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Antlion Foraging: Tracking Prey Across Space And Time
Ecology, Oct, 1999 by Philip H. Crowley, Mary C. Linton
INTRODUCTION
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Many predatory animals build traps to capture mobile prey. Well-studied examples include web-weaving spiders (e.g., Riechert and Gillespie 1986, Wise 1993), net-spinning caddisfly larvae (e.g., Hildrew and Townsend 1980, Richardson 1984, Malmqvist and Bronmark 1985), and pit-building antlion larvae (e.g., Wheeler 1930, Lucas 1982, Heinrich and Heinrich 1984). In addition to supplying food, the trap provides information about food availability at a given location and over a particular time interval. Because trap relocation ordinarily carries a significant energy cost (Griffiths 1980a, b, 1986, Lucas 1985, Linton 1995), and perhaps other costs such as increased risk of mortality (Simberloff et al. 1978, Lucas 1989, Griffiths 1992; but see Heinrich and Heinrich 1984 on risk-avoidance behavior), this approach to sampling prey over space and time can be expensive. Yet trap-building foragers may benefit substantially from tracking major shifts in prey availability through space and time. The discreteness of this trap construction - foraging - relocation process should enhance our ability to characterize foraging strategies and relate their efficacy to resource variability in time and space. In fact, many problems faced by biological systems can be reduced to the need to gather resources, distributed across space and time, as efficiently as possible.
Antlion larvae (Neuroptera: Myrmeliontidae), the focus of the present study, are generalist predators of arthropods that move along the soil surface. Larvae construct conical pits in the dry sandy or silty substrate and use them to capture prey. A large proportion of the antlion larval diet consists of ants, not because of any clear specialization or preference, but because ants are relatively abundant in the dry areas where antlions are found (Topoff 1977). Other prey frequently eaten by antlion larvae include spiders, beetles, isopods, flies, caterpillars, wasps, and mites (Turner 1915, Wheeler 1930, Heinrich and Heinrich 1984, Linton 1995).
Prey are captured when they fall into the pit, are dragged under the sediment surface, and are digested externally. Digested prey fluids are then extracted from the prey and consumed via grooved mandibles, which direct the fluids to the larva's mouth. Capture success with these prey varies; larger antlions with larger pits can generally capture a wider variety of prey types and sizes than can smaller antlions. Heinrich and Heinrich (1984) found that Myrmeleon immaculatus larvae captured an average of 8.8 prey/d in New England, whereas Cueta trivirgata in the Namib Desert captured an average of 2.3 prey/d similar of size (Marsh 1987). However, antlion larvae are capable of surviving for up to 3 months without eating (Heinrich and Heinrich 1984).
Combining the development of a predictive model with a laboratory experiment, we recently addressed the issue of foraging through space and time in a study of Myrmeleon immaculatus (Linton et al. 1991). Because antlion larvae sometimes move their pits as often as every 5 d in the few quantitative studies of their behavior in the field (Wilson 1974, Griffiths 1980b, Heinrich and Heinrich 1984, Linton 1995), we supposed that 3 d might provide a conservative estimate of the minimal time needed to evaluate a pit location. We further assumed that antlions were attempting to maintain or increase body mass; when the gain fell below the maintenance threshold for a 3-d period, an antlion would relocate its pit. The model produced the frequent pit relocations and peripheral pit distributions generally consistent with the laboratory data that we subsequently obtained (Linton et al. 1991).
One of us recently studied foraging by larvae of M. immaculatus at Sleeping Bear Dunes National Lakeshore (SBD-NL) in the northern lower peninsula of Michigan, USA (Linton 1995). Two parallel and adjacent 50-trap linear transects of artificial pitfall traps were established at each of two sites (Good Harbor Bay [GHB] and Sleeping Bear Bay [SBB]) [approximately]15 km apart. Pitfall traps consisted of a 9 cm diameter plastic cup buried flush to the soil surface and containing a downward-pointing plastic cone with its apex removed. The cone funneled captured prey into a smaller collecting cup containing preservative (Morrill 1975). These artificial traps effectively mimicked the pitfall traps of larval antlions, collecting and preserving the arthropod prey available to antlion larvae at the trap sites. The traps within each transect, spaced at 0.5-m intervals along a line parallel to the lake shoreline in vegetated dune habitat (Cowles 1899), were monitored daily over a 60-d period in each of two years (30 May-28 July 1986 and 31 May-29 July 1987, except that vandalism restricted the observations in Good Harbor Bay in 1987 to 31 May-14 July).
In addition to the space x time x taxon array of prey availability for antlions assembled from these data, pit densities, sizes, and relocation frequencies were also estimated. In this field system, antlions relocated their pits much less frequently than did the laboratory population in our previous study, rebuilding an average of 182 [+ or -] 20 cm (mean [+ or -] 1 SE; n = 42 observations) from the previous location, with an average movement interval of 67 [+ or -] 13 d (M. C. Linton, unpublished data; cf. Griffiths 1980b, Heinrich and Heinrich 1984, Matsura 1987, Matsura and Takano 1989). Analysis of the prey availability array yielded significant autocorrelations only on the smallest temporal scale considered (i.e., 1 d), but not on any of the within-transect spatial scales (0.5-25 m; Linton 1995).
