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 non-native, 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 growth rates of important invasive and native YNP grasses that are forages for ungulates under pre-industrial (280 ppm) vs modern (410 ppm) CO2 levels, and (2) a field study that compared the effect of defoliation (clipping) on 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. Together 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 and will increasingly affect forage dynamics and the sustainability of Yellowstone’s iconic ungulate herds in the Anthropocene.在人类世（Anthropocene），美国国家公园及其他保护地的生态完整性正面临威胁。针对黄石国家公园（Yellowstone National Park，简称YNP），全球变化已对其维持野生有蹄类（ungulates）大型迁徙种群的能力造成的影响，目前尚未得到充分阐明。本研究聚焦于两个尚未被充分探究的全球变化组分——大气CO₂的历史升高与外来入侵植物的扩散，探讨二者在过去200余年中如何改变黄石国家公园为有蹄类提供饲草的能力。我们开展了两项实验：(1) 生长箱（growth chamber）实验：在工业革命前（280 ppm）与现代（410 ppm）CO₂浓度条件下，测定黄石国家公园内作为有蹄类饲草的重要本土与入侵禾本科植物的生长速率；(2) 野外实验：2019年在环境CO₂浓度条件下，对比去叶（defoliation，即修剪）对湿润草原本土与入侵植物地上生长的影响。生长箱实验结果显示，现代CO₂浓度提升了本土与入侵禾本科植物的生长速率，且无论CO₂浓度如何，入侵禾本科植物的生长速度均快于本土物种。野外实验结果则表明，植物地上生长对去叶的响应呈从正到负的连续梯度，其中一类入侵物种的响应最为积极。综合来看，大气CO₂的历史升高与入侵植物的扩散——部分入侵物种曾在20世纪早中期被引种以作为有蹄类饲草——或已提升了黄石国家公园的饲草生产能力。然而，CO₂浓度升高同时也导致黄石国家公园区域出现变暖与干旱化趋势，这可能会降低草原生产力。全球变化生物学家与公园管理者面临的挑战在于，厘清全球变化的多重拮抗组分如何已然并将愈发深刻地影响饲草动态与黄石国家公园标志性有蹄类种群的可持续性。