Sometimes common sense is not actually our friend. In our society common sense is usually seen as a virtue that those so-called experts could do with more of. But common sense can fall down on the common side of things. The All Blacks have gone into every rugby world cup as the favourites to win over all. They have won only two. This is a case where ‘common’ is a statistical concept in which there is a higher likelihood of something happening, but no guarantee that it will happen. Common sense can also fall down on the ‘sense’ side of things as well. Looking at our modern lifestyle it seems obvious that we are much less active than our ancestors would have been. It’s no wonder we have issues with weight in our societies. However, data points to the fact that we are actually just as active as our hunter-gatherer ancestors and we need to look at other reasons for obesity. This is a case where ‘sense’ is a statement that needs data to confirm it. So although common sense is a useful concept, allowing us to make predictions based on experience, it can be misled through probability and through not having correct facts. Common sense is particularly a problem when we are faced with emotion-charged debates. In New Zealand one of the more emotionally charged debates is with the use of toxins to kill mammalian pest species, like rats, possums and stoats, at very large scales.

There’s a whole lot of seeds up there!

The southern beech (Nothofagus) forests of New Zealand are prone to heavy seed crops every 4-6 years (known as a beech mast). Mast events provide a huge amount of food for rodents whose populations swell over the next few months. With the rise in rodent numbers we get a similar, though slower, response by their predators, particularly the stoats (a mustelid). The following year is almost never a mast event and so rodent numbers crash as there is not enough food around. The inflated predator population will continue to eat rodents where they can but, with the rodent population decline, they often switch to other prey. In New Zealand this other prey is mainly birds, and in beech forest these are usually native species. So a mast event will usually lead to a massive reduction in bird populations the following year, when the stoats get hungry. This year has been a mast year in our beech forests. What can we do to ensure that our threatened bird species are not decimated over the next year?

The Department of Conservation has launched the ‘Battle for our birds‘ campaign. Common sense would suggest that if we could keep the rodent numbers down during the mast event, then their populations will not increase and, therefore, stoat numbers will not increase as a result. So, no  problems for birds next year. Department of Conservation are delivering the toxin 1080 to several hundred thousand hectares of key beech forest habitat throughout New Zealand this year. The toxin is dropped into the forest where is can be encountered by rodents (usually rats) who will store some of the cereal bait, eventually consuming a lethal dose. Stoats eating recently dead rats may also accumulate a lethal dose and their populations don’t increase as their food supply does not increase. Happy birds in 2015. Problem solved.

Rats: In action on the forest floor

Maybe. There are a number of studies now that have looked at the long term effects of intense control of mammal pest populations. In a seminar at the Department of Ecology, Lincoln University, Dr James Ross talks through a couple of studies that he has been involved with which looks at what happens to populations that have had pest control done. Long-story short (but you should listen to that entertaining talk!), 1080 does a great job of knocking down rodent and possum populations (and stoats as a by-product). The forests habitats respond positively to the removal of the pests, particularly the small bird species. However, different species recover at different speeds. Mice are not particularly troubled by toxin drops (as rats are usually dominant and grab the baits) and their numbers begin to grow after control as they have no competition from rats. This is not such good news for forest invertebrates that are in the food size range of mice. After a few years, the rat populations start to bounce back (mostly by re-invasions from the edges of the control areas) and at a much faster rate than stoats. So there is a period of time where rat populations will not be checked by stoat predation. So will the battle for our birds work? It depends on the timeframe and what follow up work is done. Control will certainly put a break on rat (and then stoat) populations and preserve bird populations in the following year, that’s just common sense. However, if follow up control of rats is not done after three or so years then rat populations may rebound ahead of their predators and we will have a problem similar to that of a mast season occurring, lots of rats that lead to a surge in stoats. So we might win the battle but lose the war if we are not careful. And that, perhaps, isn’t such common sense.