Biodiversity across grazing regimes
All three of our plant richness metrics, species richness (mean ± SD
(range): 20.75 ± 7.26 (6.22 – 41.11) species), the Shannon-Wiener
diversity index i.e. H’ (1.79 ± 0.56 (0.33 – 2.88)), and the index of
evenness, i.e. Evar (0.41 ± 0.11 (0.16 – 0.98)), varied
markedly among plots. However, analyses using linear mixed effect models
revealed that grazing intensity does not explain a significant amount of
this variation. Species richness varied with year
(F3,128 = 5.58, P = 0.001), as the average species
richness/plot was greater in 2004 (lsmean ± SE: 24.7 ± 1.4 species) than
2005 (20.4 ± 1.4 species; P = 0.04), 2006 (19.2 ± 1.4 species; P =
0.005), and 2007 (19.2 ± 1.4 species; P = 0.002), but was independent of
wet/dry (F1,128 = 0.001, P = 0.97), grazing
(F2,128 = 0.35, P = 0.70), and the grazing:year
interaction (F6,122 = 0.13, P = 0.99). Similarly, H’
varied with year (F3,128 = 13.34, P < 0.0001)
as plots from 2004 (lsmean ± SE: 2.13 ± 0.09) were more diverse than
plots from 2006 (1.59 ± 0.09; P < 0.0001) and 2007 (1.49 ±
0.09; P < 0.0001), and plots from 2005 (1.93 ± 0.09) were more
diverse than plots from 2006 (P = 0.02) and 2007 (P = 0.001). H’ was
also independent of grazing (F2,128 = 2.00, P = 0.13)
and the grazing:year interaction (F6,122 = 0.32, P =
0.92), but, contrary to SR, did vary with wet/dry
(F1,128 = 5.51, P = 0.02; lsmean ± SE dry plots: 1.89 ±
0.08, wet plots: 1.68 ± 0.08). Finally, Evar did not
vary with wet/dry (F1,128 = 0.68, P = 0.41), year
(F3,128 = 0.35, P = 0.78), grazing
(F2,128 = 0.91, P = 0.40), or their interaction
(F6,122 = 0.57, P = 0.76).