At high stocking rates, cattle do not functionally replace wild herbivores in shaping savanna understory community composition

Authors: WELLS, HARRY - University Of Leeds; Porensky, Lauren; VEBLEN, KARI - Utah State University; RIGINOS, CORINNA - The Nature Conservancy; STRINGER, LINDSAY - University Of Leeds; DOUGILL, ANDREW - University Of Leeds; YOUNG, TRUMAN - University Of California

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2021
Publication Date: 3/31/2022
Citation: Wells, H.B., Porensky, L.M., Veblen, K.E., Riginos, C., Stringer, L.C., Dougill, A.J., Young, T.P. 2022. At high stocking rates, cattle do not functionally replace wild herbivores in shaping savanna understory community composition. Ecological Applications. 32. Article e2520. https://doi.org/10.1002/eap.2520.
DOI: https://doi.org/10.1002/eap.2520

Interpretive Summary: Over a quarter of the world’s land surface is grazed by cattle and other livestock, which are replacing wild herbivores in many regions. In East African savannas, cattle stocked at moderate rates can functionally replace wildlife in certain ways, but it is unclear whether this is also true under high stocking rates. It is also unclear how heavy cattle grazing interacts with herbivory by large wild ungulates to affect plant communities. To evaluate the influence of cattle stocking rates on the ability of cattle to functionally replace wild herbivores and test for interactive effects between cattle and wild herbivores, we assessed herbaceous vegetation in a long-term exclosure experiment in a semi-arid savanna in central Kenya that selectively excludes wild mesoherbivores (110 - 2200 pounds) and megaherbivores (elephant and giraffe). We tested the effects of cattle stocking rate (zero/moderate/high) on herbaceous vegetation (diversity, composition, leafiness) and how those effects depend on the presence of wild mesoherbivores and megaherbivores. Our findings provide a strong caveat to previous studies, suggesting a threshold of cattle grazing intensity exists beyond which impacts change, akin to thresholds documented in other rangelands. Impacts of high cattle stocking rates were also sensitive to the presence or absence of other wild ungulates. The combination of wild mesoherbivores and cattle stocked at high rates led to increased bare ground and annual grass cover, reduced perennial grass cover, reduced understory leafiness, and enhanced understory diversity. However, these shifts were weaker or absent when cattle were stocked at high stocking rates in the absence of wild mesoherbivores. Such changes in forage quantity and quality will influence both wildlife conservation and livestock production objectives, and a better understanding these complex wildlife-livestock interactions will aid in managing mixed-use rangelands.

Technical Abstract: Over a quarter of the world’s land surface is grazed by cattle and other livestock, which are replacing wild herbivores, potentially impairing ecosystem structure and functions. It has been suggested that livestock at moderate stocking rates can functionally replace wildlife in certain ways, but it is unclear whether this is also true under high stocking rates. It is also unclear whether wild herbivore effects on plant communities moderate, enhance, or are simply additive to the effects of cattle at high stocking rates. To evaluate the influence of cattle stocking rates on the ability of cattle to functionally replace wild herbivores and test for interactive effects between cattle and wild herbivores in shaping understory vegetation, we assessed herbaceous vegetation in a long-term exclosure experiment in a semi-arid savanna in central Kenya that selectively excludes wild mesoherbivores (50-1000 kg) and megaherbivores (elephant and giraffe). We tested the effects of cattle stocking rate (zero/moderate/high) on herbaceous vegetation (diversity, composition, leafiness) and how those effects depend on the presence of wild mesoherbivores and megaherbivores. We found that herbaceous community composition (primary ordination axis) was better explained by the presence/absence of herbivore types than by total herbivory, suggesting that herbivore identity is a more important determinant of community composition than total herbivory at high cattle stocking rates. The combination of wild mesoherbivores and cattle stocked at high rates led to increased bare ground and annual grass cover, reduced perennial grass cover, reduced understory leafiness, and enhanced understory diversity. However, these shifts were weaker or absent when cattle were stocked at high stocking rates in the absence of wild mesoherbivores. Megaherbivores tempered the effects of cattle stocked at high rates on herbaceous community composition but amplified the effects of high cattle stocking rate on bare ground and understory diversity. Our results show that, contrary to previous findings at moderate stocking rates, cattle at high stocking rates do not functionally replace large wild herbivores in shaping savanna herbaceous communities. In mixed-use rangelands, interactions between cattle stocking rate and wild herbivore presence can lead to non-additive vegetation responses with important implications for both wildlife conservation and livestock production.