To help us design our detection system at Bottle Rock, we wanted to learn more about what a ‘lonely’ rat does, and where it goes, when it enters an area with no other rats. In other words, we wanted to better understand the ‘footprint’ of a rat invasion in space and time, to help ensure early detection and removal.
To do this, we caught individual ship rats from an area adjacent to Bottle Rock peninsula, fitted them with VHF transmitters, and released them one at a time into the protected area to see where they went.
We then manually tracked each radio collared rat to locate daily den sites.
At the time of the experiment, all resident rats had been removed from the Protected Area at Bottle Rock peninsula.
We designed the experiment on the basis that in a worst-case incursion scenario – an invading pregnant female – we would have 28-48 days to remove the mother before her pups became independent. This would comfortably allow up to 20 days for initial detection of an invader.
Therefore, we allowed the experiment to run for a maximum of 20 days before each rat was trapped and removed from the peninsula before a subsequent rat was released.
We placed a ‘light coastal detection’ network for each release, which evolved as we learnt more about the rats’ behaviour and movements. In the final iteration of this network, detection tools (chew cards and ‘Tun200’ traps wired open and containing inked tracking cards, both baited with peanut butter) were placed 80 metres apart along two lines running parallel to the coastline, approximately 100 metres and 300 metres inland respectively.
Four rats were successfully released and monitored using this approach.
We found that all Ship rats released behaved quite differently:
The first rat released, ‘Dr Livingstone’, adopted a range of approximately seven hectares, remaining relatively close to the release point.
The next rat, ‘Hanno the Navigator’, travelled along the coastline before adopting a den site at the tip of a small peninsula. Autopsy revealed he had developed a liking for seafood!
The other two rats (‘Calamity Jane’ and ‘Helen’) ran inland in arrow-like fashion, likely following a waterway, before exhibiting more localised ‘camped’ behaviour.
Only half of the rats were successfully detected by our ‘light detection’ system. We learned that it can be extremely challenging to efficiently detect the lone rat in a large landscape, as those released (after an initial period of exploration) generally adopted a small home range, in the order of one hectare. Therefore, the intensity of devices required to guarantee detection does not lend itself to readily scale up the ‘light detection’ system.
More optimistically, the relatively small home ranges adopted indicate that functional extinction is a likely outcome if less than one ship rat is present per, say, 100 hectares.
This finding has led us to rethink our approach to detection, and to ask the question:
“What would happen if we didn’t worry about individual invaders, but instead targeted our removal efforts on the first generation of offspring after an invader’s initial breeding event?”
Limitations of the experiment
There is inevitably a degree of artificiality with an experiment such as this one. Rats released, while caught locally just outside the ‘virtual barrier’, had not deliberately entered the peninsula through the barrier or via the sea, so the behaviour demonstrated may not be typical of an invading rat.
If we were to repeat this experiment, we would attempt to reduce the holding period from capture to release (i.e. fit a transmitter and release at night) to minimise the potential to influence their behaviour.