Scale-dependent biases in species counts in a grassland

Numbers of plant species were recorded in species-rich meadows in the Bile Karpaty Mts., SE Czech Republic. with the aim to evaluate the sampling error made by well-trained observers. Five observers recorded vascular plants in seven plots ranging from 9.8 cm(2) to 4 m(2) independently and were not time-limited. in larger plots a discrepancy of 10-20% was found between individual estimates, in smaller plots discrepancy increased to 33%. on average. The gain in observed species richness by combining records of individual observers (in comparison with the mean numbers estimated by single observers) decreased from the smallest plot (27-82% for two to five observers) to the largest one (13-25%). However, after misidentified and suspicious records were eliminated. the gain was much lower and became seale-independent two observers added 12% species, on average, and the increase by combining species lists made by three or more observers was negligible (3% more on average). It is concluded that most discrepancies between individual observers were caused by misidentification of rare seedlings and young plants. We suggest that in species-rich meadows plants should be recorded by at least three observers together and that they should consult all problematic plant specimens together in the field. to minimize errors.