Herbivore effects on productivity vary by guild: cattle increase mean productivity while wildlife reduce variability

Authors: CHARLES, GRACE - University Of California; Porensky, Lauren; RIGINOS, CORINNA - University Of Wyoming; VEBLEN, KARI - Utah State University; YOUNG, TRUMAN - University Of California

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2016
Publication Date: 1/1/2017
Citation: Charles, G.K., Porensky, L.M., Riginos, C., Veblen, K.E., Young, T.P. 2017. Herbivore effects on productivity vary by guild: cattle increase mean productivity while wildlife reduce variability. Ecological Applications. 27(1):143-155.

Interpretive Summary: The majority of rangelands worldwide are home to both wild ungulates and livestock, and in many cases livestock have at least partially replaced wild herbivores. However, few controlled experiments have addressed the individual, additive, and interactive impacts of different types of herbivores on ecosystem function and productivity. While ungulate herbivores generally reduce standing biomass, their effects on aboveground production can be quite variable. Some evidence indicates that moderate levels of herbivory can stimulate aboveground productivity. Here, we used a long-term exclosure experiment to examine the effects of three groups of wild and domestic ungulate herbivores (megaherbivores, meso-herbivore wildlife, and cattle) on productivity in an African savanna. Using both field measurements (productivity cages) and satellite imagery, we measured the effects of different herbivore guilds – separately and in different combinations – on productivity across both space and time. We found positive relationships between mean productivity and total ungulate herbivore pressure, regardless of herbivore type. In contrast, we found that variation in productivity across space and time was driven by herbivore type. Wild herbivores significantly reduced heterogeneity in ANPP across both space and time, but the same was not true for cattle. Our results indicate that replacing wildlife with cattle (at moderate densities) could lead to similarly productive, but more heterogeneous rangelands.

Technical Abstract: Ungulate wild herbivores and livestock share the majority of rangelands worldwide; yet few controlled experiments have addressed their individual, additive, and interactive impacts on ecosystem function and productivity. While ungulate herbivores generally reduce standing biomass, their effects on aboveground net primary production (ANPP) can vary by herbivore species, intensity of herbivory, herbivore species richness, and spatial and temporal context. Some evidence indicates that moderate levels of herbivory can stimulate aboveground productivity, but few studies have explicitly tested the relationships among herbivore identity, grazing intensity, and ANPP. Here, we used a long-term exclosure experiment to examine the effects of three groups of wild and domestic ungulate herbivores (megaherbivores, meso-herbivore wildlife, and cattle) on productivity in an African savanna. Using both field measurements (productivity cages) and satellite imagery, we measured the effects of different herbivore guilds – separately and in different combinations – on productivity across both space and time. Results from both productivity cage measurements and satellite NDVI demonstrated a positive relationship between mean productivity and total ungulate herbivore pressure, regardless of herbivore type. In contrast, we found that variation in productivity across space and time was driven by herbivore type; wild herbivores (primarily meso-herbivore wildlife) significantly reduced heterogeneity in ANPP across both space and time. Our results indicate that replacing wildlife with cattle (at moderate densities) could lead to similarly productive, but more heterogeneous herbaceous rangelands.