DOI: 10.5061/dryad.31zcrjdhr
title: Plant Biomass data from: Bottom-up Herbivore-Plant Feedbacks Trump Trophic Cascades in a Wolf-Elk-Grassland System

description: 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.
description: We measured annual growing season plant biomass from 2006 – 2018
 at 61 repeat sampled plots. At each plot, we measured standing biomass
 height at four subplots in the four corners of a 5m x 5m square (NE, SE,
 SW, and NW) (Fig. 2). We indirectly measured height using a
 disc-pasture-meter approach (Vartha & Matches 1977, Dӧrgeloh 2002)
 in a 0.5m x 0.5m quadrat, then visually estimated the percent cover of
 grasses, forbs, shrub, and bare ground within the quadrat. At this fifth
 subplot, in addition to the above measurements, we clipped all vegetation
 down to a grazed level (~ 2 cm). We sorted all clipped vegetation into
 live or dead grass, live or dead forb, and live or dead shrub. We weighed
 each vegetation type in the field and recorded wet weights. Each sample
 was later dried in an oven at 100 oC for 24 hours and then weighed again
 to determine dry weights. All samples were stored and weighed in brown
 paper bags, so we removed 7g (the mean weight of a dry paper bag) from the
 final dry weight for every sample. We recorded the dry weight for each
 vegetation type for each clipped subplot as well as the total dry weight
 of the clipped subplot. To estimate dry weight values for the
 drop-disc-only readings, we used the data from the clipped plots to create
 a model predicting dry weight values (e.g., Dӧrgeloh 2002, Hebblewhite et
 al. 2008).