Precision and accuracy in quantifying herbivory

1. Tissue removal by herbivores (i.e. herbivory) is a dominant interaction in most communities which has important impacts on natural and managed ecosystems. Despite the importance of herbivory, we lack a quantitative comparison of the efficacy of the most commonly used methods used to quantify herbivore damage. 2. We examined the factors that affect the precision and accuracy of visual and digital methods commonly used to quantify damage to leaves. 3. We created 224 digital leaves from four plant species. In a fully factorial design we manipulated leaf morphology and species, the location of damage (marginal or internal), estimation method (exact percentage or 25% bins), observer experience and expectancy bias (i.e., bias due to an expected result). Using 583 adult observers, we estimated the precision and accuracy of individuals' ability to visually estimate known levels of damage. In a third smaller experiment, we performed similar analyses using a digital scanner. 4. Across the first two experiments, individuals estimated damage with high precision (R-2 = 0.75 and 0.80) and accuracy (slope(actual vs estimated) = 0.88 and 0.86). However, the precision and accuracy of estimates were influenced by plant species, the location of damage, and estimation method. Inexperienced individuals also overestimated low levels of damage, and this bias decreased with experience. Digital methods were precise (R-2 = 0.98) whereas accuracy was statistically indistinguishable from visual methods (slope = 0.91). 5. Visual estimates of damage provide the fastest and most cost-effective method for quantifying herbivory, and our results show they can be precise and accurate. We use our results to provide specific recommendations for future research.