A Null Model of Patch Assessment with an Application to a Carabid Cave Beetle

Griffith, David; Brown, Joel

We develop a model of patch assessment for a forager that becomes satiated following the harvest of one prey item. This model is in contrast with most that assume the forager harvests many prey items from a patch. The models are similar in that the optimal patch-use strategy for a forager ignorant of patch quality is to spend a fixed amount of search time in each patch. When many prey items are harvested, such a fixed-time strategy leads to density-independent per-capita prey mortality. In contrast, we show that a forager that becomes satiated by the consumption of a single prey item should lead to negatively density-dependent prey mortality when using a fixed-time strategy. Deviations from this negative-dependence provided evidence for patch assessment by a forager that consumes only a single prey item. We tested this prediction in the field using a carabid cave beetle (Neaphaenops tellkampfi) that is a predator on the eggs of a cave cricket (Hadenoecus subterraneus). At a study site within Great Onyx Cave, Mammoth Cave National Park, Kentucky, USA, we varied cricket egg densities within semi-permeable exclosures that permitted beetle entry. We found that the beetles removed a density-independent proportion of eggs. This deviation from the model's prediction is consistent with field observations of foraging beetles, which concentrate foraging efforts around oviposition mounds of crickets. We believe that beetles estimate patch quality by the presence of substrate disturbances left by crickets when they oviposit. The model is useful as a null model not only for cave beetles, but other foragers which may become satiated by a single prey item.

A Null Model of Patch Assessment with an Application to a Carabid Cave Beetle

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