Visual versus random-point percent cover estimations: 'objective' is not always better

Abstract

We compared the accuracy and repeatability of 2 common methods of estimating percentage covers of sessile organisms: visual estimates and random-point-quadrats (RPQ). Comparisons of estimates were made using both quadrats in the rocky intertidal zone and simulated quadrats drawn on a computer, where estimates could be compared with true, digitized percent cover values. In each case, visual estimates were found to be more repeatable (less within-and among-observer variation) and more accurate (closer to the true value as determined by digitizing) than the RPQ method. RPQs using 100 points were more accurate and less variable than those using 50 points, but were still less accurate (and much slower to carry out) than visual estimates. The RPQ method often missed rare species (<2 % cover) altogether, but when it 'hit' them, values were usually overestunated. Visual estimates also tended to overestimate percent covers of species (although less than the RPQ method), especially uncommon ones. Thus although the probabilistic RPQ method is supposedly more objective and is statistically valid, visual estimates may give a more accurate representation of relahve coverage of sesslle organisms, and can reduce overall s a m p h g error because they make increased sample sizes possible. Use of small subdvisions in quadrats, pre-field observer training, and a conscious effort to avoid bias are necessary to make the visual method valid and accurate.

In the eye of the beholder: bias and stochastic variation in cover estimates

“…Økland 1988Økland , Kercher et al 2003. The reliability of visual cover estimates has been the focus for a number of studies (Sykes et al 1983, Jukola-Sulonen and Salemaa 1985, Floyd and Anderson 1987, Kennedy and Addison 1987, Mitchell et al 1988, Dethier et al 1993, Bråkenhielm & Qinghong 1995, Carlsson et al 2005, Vittoz & Guisan 2007. Still, the contribution of bias due to individual observers versus other unexplained sources of error has rarely been reported (van Hees & Mead 2000, Milberg et al 2008.…”

Influence of a breakwater on nearby rocky intertidal community structure

“…An arbitrary value of 0.5% cover was given to species whose position did not match any of the intersections. Barnacle cover was estimated by giving a score from 0 (absent) to 4 (100% cover) in each of the 25 sub-quadrats (Dethier et al, 1993), while mobile animals (limpets and littorinids) were counted and expressed as density per quadrat. Other animals present (e.g.…”

Salt marsh stabilization affects algal primary producers at the marsh edge

“…In each survey, macroalgae and marsh plants were identified and percent cover was estimated in 0.5 m 2 quadrats at the low marsh edge. Visual percent cover estimates have been shown to be a repeatable and efficient sampling method (Dethier et al 1993). At each site on each sampling date, we surveyed three replicate transects.…”