Grazing in natural communities can influence the maintenance of biological diversity. However, the direction and magnitude of grazing effects depends on numerous factors, including the intensity and duration of grazing, reflected by stocking rate. For example, the relationship between grazing intensity and diversity within vascular plant communities is often unimodal with peak diversity at intermediate levels of grazing; typically attributed to competitive dominance of some species at low grazing intensity and the elimination of species at high grazing intensity. As biodiversity is an inherently multi-taxonomic concept that extends beyond any specific taxonomic group, our objective was to address the broader question of grazing impacts on biodiversity by assessing the relationship between stocking rate and species richness of six major taxonomic groups.
We collected measures of stocking rate and diversity over a large area (c. 200000 km2) of Alberta, Canada. We estimated stocking rate on 78 focal pastures using data on cattle number, pasture size, and times and duration of grazing, gathered from a producer survey. Species richness of six taxonomic groups, vascular plants, mosses, lichens, mammals, birds and mites, were censused in each focal pasture using standardized protocols of the Alberta Biodiversity Monitoring Institute.
Results/Conclusions
The relationship between stocking rate and diversity varied among taxonomic groups even though diversity was positively correlated among some taxa, such as vascular plants, mosses, lichens and mites. An overall unimodal relationship between stocking rate and vascular plant diversity was detected. However, partitioning the study sites into agro-climatically unique regions, the relationship between stocking rate and diversity of vascular plants was negatively linear in northern boreal sites, and positively logarithmic in more arid, southern grassland sites. This variation among regions potentially reflects differences in pasture management and climatic conditions.
Observed differences in the relationship between stocking rate and species richness among the different taxonomic groups suggests that understanding the effects of ecological processes on biodiversity requires a multi-taxonomic approach since the response of one taxonomic group is not necessarily predictive of others. Additionally, these results indicate there may be tradeoffs in diversity among taxonomic groups at different stocking rates. Finally, these findings demonstrate that management decisions pertaining to diversity need to account for regional variations as the responses of various taxonomic groups can depend on geographic location.