Sampling method and sample size affect diversity and indigenous dominance estimates in a mixed grassland community

Estimates of vegetation attributes measured by sampling often inform scientific inference, management actions, and policy decisions. However, different sampling methods and sample sizes (i.e. number of plots) can yield significantly different estimates of vegetation attributes. This occurs because the abundance distributions and spatial distributions of species in the plant community influence their probabilities of detection and estimates of their abundances. We predicted that different sampling methods and sample sizes would produce significantly different estimates not only of vascular plant species diversity, but also of indigenous dominance (the level of indigenous influence) in mixed vegetation where indigenous and exotic floras have different abundance or spatial distributions. To test our predictions we applied three sampling methods to 24 plots in grassland and cushion vegetation in a 1058-ha scientific reserve in the Upper Waitaki (Mackenzie) Basin, New Zealand. Our methods sampled ground areas from 0.65 to 400 m², and included two variants of common ‘subsampling’ approaches, which assessed only discrete subunits within larger plots. Indigenous plant species were both less abundant on average and more spatially-clustered (i.e. less evenly dispersed across plots) than exotic species. The two subsampling methods were less likely to detect less abundant and more spatially clustered species, leading to lower ratios of indigenous to exotic species recorded, and lower estimates of indigenous dominance of composition (% of species indigenous). Numbers of indigenous species accumulated more rapidly with increasing sample size than numbers of exotic species, so that indigenous dominance also increased with the number of plots sampled. We conclude that properly measuring species diversity and indigenous dominance in mixed indigenous-exotic plant communities requires both the searching of sizeable plots and use of rarefaction rather than plot-averaging of statistics. We suggest greater use of rarefaction and more consideration of species’ detection probabilities in sampling New Zealand’s mixed indigenous–exotic plant communities should improve the reliability, transparency and comparability of measures of diversity and may also provide new ecological insights.