Sequential-encounter prey choice and effects of spatial resource variability

Models of diet choice generally assume that resource attributes such as encounter rates, energetic values and handling times are fixed constants. We present a model where either encounter rates or profitabilities of one food type vary randomly through space according to a Markov process. As in the standard diet model, we assume sequential encounter with prey items, mutually exclusive search and handling and we equate enhanced fitness with maximization of the long-term rate of energy gain. The ability of a forager to perceive and respond to the variability determines whether the forager tracks variability or averages over it. The results of this model differ from those of the standard diet model. In particular, the overall proportion of a given food type in the diet does not always match the proportion suggested by the “zero-one” rule of the standard diet model. Also, under some conditions, a forager is predicted to specialize on a prey type of lower average profitability, and to decide on the inclusion of a given prey type based on the abundance of a lower profitability prey. These are never predictions of the standard model. In general, tracking variability results in a greater weighted long-term rate of energy intake than averaging when encounter rates are variable. However, when profitability varies stochastically, a trade-off occurs between the cost of recognizing different kinds of variable items and the increased foraging efficiency gained when tracking variability. This trade-off determines whether tracking or averaging yields the greater long-term rate of energy gain for the forager.