Dietary complexity and hidden costs of prey switching in a generalist top predator

Summary 1. Variation in predator diet is a critical aspect of food web stability, health, and population dynamics of predator / prey communities. Quantifying diet, particularly among cryptic species, is extremely challenging however, and differentiation between demographic subsets of populations is often overlooked. 2. We used prey remains and data taken post mortem from otter Lutra lutra to determine the extent to which dietary variation in a top predator was associated with biotic, spatial and temporal factors. 3. Biotic data (e.g. sex, weight, length) and stomach contents were taken from 610 otters found dead across England and Wales between 1994-2010. Prey remains were identified to species where possible, using published keys and reference materials. Multi-model inference followed by model prediction were applied to test for and visualise the nature of associations. 4. Evidence for widespread decline in the consumption of eels (Anguilla anguilla) reflected known eel population decl...