Effectiveness of aerial poison drops

Our previous blog post showed a simple model to allow you to work out trap interaction rates. The goal of this post is to show that this same simple model can also be used to explain the effectiveness of aerial poison drops. We are not making any comments about the value of poison drops as a tool but just showing how a simple model works for different elimination methods. We like the fact that a fairly simple tool can help explain the relative merits of different elimination methods. The real value starts to appear when we tweak some of the other parameters in the model - we think it gives us a clear idea about useful ways to improve predator elimination.

Let's check the terms we defined in the first blog:

  • 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 poison pellets dropped.  Baits are usually about 6–8 g in weight and spread at two to five kilograms per hectare.
  • Kill Rate: The % of animals that are eliminated when they consume the poison.  In most cases, this is close to 100% but some may get partial doses or refuse the poison altogether.
  • Interaction Rate:  The chances of a predator interacting with a pellet.  Coming across it and consuming at least some of it.

The graph below show the expected result given our example inputs.

In this case the predator numbers drop to a very low number very rapidly which seems to reflect most people's experience in the real world. We know it doesn’t go to absolute zero in most cases but certainly has a higher knock down rate than trapping and this model explains it by the higher number of chances to interact with elimination "device" (poison). In the real world there is often re-invasion and breeding of surviving populations so this usually leads to the poison drop having to be repeated.

We have now made the full model available - you can download your own copy here.  This version of the model has tabs that show results for aerial drops, improved kill rates, auto resetting traps, automated lure dispensers, and high interaction traps as well as some supplementary material we've used to clarify our thinking around how this model should be used and its results understood. We encourage you to have a play with each of the tabs and please do let us know if you get any interesting results - email us at blog@cacophony.org.nz

What this means

The aerial drop model we discussed in this article shows that aerial drops work well due to the high number of "devices" (pellets).  Even at low interaction rates, the poison drops are effective at drastically reducing the target populations.  This reflects what we see in real life and this is why such methods remain a weapon in DOC's armoury.  Having the model show the expected real-world results gives us further confidence that the model is useful for understanding different elimination strategies.

There may be nothing particularly insightful in today's little analysis but it does confirm that this simple model helps us understand how different elimination methods work.  Where this becomes important is when we attempt to explain why it seems likely many common trap developments won't provide the leap forward we're all hoping for (more on that in the next blog post).  But if that feels like bad news - don’t worry.  There are a number of things that do look wildly useful and the whole point of this series of articles is to try to get more people thinking about and working on these useful things.

As always, we welcome your feedback so don't hesitate to get in touch or leave a comment below.

The model we shared 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.


Publication Date: 
Friday, 13 March 2020