Ungulate herbivores promote beta diversity and drive stochastic plant community assembly by selective defoliation and trampling: From a four-year simulation experiment

Ungulate herbivores shape grassland plant communities at multiple scales,
ultimately affecting ecosystem function. However, ungulates have complex
effects on grasslands, including defoliation, trampling, excreta return,
and their interactions. Moreover, the effects of ungulate density on
grasslands are regulated by these three mechanisms. Nevertheless, how
these three mechanisms affect biodiversity at multiple scales and
community assembly remains poorly understood. Here, we conducted a 4-year
novel field experiment to disentangle the effects of defoliation,
trampling, and excreta return by ungulates on plant community assembly in
a temperate grassland in Inner Mongolia, China. This experiment set two
different scenarios: moderate ungulate density (Moderate, characterised by
selective defoliation and moderate trampling) and high ungulate density
(Intense, characterised by non-selective defoliation and heavy trampling),
including different combinations of defoliation, trampling, and excreta
return in each scenario. We found that defoliation and trampling increased
stochasticity in community assembly and promoted alpha and beta diversity
under both scenarios. Specifically, defoliation promoted the coexistence
of species with multiple resource acquisition strategies (higher
functional trait diversity) by reducing interspecific competition;
trampling tended to facilitate random species colonisation. Conversely,
excreta return favoured grasses, promoting deterministic assembly and
impacting species coexistence. Notably, selective defoliation in the
moderate scenario led to a dominance of stochastic processes during
community assembly, whereas non-selective defoliation still did not change
the dominance of deterministic processes. Further, communities subject to
selective defoliation were insensitive to changes in soil properties
caused by trampling and excreta return, maintaining a high-level beta
diversity and the stochastic of community assembly. Synthesis: Our study
provides important insights into the mechanisms by which ungulate
herbivores influence plant community assembly, suggesting that defoliation
and trampling have the potential to drive stochastic processes, while
excreta return plays the opposite role. Our study also suggests that
selective foraging by ungulates acts as stronger stochastic forces during
community assembly compared to non-selective defoliation. These results
imply that considering ungulate feeding preferences and foraging behaviour
in grassland management will help prevent biodiversity loss and biotic
homogenisation.