DOTS ON THE ROCKS - A COMPARISON OF PERCENT COVER ESTIMATION METHODS

Several percent cover estimation methods have become ''standards'' in ecological research conducted in rocky intertidal habitats. We compare the results of five methods for estimating cover (visual estimation, evenly spaced dots, random dots, and stratified random dots on transparent plastic plates, and electronic digitizing of photographic images) for repeatability (precision), robustness against observer bias, and sensitivity (ability to detect rare species). Digitizing of photographic images was found to be most precise for species covering > almost-equal-to 30\% of available space. The estimates derived from the other techniques were generally not significantly different from each other for percent coverage of this magnitude. The methods were not significantly different in their abilities to estimate percent cover of less abundant species although each of the dot methods frequently failed to detect species covering < almost-equal-to 1 \%. Because they require less time in the field, a combination of visual estimation and photographic documentation seems particularly effective for intertidal surveys. Observer training and validation of visual estimates by comparison to those derived from electronic digitizing of photographic images allow surveys of large numbers of plots while maximizing both coverage and reliability.