Optimal foraging: Levy pattern or process?

Many different species have been suggested to forage according to a Levy walk in which the distribution of step lengths is heavy-tailed. Theoretical research has shown that a Levy exponent of approximately 2 can provide a higher foraging efficiency than other exponents. In this paper, a composite search model is presented for non-destructive foraging behaviour based on Brownian (i.e. non-heavy-tailed) motion. The model consists of an intensive search phase, followed by an extensive phase, if no food is found in the intensive phase. Quantities commonly observed in the field, such as the distance travelled before finding food and the net displacement in a fixed time interval, are examined and compared with the results of a Levy walk model. It is shown that it may be very difficult, in practice, to distinguish between the Brownian and the Levy models on the basis of observed data. A mathematical expression for the optimal time to switch from intensive to extensive search mode is derived, and it is shown that the composite search model provides higher foraging efficiency than the Levy model.