Shrunk coexistence: Cattle exclusion and nutrient addition intensify competition between native and exotic grasses with low phenological overlap

Exotic plants are becoming increasingly common worldwide, often driven by
ecological factors such as domestic livestock grazing and soil
fertilization. In temperate grasslands, native C4 and exotic C3 grasses
are generally expected to coexist due to their distinct temporal niches,
which should reduce direct competition. However, their differing abilities
to exploit limited resources or tolerate vertebrate herbivory can create
fitness imbalances, favoring exotic forage species over native species and
potentially undermining their long-term coexistence. Few studies have
explored how seasonally distinct groups of native and exotic plants
interact, especially under the simultaneous manipulation of biotic
factors—such as grazing and species interactions—and abiotic factors—such
as soil nutrients. We conducted short- and long-term manipulative
experiments in a temperate Pampa grassland in Argentina to investigate how
herbivore exclusion, fertilization, and their combination influence plant
invasion by altering interactions between native warm-season (C4) and
exotic cool-season (C3) grasses. Despite their phenological differences,
native C4 and exotic C3 grasses interact, and their interactions were weak
under grazing. However, with herbivore exclusion and nutrient addition,
interactions shifted towards strong competition, particularly in spring.
This competition resulted in a slight increase in exotic grass cover over
natives after two years. However, fourteen years after the onset of
longterm herbivore exclusion, exotic grasses, particularly Festuca
arundinacea, had become dominant, suggesting potential risks to the
coexistence of native and exotic species. Our study uniquely reveals that
even when niche differentiation is expected to prevent direct competition,
biotic and abiotic changes can trigger interactions that favour exotic
species' dominance, challenging assumptions about the stability of
coexistence between phenologically distinct grasses.