Friday, September 3, 2010


Finally! I'm going to explain what exactly it is we've been doing with rats for the past couple weeks. As I mentioned in my earlier post (July 26th), the goal of the project I'm working on this summer is to examine how the invasive species Rattus rattus (the common rat) affects the food web structure of native forests here in Hawaii. We spent the first 5 weeks collecting insects to get a pre-rat control insect diversity and concentration level. Next summer, whoever is working on the project will begin eliminating rats from 16 of the 32 kipuka. Part of the work we're doing with rats this summer is a test rat exclusion; in an isolated kipuka that isn't part of the project, we're attempting to see how effective we are at eliminating the rats. It's miraculous no one broke any fingers over the last two weeks- two to three times a week we go out to the kipuka, see if any rats have been killed (and collect them if they have been: eww!), and then replace the bait and reset the traps. On an average day, I reset about 50 snap traps, so in the past few weeks I've reset nearly 200 of them. And no broken fingers!

I've also been fortunate in that on the days I've checked the traps, we've only caught one rat. Lucky Jenny, has had the "opportunity" to witness the demises of the other 13 rats we've caught. Since the first day, we've only caught 2 or 3 rats and lately we've started catching rat's prey, mice. Turns out there were way fewer rats than anyone predicted in the kipuka (or the rats are really smart)- I predicted that we would catch 85. Oops. The final process is putting out tracking tunnels, little ink cards baited with coconut to attract rats, to see if there are anymore rats hiding out in the kipuka, which we did earlier this week.

Our other task with rats has been much more exhausting. Since we're eliminating rats from some kipuka, it's important to know if rats travel outside of and between kipuka. Otherwise, killing the rats in one kipuka may simply prompt other rats nearby to come in and replace the dead rats. To do this, we've been attempting to capture rats in live traps, place radio transmitters on their necks, and then go out once a week and see where the rats are in the day and at night, with the hope of learning how much the rats move.

The hardest part has actually been catching and collaring the rats. In some kipuka we haven't even caught rats, in others, the rats have been too small or they've killed by predators before we could collar them, and sometimes the stress of being collared has been too much for the rats. Additionally some of the rats we've captured and collared have stopped moving deep in the lava- they've either been killed or shaken off their collar. Below are a few pictures of the collaring process:

We first anesthetize the rat, and then while it's knocked out, we weigh the rat (to make sure the collar isn't too heavy), put the collar, and take some measurements on the rat.

Day-tracking the rats is fairly easy since the rats are nocturnal- during the day we're simply tracking down the location of their dens. We tune into specific rats' frequencies with our receivers and keep walking till we find where the signal is the strongest. The biggest surprise has been in one of the kipuka where we learned after frustratingly searching for a couple hours that the rats dens are sometimes up in the trees.

Night-tracking is the 'fun' part. The rats are active, so approaching them isn't an option. Instead Jenny, Devin, and I get close enough to get a strong radio signal, spread out, and then take a GPS point and bearing on where the signal is strongest. Afterwards, Devin imports the bearings and location into a piece of software and it spits out a 95% confidence interval of where the rat was. Things are rarely that simple though. Since the rats are active at night, we're often tracking a moving rat which causes the location of our signal to change drastically, and the topography of the kipuka and matrix causes odd distortions of the radio signal, making things even more difficult. And there's all the human error: Oh! I'm supposed to read the number on the bottom of the compass to get the bearing? Oops, I forgot to take a GPS point!, and Hmm... our bearings don't intersect.

The moral of the story from tracking and controlling rats this summer is science is never perfect. We way over-estimated the number of rats and capturing and keeping track of the rats has been way harder than we imagined. Regardless, we are learning more and more about how rats function in the kipuka and matrix, and that's science!

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