Detectability of species of Carex varies with abundance, morphology, and site complexity

Prepared by Jacqueline M. Dennett and Scott E. Nielsen

Imperfect detection, the failure to observe a species where it is present, is inevitable in biological surveys, yet the degree to which this causes bias against certain taxa remains poorly understood. Some evidence suggests that plants like grasses, sedges, and rushes (graminoids) may be missed in surveys more frequently than other plants, presumably due to their thin, plain appearance and the difficulty many observers have in recognizing species in the field. We should, however, expect substantial variation in detection among graminoids based on growth form, as some species are tall with relatively distinct seed heads that make them more noticeable. In addition, we expect that species’ abundance and site structure, for example dense shrub vegetation, will also affect an observer’s ability to detect plants. In our study, we evaluated whether graminoids were more likely to be overlooked relative to other plants in different boreal forest habitats, and we used Carex (sedges) as a model group to determine how species appearance, abundance, and site structure affects detection failures and delays. Two observers completed independent surveys at 50 transects across northeastern Alberta, Canada. We used field measures of morphology to group sedges into six categories based on their appearance, allowing for broad inferences on the effect of species form.

We found no evidence of an overlooking bias toward graminoids as a group, but observers were least reliable at detecting graminoids at sites with high forb and shrub cover, where larger, leafy plants obscure them. Overall, our detection probabilities for sedges were high relative to published estimates for other vascular plants, ranging between 0.82 and 0.99. Tall sedges with wide leaves and large reproductive structures were almost never overlooked, but we showed a clear detection bias toward short sedges with a small inflorescence. As expected, abundance was the greatest determinant of detection success with failure to detect sedges most likely when species cover was <5%. Likewise, delays in detection within a site (observing a species after its first location on a transect) were most likely to occur for species when the cover was <25%. Site structure variables were not supported in explaining detection failure; instead, it is morphology and abundance that determine the failure. In contrast, detection delays were not influenced by morphology, but were affected by site structure variables, suggesting that observers may be distracted and therefore slower to observe species in dense search environments.

A takeaway from our study is the importance of survey effort. We spent 0.15 minutes/m² per observer across large plots (100 × 2 m), plus additional time at all sites. While our overall success was high relative to other published studies, we did not achieve 95% detection probabilities for half of the morphological groups considered. Surveys for sedges which use lower effort over even larger plot sizes, such as the majority of environmental impact assessment surveys in our region, are likely to have poorer results than what was shown here. Using a greater number of observers or allocating greater search time to survey plots are recommended to achieve high detection success of graminoids.

This is a plain language summary for the paper of Jacqueline M. Dennett and Scott E. Nielsen published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12713).