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Antlion Foraging: Tracking Prey Across Space And Time
Ecology, Oct, 1999 by Philip H. Crowley, Mary C. Linton
FILTERING THE PITFALL TRAP DATA
The 170 different size and taxonomic categories of arthropods identified from the trap transects were combined into 26 prey types, based on apparent similarity of handling by captive antlion larvae and on similarity in average biomass extracted by the larvae from the prey. Antlion profit from a prey type is affected by capture success, by the amount of biomass that is extractable, and, at unusually high prey densities, by handling time. Sclerotized exoskeletons and cuticles, which vary considerably in thickness among prey types, help to reduce the prey's profitability to antlion larvae via all three of these factors.
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The average amount of food antlion larvae could extract from prey items found in the pitfall traps was determined in feeding experiments with captive antlions. Four types of prey accounted for a large percentage of the total number of prey (slightly more than 70%) and of the total extractable biomass (66%) captured in the traps, but two of these, scarab beetles (2% by numbers and 12% by biomass) and small/medium spiders (7% by numbers and 24% by biomass), were rarely captured by antlions in the field (M. C. Linton, personal observations). We therefore focused our simulation analysis on the availability of the other two prey types, caterpillars and grubs (hereafter pooled as "caterpillars," which contributed 2% of the total prey numbers but 18% of the biomass in the traps) and ants (60% by number and 12% of the biomass). All three instars of antlion larvae can capture these two prey types. Caterpillars represent a windfall of food (much more extractable biomass than the typical prey item) and may provide enough energy for an antlion larva to molt into the next instar or pupate from the final instar. Yet these larvae can grow steadily on as few as four average-sized ants per day (Linton 1995).
The "search path" width of a foraging antlion larva is the diameter of its pit, which is the extent of pit edge orthogonal to the direction of prey movement. We thus expected the capture rate of a pit to be proportional to pit diameter. Because antlion pits were not as wide as the transect traps used to determine prey availability, the daily trap captures overestimated the amount of prey available to individual antlions. Moreover, there was a seasonal increase in average pit size, reflecting antlion growth. We therefore generated expected pit diameter for each day from a linear regression of pit diameter against sampling date; the expected pit diameter, divided by the trap diameter, was then multiplied by the total number of prey to estimate the number available to an individual antlion pit on a given date.
To account for handling-time effects on the overall foraging rate of the antlion larvae, we further filtered the available prey estimates for each trap, using a standard type-2 functional response relationship (Holling 1959) and expressing the gain rate G in milligrams consumed per antlion per day:
G = [Epsilon]P/(1 + tP) (1)
