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.