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Antlion Foraging: Tracking Prey Across Space And Time
Ecology, Oct, 1999 by Philip H. Crowley, Mary C. Linton
Our simulation results suggest that antlions may relocate their pits only infrequently at Sleeping Bear Dunes National Lakeshore because of high relocation costs. Once these costs are reduced sufficiently, the optimal strategy shifts sharply in favor of a very brief site assessment, lower relocation threshold, larger displacement, and frequent relocations. This dramatic change can be understood as a response to spatial and temporal autocorrelations in foraging gain rate that, although largely undetectable by standard statistical methods, are nevertheless present and important.
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Ideally, an antlion's pit should be relocated whenever the expected gain from moving exceeds the cost. Even a positive temporal autocorrelation of only one day indicates that a low foraging gain today will tend to be repeated tomorrow, making immediate relocation advantageous whenever the overall expected daily gain for the habitat exceeds today's gain by more than the relocation cost. To ensure that the new location is less likely to suffer from the low prey availability of the current site, however, displacement should be great enough to reduce or eliminate any positive spatial autocorrelation with the current site, as long as movement costs are not excessive. Thus, the characteristics of the optimal strategy when relocation costs are very low are consistent with the spatiotemporal patchiness of prey availability apparent in Fig. 5. With significant relocation costs in place, however, it is simply too expensive for antlions to take full advantage of the information provided by the autocorrelation structure of prey availability in the habitat.
Note that this interpretation hinges on the magnitude of relocation cost relative to expected net foraging gain from relocating. This means that increasing the variance of gain rate over space and time has an effect on characteristics of the optimal foraging strategy comparable to reducing the relocation cost. Moreover, greater prey availability is often correlated with greater variance of prey availability, as in Tables 1 and 2. We might therefore generally expect to find more frequent trap relocations associated with greater prey availability, a pattern apparent in site and site-year comparisons for Table 2, where relocation costs are excluded, but not for Table 1.
This seemingly counterintuitive association between more frequent trap relocation and higher prey availability has been observed in the long-jawed orb-weaving spider Tetragnatha elongata, and has been attributed to two entirely different mechanisms. Comparisons of creek- and lakeside populations of T. elongata showed that prey are more abundant near the lake, where spiders relocate their webs almost every night, whereas individuals in the creek population maintain the same web for at least several days before relocating (Caraco and Gillespie 1986, Gillespie and Caraco 1987, Smallwood 1993). Caraco and Gillespie explain the difference based on the distinction between risk-prone (creek) and risk-averse (lake) behavior by spiders maximizing their chances of being able to produce an egg sack within a season of fixed duration. In contrast, Smallwood emphasizes the importance of site-specific differences in the intensity of aggressive interactions among conspecifics; higher prey availability draws more spiders, which therefore interact and move around more often. Having life cycles of flexible duration (Wheeler 1930, Furunishi and Masaki 1982) makes antlions less likely candidates for risk-sensitive foraging; sand-tossing associated with pit-building, however, can apparently increase relocation frequencies at very high densities (P. Dillon, personal communication), in closer accord with the mechanism proposed by Smallwood. At the relatively low antlion densities and prey availabilities typical of temperate zone antlion habitat, however, it seems likely that it is the costs of pit relocation that will generally keep relocation frequencies low.
