Isotopic analysis reveals landscape patterns in the diet of a subsidized predator, the common raven

1. Anthropogenic subsidies to native predators can have cascading effects
on sensitive prey populations, but the spatial mechanisms behind these
effects are often unknown. 2. We used a stable isotope mixing model to
reconstruct spatially naïve assimilated diets of common raven (Corvus
corax) chicks and then used regression analysis to investigate landscape
patterns in assimilated chick diet, with particular respect to the eggs
and chicks of greater sage-grouse (Centrocercus urophasianus). 3.
Assimilated raven diets were primarily composed of mammal carrion,
followed by anthropogenic food and sage-grouse eggs and chicks. 4. Raven
diets showed landscape gradients, whereby raven chicks in nests near
active greater sage-grouse breeding leks consumed a higher proportion of
sage-grouse eggs, sage-grouse chicks, and insects in their diet and less
mammal carrion. A majority of raven nests on anthropogenic nesting
structures (78.7%) were within 5 km of the nearest sage-grouse lek. Ravens
nesting in high-probability greater sage-grouse nesting habitat consumed
more insects and plants and less mammal carrion. 5. In landscapes devoid
of natural raven nesting substrates, such as our study area, anthropogenic
nesting substrates can ‘anchor’ breeding ravens nearer to greater
sage-grouse leks, with concomitant increases in raven predation on greater
sage-grouse nests. Curtailment of anthropogenic nesting substrates within
5 km of a sage-grouse lek may have a disproportionately positive impact on
sage-grouse populations. More generally, these findings highlight that the
spatial arrangement of anthropogenic subsidies can result in indirect
interactions between humans and predators with direct implications for
predators and prey.