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Antlion Foraging: Tracking Prey Across Space And Time
Ecology, Oct, 1999 by Philip H. Crowley, Mary C. Linton
3. Threshold, the weighted-average foraging success during the interval, below which the antlion relocates its pit. - After each day's foraging, if the antlion has remained at the same location for at least the duration of the interval, then the weighted-average gain over the interval is compared with the threshold, and subthreshold gains trigger immediate pit relocation. This means that, all other things being equal, higher thresholds increase the relocation frequency, and lower thresholds reduce this frequency. Most threshold values that we used were derived by multiplying the overall mean mass gain per day (2.34 mg) by powers of two; we also used two extreme values, ensuring in these cases that the weighted-average gain was either never (zero) or always (300) below threshold. The values were 0, 0.146, 0.293, 0.585, 1.17, 2.34, 4.68, 9.36, 18.72, and 300 mg.
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4. Displacement, the number of "steps" taken by an antlion in a random walk to a new pit location. - Pit relocation simply shifted an antlion to a different pitfall trap position along the transect in 0.5-m steps. Each step was taken with equal probability in either direction along the transect. An antlion attempting to move off the end of a transect would instead spend a step and remain at the end location. This rule ensures that an antlion starting from an unspecified, random location is equally likely to occupy any of the 50 pitfall trap locations at any future time. There is both a pit-rebuilding cost (0.781 mg), estimated using a relationship derived by Griffiths (1980a) for Morter obscurus, and a movement cost per step (0.065 mg per 0.5 m), roughly estimated by assuming that a typical movement path length in the field of 6 m corresponds to approximately the same energy expenditure as rebuilding a pit (M. C. Linton, unpublished data); these reduce the net energy gain during the day that an antlion relocates. We assume that the time spent moving and rebuilding has a negligible influence on the time available for foraging. Displacements that we evaluated were 1, 3, 5, 10, 15, 20, 30, 50, and 100 steps. Actual displacements achieved (because longer random walks generally include much doubling back) were approximately proportional to the square root of the displacement number.
IMPLICATIONS OF AUTOCORRELATION
Fig. 1 summarizes our expectations about relationships between these components of foraging strategy and scales of autocorrelation. For spatial patterns, when the scale of spatial autocorrelation in prey availability is large, then trap relocation distances should be extensive (i.e., large displacement), to escape what may be a relatively large unprofitable vicinity. Because long-distance relocations may prove costly in terms of movement energetics and predation risk, small-scale spatial autocorrelation is likely to result in smaller trap displacements when relocation is necessary. For temporal patterns, when the scale of temporal autocorrelation in prey availability is large, only a relatively small window of time may be needed to estimate near-future foraging profitability, whereas the high variability associated with small-scale temporal autocorrelation should necessitate a longer assessment window.
