Rancho Seco vernal pool community data

Disturbance often increases local-scale (α) diversity by suppressing
dominant competitors. However, widespread disturbances may also reduce
biotic heterogeneity (β diversity) by making the identities and abundances
of species more similar among patches. Landscape-scale (γ) diversity may
also decline if disturbance-sensitive species are lost. California’s
vernal pool plant communities are species-rich due in part to two scales
of β diversity: (1) within pools, as species composition changes with
depth (referred to here as vertical β diversity), (2) between pools, in
response to dispersal limitation and variation in pool attributes
(referred to here as horizontal β diversity). We asked how grazing by
livestock, a common management practice, affects vernal pool plant
diversity at multiple hierarchical spatial scales. In terms of
abundance-weighted diversity, grazing increased α both within local pool
habitat zones and at the whole-pool scale,  as well as
γ  at the pasture scale without influencing horizontal or
vertical β diversity. In terms of species richness, increases in α
diversity within habitat zones and within whole pools led to small
decreases in horizontal β diversity as species occupancy increased. This
had a dampened effect on species richness at the γ  (pasture)
scale without any loss of disturbance-sensitive species. We conclude that
grazing increases species richness and evenness (α) by reducing
competitive dominance, without large disruptions to the critical spatial
heterogeneity (β) that generates high landscape-level diversity
(γ).