Plant Biomass data from: Bottom-up Herbivore-Plant Feedbacks Trump Trophic Cascades in a Wolf-Elk-Grassland System

Top-down predator-prey effects that alter the abundance, biomass, or
productivity of a population community across more than one link in a food
web are referred to as trophic cascades. While these effects have been
extensively studied in aquatic environments, fewer studies have examined
trophic cascades in terrestrial ecosystems. And fewer still terrestrial
studies have tested for trophic cascades between vertebrates and grassland
vegetation. Across the globe, grassland plant biomass is driven by both
precipitation and non-linear positive feedbacks between grazing and plant
productivity, as predicted by the Intermediate Grazing Hypothesis. Yet
little is known about the role that apex carnivores play in regard to
trophic impacts on grassland biomass. We utilized a long-term dataset
collected over the last two decades on a montane rough-fescue grassland
adjacent to Banff National Park, Alberta, to test whether top-down effects
regulate grassland biomass in a wolf-elk system. First, we measured annual
growing season plant biomass from 2006 – 2018 at 61 repeat sampled plots
in the grassland. Next, we measured wolf predation risk using a previously
developed wolf resource selection function created from GPS radiocollar
data from 5 wolf packs. Finally, we measured grazing intensity using
Brownian Bridge Movement Models derived from GPS radiocollar data from 131
unique elk. We then tested top-down, bottom-up and abiotic hypotheses for
grassland biomass over time in program R. The top model incorporated
precipitation and positive non-linear effects of elk use, excluding
predator effects and thus failing to support the trophic cascade
hypothesis. This may be due to the observational nature of this study, or
predation effects in this system may be obscured by human use.
Alternatively, our results also support the hypothesis that intermediate
grazing may outweigh the benefits of predation in grassland systems. Our
study serves to help fill a gap in trophic cascade literature, and
emphasizes that positive feedback between grazers and grasslands may trump
top-down effects. Understanding when trophic cascade theory is or is not
applicable is vital for carnivore management, conservation, and
reintroduction efforts across North America.