Large carnivores affect lower trophic levels by influencing ungulate herbivory directly via reducing ungulate numbers and indirectly by altering ungulate behavior due to predation risk. Predation risk factors act at different spatial scales, where carnivore distribution shapes predation risk at the landscape-scale and impediments (objects blocking view and escape routes) affect predation risk at a fine-scale. The relative importance of these two different spatial scales and whether they interact in affecting predator-induced risk effects has often been ignored in trophic cascade studies. Previous work in our study site, the Białowieża forest (Poland), showed that deer behavior is affected by both fine-scale risk (tree logs) and landscape-scale risk (distance to wolf (Canis lupus) core). We studied how and till what distance tree logs affect ungulate browsing intensity and how this is modified over a landscape-scale predation risk gradient (distance from human settlements to wolf core) in the primeval Białowieża forest, Poland.
We found that landscape-scale and fine-scale risk factors strongly interacted in determining the strength and magnitude of carnivore induced risk effects on lower trophic levels. In areas with low perceived risk, tree logs reduced browsing intensity in small patches (≤4 m from logs), whereas in high risk areas larger patches with a reduced browsing intensity were found (≤16 m from tree logs). Moreover, the magnitude of these effects changed over a landscape-scale with the strongest reduction in browsing intensity around tree logs in high risk areas (up to 37%) and the smallest in low risk areas (<20%). These results suggest that risk factors operate at different spatial scales, strongly interact and determine the functional role of large carnivores in affecting ecosystem processes. These interactive effects should be incorporated in predator-induced trophic cascade studies, to understand patterns of tree regeneration in ecosystems where large carnivores and herbivores live together.