COS 234-1 - A case for the grazing optimization hypothesis: Intensive grazing increases plant productivity

Background/Question/Methods

The grazing optimization hypothesis postulates that intense herbivory stimulates energy flow through successive trophic levels, including gains in net primary plant production. Beyond its substantial theoretical influence, this concept has inspired applied intensive grazing management systems (such as ‘Adaptive Multi-paddock Grazing’; hereafter AMP) despite a limited understanding of its real-world applicability. To address this knowledge-gap, we conducted a large-scale exploration of the translatability of this concept across the northern great plains by comparing pairs of AMP ranches with neighboring ranches using regionally representative grazing management. Our core questions were: 1) does intensive AMP grazing management correspond to greater plant production?; 2) does intensive management homogenize the diversity and production of the plant community?; and 3) what management attributes distinguish the intensive management of AMP ranches from regionally typical management?

To answer these questions, we compared plant production and community composition between intensively managed AMP ranches and their regionally typical neighbours. These pairs (n=18) were located across a broad expanse of western Canada. Aboveground net primary production (ANPP), root biomass, litter biomass, biomass removal, and plant community abundance and richness were collected from 9 sites within each ranch and analysed using linear mixed modelling to assess the effects of grazing management.

Results/Conclusions

Consistent with the grazing optimization hypothesis, there was significantly greater ANPP (+20%), litter mass (+26%), and total biomass (+14%) within intensively grazed AMP grasslands compared to their regional neighbours. This increase occurred alongside no detectable difference in plant community composition, functional plant group abundance, or root biomass between pairs, and despite marginally lower alpha and gamma diversity in grasslands subject to AMP grazing. Remarkably, this increase in standing biomass occurred despite far greater proportional and absolute biomass removal within intensively managed AMP systems compared to their paired neighbours. Complicating interpretations of the drivers of these differences, no differences were found in the evenness, beta diversity, or coefficient of variation for biomass metrics as a function of grazing management. Our observational work validates the applied expression of the grazing optimization hypothesis across these northern great plains grasslands and we emphasize the need for a mechanistic assessment of the drivers behind these differences.