PS 10-124 - Looking beyond the bison: Small vertebrate and invertebrate herbivores also substantially impact tallgrass prairie plant community structure and function

Abstract: The loss of bison from North American grasslands has been shown to dramatically increase standing plant biomass with a subsequent decrease in plant species richness and evenness. However, feedbacks from smaller herbivores on plant biomass and community composition in grasslands are poorly understood, with the general expectation that the role of these small herbivores is only significant in outbreak years. Here we examine the relative impacts of different size-classes of herbivores over time in a tallgrass prairie ecosystem to assess the relative and interactive effects of these groups in top-down control of grassland structure and function. Working across two burn regimes (annual and four-year) at the Konza Prairie Biological Station in eastern Kansas, we have imposed four years (2019-2022) of herbivore removal treatments. Specifically, bison and invertebrate removals are crossed in a fully factorial design, with small mammal removals additionally crossed with invertebrate removals and nested within the bison removals. In all plots, we collect aboveground plant biomass and plant, invertebrate, and small mammal community composition on a yearly basis.

We find that bison have the greatest impact on aboveground plant biomass, with bison removal increasing aboveground plant biomass by an average of 32-48% across the landscape. However, small mammals and invertebrates also significantly impact aboveground plant biomass, with their removal resulting in an increase in aboveground plant biomass of 17% and 25%, respectively. This plant biomass response is primarily driven by an increase in graminoid biomass with bison removal. In contrast, the plant biomass response to small mammal and invertebrate removals is primarily driven by an increase in forb biomass. The feeding preferences of these different size-classes of herbivores that underlie the observed biomass shifts can also have large impacts on plant community composition. We find that the removal of either bison or invertebrate herbivores resulted in a decrease plant species richness. Additionally, the removal of bison resulted in a significant decrease in plant species evenness, with the C₄ graminoid Andropogon gerardii coming to dominate bison removal plots. In contrast, the removal of small mammals and invertebrate herbivores resulted in an increase in plant species evenness as forb species were released from top-down control. As herbivore populations in grasslands are under continual threat from land use and climate change, understanding the potential consequences of their loss on plant community structure and function is critical for future prediction and conservation efforts.