Interobserver error in grassland vegetation surveys: sources and implications

Aims Observer error is an unavoidable aspect of vegetation surveys involving human observers. We quantified four components of interobserver error associated with long-term monitoring of prairie vegetation: overlooking error, misidentification error, cautious error and estimation error. We also evaluated the association of plot size with pseudoturnover due to observer error, and how documented pseudochanges in species composition and abundance compared with recorded changes in the vegetation over a 4-year interval. Methods This study was conducted atTallgrass Prairie National Preserve, Kansas. Monitoring sites contained 1 0 plots; each plot consisted of a series of four nested frames (0.01, 0.1, 1 and 1 0 [m.sup.2]). The herbaceous species present were recorded in each of the nested frames, and foliar cover was visually estimated within seven cover categories at the 1 0 [m.sup.2] spatial scale only. Three hundred total plots (30 sites) were surveyed, and 28 plots selected at random were resurveyed to assess observer error. Four surveyors worked in teams of two. Important Findings At the 1 0 [m.sup.2] spatial scale, pseudoturnover resulting from overlooking error averaged 1 8.6%, compared with 1.4% resulting from misidentification error and 0.6% resulting from cautious error. Pseudoturnover resulting from overlooking error increased as plot size decreased, although relocation error likely played a role. Recorded change in species composition over a 4-year interval (excluding potential misidentification error and cautious error) was 30.7%, which encompassed both pseudoturnover due to overlooking error and actual change. Given a documented overlooking error rate of 1 8.6%, this suggests the actual change for the 4-year period was only 1 2.1 %. For estimation error, 26.2% of the time a different cover class was recorded. Over the 4-year interval, 46.9% of all records revealed different cover classes, suggesting that 56% of the records of change in cover between the two time periods were due to observer error. Keywords: cautious error, estimation error, misidentification error, observer error, overlooking error, pseudoturnover [phrase omitted]