Food-limitation in a generalist predator

Area of Science:

• Ecology
• Wildlife Biology
• Conservation Biology

Background:

• Generalist predators like the northern goshawk (Accipiter gentilis) are difficult to study under food limitation due to prey switching.
• Robust studies require documenting the full prey base and observing situations with simultaneous declines in multiple preferred prey species.

Purpose of the Study:

• To investigate how food-supply limits a generalist avian predator, the northern goshawk (Accipiter gentilis).
• To analyze dietary shifts in response to varying food supplies using abundance-biomass comparison curves (ABC curves).

Main Methods:

• Documented the entire prey base of the northern goshawk population.
• Analyzed dietary changes using abundance-biomass comparison curves (ABC curves) during a period of prey scarcity.
• Observed population dynamics and breeding success during a natural food limitation event.

Main Results:

• A simultaneous crash in multiple principal prey species led to a marked decline in both breeding and non-breeding northern goshawk populations.
• Nest failures increased significantly, though brood size of successful pairs remained unaffected.
• Dietary analysis revealed a decrease in the dominance of principal prey species and an increase in small-bodied prey and intra-guild predation as food stress intensified.

Conclusions:

• Food limitation, driven by the decline of multiple prey species, significantly impacted northern goshawk populations.
• The study demonstrated an 'acceptance threshold' for territory choice in raptors, as breeding ceased despite the presence of floaters.
• Management implications highlight that reduced prey availability can rapidly depress raptor populations without buffering from non-breeding individuals.