FA08LabManual_SquirrelForaging

FA08LabManual_SquirrelForaging - Lab #5: Fox Squirrel...

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Lab #5: Fox Squirrel Foraging and the Landscape of Fear GSI: Erin Meyer BIO 1B, Fall 2008, Section 130 FOX SQUIRREL FORAGING AND THE LANDSCAPE OF FEAR Animals (and plants) must forage for food and other resources when they are scattered around a heterogeneous environment. Foraging by any organism involves trading off the gains made by collecting food against the costs of doing so, including costs of energy spent moving and coping with stressful environments, and their exposure to injury or death by competitors or predators. To understand how and why a forager chooses an area, we must understand both the food availability and how a forager perceives the level of risk across a particular landscape. Food availability varies from site to site, but so too do environmental conditions that foragers experience (e.g. temperature, wind) and the risks to which they are exposed (e.g. predators). We can sometimes estimate short-term food availability from the density of food or the ease with which a forager can extract it from the local environment. Food availability is also a variable that can fairly easily be controlled or altered in an experimental setting. However, evaluating how a forager perceives risk is difficult, especially in different types of environments. Joel Brown, a behavioral evolutionary ecologist at the University of Illinois, Chicago, determined a way to measure “the landscape of fear” for various animals. (Brown’s “landscape of fear” measurement is a proxy for how foragers perceive risk in their environment.) Many animals can be considered “central place foragers”. These animals venture out from a relatively safe place (e.g. a burrow for a rabbit, gopher, or crayfish; a tree for a squirrel or a bird) into a more hazardous place to find food (e.g. an open field). Brown reasons that foragers might deplete patches more thoroughly in areas that they consider safe, and thus where they spend a lot of time. In patches they perceive as more dangerous, foragers might abandon the site earlier, “giving up” on foraging in that location. In the latter case, foragers will leave behind a higher density of food. If food densities at various sites were initially similar (before foraging occurred), the “Giving Up Density” (GUD) of food at a site after an animal quits foraging might indicate how risky the animal considered that particular landscape position to be. The fox squirrel ( Sciurus niger rufiventer ), native to the Midwest United States, was introduced to the Berkeley campus around 1926, and has adapted very successfully to this environment (Boulware 1941). It currently nests in campus blue gum trees ( Eucalyptus globules ) imported from Australia, and eats their nuts (Boulware 1941). Fox squirrels often collect gum nuts and
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FA08LabManual_SquirrelForaging - Lab #5: Fox Squirrel...

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