QUANTIFYING THE EFFECT OF PREDATION RISK ON FORAGING BULLIES - NO NEED TO ASSUME AN IFD

Previous studies of the effects of predation risk on patch choice have assumed the distribution of foragers to be Ideal Free. However, recen t investigations have revealed systematic departures from the Ideal Fr ee Distribution. In this study we do not assume that the distribution of foragers fits an Ideal Free Distribution. Instead we use a method t hat separates the effects on patch choice of predator avoidance (site bias) from effects due to the resource ratio. Our experiment examines the distribution of foraging upland bullies (Gobiomorphus breviceps) b etween two resource sites over a series of five different food input r atios. We had two conditions, a predator-free and a predator condition . The bullies exhibited a strong bias (preference) towards Site 1 when there was no predator (quinnat salmon, Oncorhynchus tshawytscha). Whe n a salmon was present at Site 1 they exhibited a strong bias towards the alternative resource site (Site 2). We estimated that it would be necessary to change the relative food availabilities by a factor of ap proximate to 28 to remove the effect of the presence of the predator. The slope of the distribution of bullies (their sensitivity to resourc e profitability) deviates from the Ideal Free prediction for both cond itions. A trend in the slope of the distribution of the bullies betwee n the predator-free and predator conditions suggested that the presenc e of a predator may alter the behavior of the bullies, By acknowledgin g that the distribution of foragers may deviate from Ideal Free, we de monstrate that it is possible to separate the effects of predator avoi dance from a change in their ability to discriminate between resource sites caused by the presence of a predator.