An energetic evaluation of a predator-prey (leech-amphipod) couple in Montezuma Well, Arizona, USA

1. The highly stable conditions in Montezuma Well, Arizona, have contribute d to a very close predator-prey relationship between the endemic leech, Erp obdella montezuma, and its endemic amphipod prey, Hyalella montezuma, which forms nearly 90% of the leech diet. 2. Using a simple bioenergetic model, it was determined why a benthic oppor tunistic foraging strategy, typical of most other erpobdellids, is inferior to the specialized foraging behaviour utilized by E. montezuma and why rep roduction by the leech is restricted during certain times of the year. 3. Our model demonstrated that to attain a biomass in excess of 2000 mg (ma ximum field size) during periods of low amphipod densities in the winter E. montezuma must feed on alternative prey types and have a foraging activity < 4 h per day. 4. Leech reproduction 4-6 months prior to the onset of winter food shortage s of prey increases juvenile and adult survivorship with highest reproducti ve success for E. montezuma occurring in June-July. 5. Rather than being an obligate forager on H. montezuma, E. montezuma expl oits a periodically abundant amphipod during most of the year in addition t o alternate prey types during the winter thereby achieving an overall highe r rate of energy gain than could be achievable by other closely related spe cies of erpobdellids utilizing an opportunistic foraging approach.