Feast or famine: How is global change affecting forage supply for Yellowstone's ungulate herds?

The ecological integrity of US national parks and other protected areas are under threat in the Anthropocene. For Yellowstone National Park (YNP), the impacts that global change has already had on the park's capacity to sustain its large migratory herds of wild ungulates is incompletely understood. Here we examine how two understudied components of global change, the historical increase in atmospheric CO2 and the spread of nonnative, invasive plant species, may have altered the capacity of YNP to provide forage for ungulates over the last 200-plus years. We performed two experiments: (1) a growth chamber study that determined the growth rates of important invasive and native YNP grasses that are forages for ungulates under preindustrial (280 ppm) versus modern (410 ppm) CO2 levels and (2) a field study that compared the effect of defoliation (clipping) on the shoot growth of invasive and native mesic grassland plants under ambient CO2 conditions in 2019. The growth chamber experiment revealed that modern CO2 increased the growth rates of both invasive and native grasses, and invasive grasses grew faster regardless of CO2 conditions. The field results showed a continuum of positive to negative responses of shoot growth to defoliation, with a subgroup of invasive species responding most positively. Altogether the results indicated that the historical increase in CO2 and the spread of invasive species, some of which were planted to provide forage for ungulates in the early and mid-1900s, have likely increased the capacity of forage production in YNP. However, rising CO2 has also resulted in regional warming and increased aridity in YNP, which will likely reduce grassland productivity. The challenge for global change biologists and park managers is to determine how competing components of global change have already affected and will increasingly affect forage dynamics and the sustainability of Yellowstone's iconic ungulate herds in the Anthropocene.

Density-dependent plant growth drives grazer stimulation of aboveground net primary production in Yellowstone grasslands.

The mechanisms by which grazing animals influence aboveground net primary production (ANPP) in grasslands have long been an area of active research. The prevailing wisdom is that grazing can increase ANPP by increasing the availability of growth-limiting resources such as nitrogen and water, but recent theory suggests that the density-dependent growth of grassland vegetation can lead to grazer-stimulation of ANPP simply by removing shoot biomass and increasing relative growth rate (RGR). We compared the relative roles of resource availability and density-dependent growth in driving positive responses of ANPP to grazing in Yellowstone National Park. We measured the effects of clipping (50% simulated grazing intensity) and natural grazing on soil nitrogen availability, soil moisture, and shoot growth over 2 months in two grassland plant communities (mesic and dry) grazed primarily by bison. Clipping increased RGR by over 100% in both grassland types but had no effect on N availability or soil moisture during the same growth periods. Clipping stimulated ANPP only at mesic grassland, and the magnitude of this effect was strongly related to the initial plant biomass at the time of clipping relative to estimated peak biomass, supporting the density-dependent framework. Bison grazing had qualitatively similar effects on ANPP and RGR to clipping with no accompanying effects on N availability or soil moisture. Our results show how grazing can stimulate ANPP independent of a direct influence on resource availability simply by exploiting the dynamics of density-dependent plant growth.

Episodic herbivory, plant density dependence, and stimulation of aboveground plant production.

Herbivory is a major energy transfer within ecosystems; an open question is under what circumstances it can stimulate aboveground seasonal primary production. Despite multiple field demonstrations, past theory considered herbivory as a continuous process and found stimulation of seasonal production to be unlikely. Here, we report a new theoretical model that explores the consequences of discrete herbivory events, or episodes, separated in time. We discovered that negative density (biomass) dependence of plant growth, such as might be expected from resource limitation of plant growth, favors stimulation of seasonal production by infrequent herbivory events under a wide range of herbivory intensities and maximum plant relative growth rates. Results converge to those of previous models under repeated, short-interval herbivory, which generally reduces seasonal production. Model parameters were estimated with new and previous data from the Serengeti ecosystem. Patterns of observed frequent and large magnitude stimulated production in these data agreed generally with those predicted by the episodic herbivory model. The model thus may provide a new framework for evaluating the sustainability and impact of herbivory.