At my house we have our own pest mammal species - teenage boys. Packs of them occasionally descend on our home and browse their way through the pantry. If we were to forbid them from our home would this be beneficial to our food stocks? Or would our resident teenagers simply consume more? Perhaps there are certain kinds of foods that might be eaten more in packs (chips and fizzy drinks) than in lower densities (2-minute noodles)?
When you have a pest species that forages on plants it seems like a no-brainer that controlling that species will be helpful to the plant species in an area. But is it helpful to all plant species, or all individuals, in all areas? In New Zealand we have a major pest species, the brushtail possum, that browses on plants as part of its diet. There are so many of these pests that they can have a major impact on our native flora. Because of this, possums are often controlled (through trapping and poisoning) over vast areas. However, we are not really sure how this reduction of pest numbers actually impacts on plant damage. We simply assume that it will help.
A group of researchers led by Richard Duncan from Lincoln University and colleagues from Landcare Research have looked at this issue in possums. They were interested in whether possum density affects browse damage in native forest. They needed to find a species in the Goldilocks zone: not too palatable, not inedible, just right. In our teenage boy example there would be no point in looking at impact on chocolate biscuits (as they will be all eaten if even one boy is around) or salad (as things would have to be very desperate for a teenage boy to consider that food). Perhaps tins of spaghetti or baked beans would be a good food to monitor. In much the same reasoning, Richard chose kamahi (Weinmannia racemosa) as their indicator species. This is a species that possums will browse but is not a particular favourite.
Richard and his colleagues measured browse damage as well as trapping areas to find out the density of possums. Measurements were made at 21 sites around the North Island of New Zealand over 8 years. Possum numbers fluctuated a great deal over this time. In a paper published in Austral Ecology they found that where there were more possums there was also more browse damage but that damage was not consistently spread among individual kamahi at a site, nor was it consistent between years - damage was very patchy. Within any forest area, regardless of the density of possums there were kamahi with extensive browse damage. What the researchers concluded was that reducing the density of possums might reduce the overall level of browsing (fewer trees will be affected) but would not impact on heavily browsed kamahi (possums will still cause a lot of damage for certain kamahi). This has obvious implications for looking at which areas to put possum control into.
Does this help with our teenage boy infestation? Well it does suggest that larger packs will generally eat more but that certain foods will be heavily impacted regardless of the numbers of boys about. Which seems to fit our food shop - no matter how many 2-minute noodles we buy each week they are always consumed!