a polite kiwi way of saying slaughtered) in vast numbers around the country. As we have gotten better at controlling possum populations we have become interested in what happens to their behaviour as population densities decrease.

Possum control seeks to preserve native areas

Belinda Whyte has just completed a PhD in the Centre for Wildlife Management and Conservation at Lincoln University. Belinda’s research directly examined possum behaviour in populations of different density and her first paper, with her advisors James Ross and Helen Blackie, has been published in Wildlife Research. Belinda’s research was mostly centred in the Malvern area of central Canterbury. She had two sites which had low possum densities (1-2/ha) and one site with high densities (7/ha). Live-trap cages were placed in the sites and possums were captured and fitted with GPS collars that allowed Belinda to follow the movement of the individual over the following months. Three times a week individuals were tracked to their den sites (areas where they slept during the day). Population density seemed to make a difference. The two low density sites had possums that behaved similarly to each other but both differed from the high density site. At the low density sites possums had much larger home ranges (9-12 ha compared to 2 ha), more home range overlap (26% compared to 6%) and more den sites (4 compared to 3) than high density possums. So, in low densities possums tend to move around further, potentially interact more with other possums and have several places of safety. These outcomes have important implications for control. By reducing a possum population you will have fewer individuals that can carry and spread TB or eat your favourite native bird species eggs. However, this gain is tempered by the fact that surviving possums will forage further afield, potentially coming in contact with more cattle, and that they are more likely to bump into neighbouring possums and spread the disease amongst the surviviors. So while controlling possums is a useful thing to do it is not as effective as it could be because they change their behaviour as densities decline.

So teenage boys and possums seems to have some behaviours in common! Home range behaviours change depending on density of the local population. Let’s just hope that urine marking of territory is not one of those behaviours in common or I might not want to go back to my den tonight.

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