A Christmas tale of density dependence….

We all know the scenario. Christmas Day and you pack up the kids and wife to head to the grandparents for dinner. Lincoln to Ashburton is a fast 50 minutes of straight, flat driving over the Canterbury Plains, only enlivened by the wide Rakaia River (and giant salmon statue) and the Book Barn in Chertsey (which is literally a barn, held up by thousands of cheap and interesting books). There are plenty of passing lanes and the traffic hums along at the speed limit. But this is Christmas day. It is 30 degrees Celsius. Everyone is full of festive cheer (for good and bad). And it seems that everyone in Christchurch has remembered family in Ashburton that they need to visit. The roads are packed. The passing lanes allow you to make no real progress in a constant flow of traffic. The speed of the trip is much lower and therefore slower. On a normal day, the average speed of traffic would depend on the fastest drivers, as they can drive at whatever pace (within the speed limit) they choose to, and they can pass slower drivers. But on days like Christmas, the average speed of traffic depends on the slowest drivers as it is almost impossible to pass them. As we crawl along at 60-70 km/hr I have plenty of time to reflect on the effect of density dependence. As the density of traffic increases, slow drivers have more influence, as traffic density decreases, fast drivers have more influence. For some reason, explaining this to my impatient carload of teens and wife does not seem to be well received. Are we there yet?

How about we look at a different example? Ignoring the indifference from the back seats I plunge on. Who would win a fight between a possum and a rat? The possum. Why? Because a possum is much bigger than a rat. So if we put baits in a forest because we are looking to control possum numbers and then – what? why? Well possums are evil SOBs who destroy our forests, eat our native wildlife, spread disease into our cattle and probably laugh while they’re doing it. So we want to kill them all. If we put cereal baits in the forest, they get eaten by possums and the toxin hidden inside the bait completes the job. Of course rats also like to eat those baits and sometimes they do (or at least take the baits away to eat later – yes, like a teenager), but a) we don’t really care as rats are nearly as bad on our public enemy scale and 2) possums will usually win encounters with rats and get the baits. But of course life is never this simple. Things change when you change densities. How do I know? My colleague (and neighbour at work) James Ross, and former PhD student Shona Sam, from Lincoln University have done some research on this. What do you mean “oh no this isn’t going to end up in one of your blogs is it Dad”? Course not.

Anyway. James and Shona put out baits with trail cameras and found when rats are in very high densities that they are likely to take over 90% of the available baits and that possums seldom find or eat any baits. If rat control is put in place, reducing rats by 80% or more, then virtually all of the baits are available for the possums and many possums will find and consume a lethal dose. Why does this happen? No it’s not because the possums are scared when there are of lots of rats and back off. Rather, the cameras showed that rats start searching for food at least a couple of hours before possums do. This doesn’t matter much when there are few rats and plenty of baits. However, when there are lots of rats then they are more likely to have found the baits and consumed them before the possums even start looking. So there you go. Controlling possums successfully is influenced by the density of rats. For those in the real world who have to control possums this means that they may have to do some rat control before they do possum control to achieve the kill rates that they desire. And that is a good example of density dependence. Just like the car speeds. What do you think of that?

The sound of dozing teens is a beautiful thing on a long, hot, Christmas drive to Ashburton.

Note: This research is published in the Proceedings of the 26th Vertebrate Pest Conference. For a copy of the paper contact James .