Experimental Design

One thing I enjoyed about my summer in the Yukon was the sort of conversation that can be found in a remote camp of a dozen or so young biologists. While the squirrels being studied at camp were certainly a dominant topic, we would also spend time talking about other research projects we had heard about or been involved in. On a particularly wild night, we watched an episode of BBC's Planet Earth 2 on an old laptop and I have never seen such an excited and vocal audience for a documentary.

If anyone at camp suggested an untested theory or hypothesis, the next question was 'how would you design an experiment to find that out?" For example, if someone proposed that squirrels bark when they hear other squirrels barking, a simple experiment might be to play recordings of squirrel barks in the forest and see if real barking increases in frequency.

There were many good experiment designs discussed, but perhaps the most brilliant was a story we found written in a textbook written by some of the scientists who had started doing ecology research in Kluane (the part of the Yukon we were in) over 30 years ago.

The question to be answered was 'is the population of snowshoe hares limited by food availability?' A standard experiment to answer this question is a 'food add'-- if you put more food in the hares' environment, and the hare population then increases relative to an environment with no added food, there is evidence that hare population is limited (at least in part) by food availability.

There was, however, a problem. Snowshoe hares in Kluane share an environment with bears, and the containers of food pellets put out for the hares were being eaten by said bears. To prevent this, a series of clever containers were made to allow hares access to the food while keeping bears from eating everything and ruining the experiment: metal bars at the mouth of a container, for example, could create a barrier for bears that hares could slip right through. Unfortunately, the bears were able to break everything that the researchers could (cost-effectively) come up with.

The key design question, then, for this food add experiment is how to provide food for hares without providing food for bears. How might it be done?





It's not the easiest question, considering the variety of things that bears will eat.





The brilliant solution that was eventually arrived at was to scatter the food pellets over the forest floor instead of depositing them in containers as is typical in many food adds. This works because of Optimal Foraging Theory, which says that since animals need to spend time and energy to acquire food, they will only make an effort if the reward for foraging is worth their investment. When the food pellets were all together in containers, it was worth a bear's time to break into said container, but if eating all those pellets required picking them up one by one off the ground, it was no longer sustainable foraging for a bear. For a hare, however, finding and eating pellets one by one was a great deal, and thus the experiment was successfully conducted. The design was simple, elegant, and made use of a broad ecological theory for fine-tuning; that sort of experiment is something to aspire to.