How Often Do Predators Interact with Traps?
Over the last three years we have had various versions of our thermal cameras in front of all the main types of traps. What we have found is that a large number of predators seem to just walk on past irrespective of what type of lures and set ups we use. We have detailed videos of rats and possums running all around traps but not always interacting with them. Typical reaction to this is that we must be doing something wrong but after watching tens of thousands of videos we suspect this is more of an issue than just us being hopeless trappers.
Today we introduce a model we have developed to allow you to calculate the interaction rate of your traps. Given inputs such as interaction rate, elimination rate, number of devices, and predator population the model creates a graph showing the likely impact on the population. We think the results are not only intriguing but telling. They suggest it might be time for all of us to adjust our view of the importance of some of the factors involved.
Our evidence shows that predators are not interacting with traps as much as we would all like them to. Our goal is total elimination of an introduced predator population so we need to work out what a low interaction rate might mean for the approach we (and the rest of Aotearoa) are taking to solving the problem.
Some Definitions
To understand the model, you need to understand the four input factors (don't worry, you know them already, we're just defining them so we're all talking about the same thing). We define them as the following:
Predator Population: The number of target predators in the target area. This is an estimate of how many predators per hectare (in our example, we assume 10 hectares).
Number of Elimination Devices: The number of devices deployed in the target area (we prefer the term “elimination device” – it’s more generic than “trap”).
Kill Rate: The elimination success rate of each device. Defined as the % of animals that are eliminated when they trigger the trap. In most cases, this is close to 100%.
Trap Interaction Rate: The number of times a predator interacts enough to trigger a trap in a given time and area.
In our model, we have chosen to define the interaction rate based on the chance a predator interacts with a trap in a 10 hectare area over one week. This time and area is arbitrary and you could choose different ones if you want but obviously given infinite time it is likely to interact or infinite space it never will (almost certain to interact given a tiny constrained space).
What the Model looks like
Get your own copy of the model here.
Changing the inputs on the left (highlighted in yellow) will change the graph on the right. We've found that the best way to start is to adjust the Trap Interaction Rate and see what happens to the graph. If you're already actively trapping an area, what you're looking for is a graph on the right hand side that looks like your real-world experience. Play with the numbers a bit and you'll see how the graph changes. Once you get something that looks real to you, look at the interaction rate - is it lower than you expected? We'd love to see your results so please do share them in the comments below or you can email them to us at blog@cacophony.org.nz.
How The Model Works
The example shown below assumes a high density of possums at four per hectare and suggests the trapping effort would kill 75% of the population in the first year which is reasonably optimistic.
Obviously, in the real world, you have population growth, population movements, re-invasion, and lots of other complexities but our goal here is to show the impact of the interaction rate. The model is based on a best case auto-resetting, auto re-baiting device and the interaction rate still ends up being about 0.4%. That is a 0.4% chance of a predator interacting with a trap per week per 10 hectare area. This seems very low but try some other numbers - whatever you feel reflects reality. We tried an interaction rate of 10% and it shows all predators gone in a week which clearly doesn’t happen. Also worth taking note of is the calculation that shows the number of catches per trap per year (you probably know this for your traps). We have some good data from trap.nz that shows this number for sixty of the best monitored projects around New Zealand (see below).
Most people we talk to seem to think their trap networks do better than this but a number of 6 catches per trap per year would mean that if you checked them all every 2 months every trap would have a predator in it. In most of these projects the catch rate per year is fairly constant which suggests there is a persistent predator population.
What it all means
We've been working on this for a long time now so it feels obvious to us but we're aware this may have been a lot to take in so let's summarise:
We have video evidence to suggest that the vast majority of animals just walk on past traps without interacting with them
We now have a simple model that takes real world data from 60 projects around New Zealand
The model confirms that, for real world projects to be showing the results they are, the trap interaction rate must be very low
Let's take a moment to think about that. A substantial army of trappers are putting in an enormous amount of effort to place and maintain networks of traps. We suspect everyone agrees that all that valiant effort is achieving a suppression of the predator population, not an elimination. Why is that? Is it that we're all terrible trappers? We think not. Aotearoa probably has a more skilled set of trappers than any other country. Those trappers have a rich set of efficient elimination devices available to them. Thanks to DOC, we have tried & tested protocols for the use and deployment of all those devices.
So...if it's not us, it must be them! If the predators are rude enough to mostly ignore all these lovely devices we've been delivering to their back yards, then it seems likely that we're only impacting a small percentage of the population.
So what should we do? The model suggests that increasing the kill rate of devices can only ever be part of the solution. And it feels like we already have a set of pretty good elimination devices. What will make all the devices (existing and future) more efficient would be to increase their interaction rate. We think that's the most important lesson for us to learn. There is not a lot of point just automating something most predators continue to walk past. If predators don't interact with traps, better traps won't solve the problem.
Everything suggests that increasing the interaction rate could have an enormous impact. And an enormous impact is what we're all after. I'm sure we're all tired of being reminded that elimination is a hard problem to solve. We think we might've found a way to make solving that problem substantially easier.
More on this in the next blog posts.
The model we introduced today is a gift from us here at Cacophony to trappers and trapping communities worldwide. Please use the model at will and feel free to share it.
We'd love to see your results so please do share them in the comments below (tell us your interaction rate) or you can email them to us at blog@cacophony.org.nz.
