Quantifying the Effect of Predation Risk on Foraging Bullies: No Need to Assume an IFD
1994
Kennedy, Martyn | Shave, Cathy R. | Spencer, Hamish G. | Gray, Russell D.
Previous studies of the effects of predation risk on patch choice have assumed the distribution of foragers to be Ideal Free. However, recent investigations have revealed systematic departures from the Ideal Free Distribution. In this study we do not assume that the distribution of foragers fits an Ideal Free Distribution. Instead we use a method that 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) between two resource sites over a series of five different food input ratios. We had two conditions, a predator—free and a predator condition. The bullies exhibited a strong bias (preference) toward Site 1 when there was no predator (quinnat salmon, Oncorhynchus tshawystcha). When 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 °28 to remove the effect of the presence of the predator. The slope of the distribution of bullies (their sensitivity to resource profitability) deviates from the Ideal Free prediction for both conditions. A trend in the slope of the distribution of the bullies between the predator—free and predator conditions suggested that the presence of a predator may alter the behavior of the bullies. By acknowledging that the distribution of foragers may deviate from Ideal Free, we demonstrate that it is possible to separate the effects of predator avoidance from a change in their ability to discriminate between resource sites caused by the presence of a predator.
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