Methodological Detail from David Lewis' Ph.D. Dissertation and Lewis and Magnuson (2000)
We surveyed each lake once for snails and several variables that potentially influence snail distribution. Prior to sampling, lake habitat was determined by mapping around the entire shoreline of a lake at 30 m intervals or by consulting habitat maps (Petrie et al. 1993) that we confirmed for accuracy. Sample sites were chosen randomly, stratifying by habitat type. Snails were sampled from four habitat types in each lake: sand, cobble, woody debris, and macrophytes. The first three categories were sampled at three to five locations apiece in each lake. Sand and cobble sites were usually free of structure such as coarse woody debris and macrophytes. Preliminary sampling indicated that haphazardly located quadrats resulted in a gross underestimation of species richness. We sought to ensure that the search area for these patchily distributed organisms was appropriately scaled, depending on local snail density. Therefore, we used search time as the standard for effort, and each site was searched using SCUBA over a depth range of 0.5 - 4 m for 8 minutes. Timed search has been successfully applied to snail sampling in other physically complex habitats (Tattersfield 1996). For the macrophyte habitat, we sampled up to five sites in each lake, if available, by sweeping a D-net through 1.5 m2 (bottom surface area) of submerged macrophytes per site. Fewer sites were sampled from lakes that had only one or two habitat types per lake, and in which only one patch of a particular habitat could be found. Snails were identified to species according to Burch (1982) with supplemental information from Baker (1928) and Clarke (1973).
We analyzed sampling efficiency for each lake by estimating a potential maximum species richness. For each lake, we randomly ordered the individual samples and constructed a cumulative species richness sampling curve. This was conducted 100 times and a mean sampling curve was calculated. The mean sampling curve was then analyzed with asymptotic (cumulative richness = 1 C [1 - exp(- 2 C sample sequence number)]) and Walford (1946; see Ricker 1975) growth curves to estimate a potential maximum species richness.