Choosing Appropriate Taxonomic Units for Ecological Survey and Experimentation: the Response of Aristida to Management and Landscape Factors as an Example.

The use of appropriate taxonomic units to describe herbaceous vegetation is an issue of increasing importance due to developing interest in the effects of grazing and other management on plant diversity in pastures. We use data for the genus Aristida as an example to examine patterns of ecological response to grazing, pasture sowing and landscape position in different taxonomic units. Responses of Aristida at the level of genus, section and species were compared over the gradients. There was strong evidence of differential responses to these factors, both between sections within genus, and between species within sections. The planning and execution of botanical field descriptions involve striking a balance between taxonomic accuracy and precision that will best address the objectives of a particular project. The species of the genus Aristida have a range of ecological responses, the variety of which can not be captured in data sets which record Aristida as a single entity. As a result, management recommendations for the genus in its entirety are likely to be inaccurate. Although data collection might ideally record individual species, this is not always practicable. In any case, there may be many rare species in the data set, with occurrences too low to detect ecological pattern. Delineation of rare species may only be important if diversity issues are being examined. In other circumstances the use of an intermediate unit (e.g. section or species groups) may be appropriate. The latter approach may be required if there are practical limitations to the collection of data at specific level for all groups, or if the research objectives do not require a high level of precision.