A Tale of Two Scales: Landscape-Level Heterogeneity Impacts Local Heterogeneity and Beta Diversity

Background/Question/Methods

Increasing environmental heterogeneity is expected to impact various aspects of diversity. This pattern has been confirmed by numerous experimental and observational studies. For plant communities at relatively large spatial scales (~256 hectares), heterogeneity is likely driven in part by differences in existing vegetation structure that are created through disturbances such as fire or grazing. While any disturbance that removes plant biomass can create structural heterogeneity that impacts plant community diversity, it is likely that the type of disturbance influences the direction and magnitude of those impacts. We utilized a block design with four replicates to determine the impact of different disturbances on heterogeneity and beta diversity. We established pastures managed with either patch-burn grazing, season-long grazing, or a modified twice-over rest-rotational grazing system (MTORG). Patch-burn pastures create structural heterogeneity by burning a quarter of each pasture every spring and then having cattle preferentially graze the recently burned areas. MTORG pastures create structural heterogeneity by altering the grazing intensity of each paddock within a pasture through interior fences. We sampled plant community composition in each pasture for five years to assess the spatial variation in species composition (beta diversity and vegetation patch size) and niche availability of each management practice.

Results/Conclusions

We found that increasing landscape-level heterogeneity through patch-burn grazing, but not MTORG, increased beta diversity when compared to a relatively homogenous landscape (season-long grazing). However, vegetation patch size varied among treatments. Patch-burn grazed pastures had smaller patches with higher species turnover and less spatial variation (7.2 meters across) than either MTORG pastures or season-long pastures (both 7.4 meters). Additionally, the niche availability of patch-burn pastures was equal to or greater than that of MTORG pastures and both had greater niche availability than season-long pastures. A severe drought helped to equalize differences in patch size in years four and five, and led to more niche availability in MTORG pastures than either patch-burn or season-long pastures in year five. While both patch-burn grazing and modified rotational grazing create landscape-level heterogeneity, only patch-burn grazing was meaningful for beta diversity. Patch-burn grazing created a landscape with more varied vegetation patches and greater niche availability than modified rotational grazing or season-long grazing. These results suggest that even though grain size and extent may be similar, how that heterogeneity is created is important for variation in species composition and niche availability. In other words, not all heterogeneity is created equal.