Statistical power comparison of two sampling protocols for riverine snails

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Abstract:

The authors compared the statistical power of two alternative sampling designs to detect changes in threatened and endangered snail species populations in the Mid-Snake River, Idaho. The goal was to determine which sampling approach would have the best chance of detecting a change associated with different hydroelectric project management scenarios. Data was summarized as 1) the average number of snails collected across quadrats (density/m2), and 2) the proportion of quadrats that had snails present. The authors calculated the minimum detectable difference that each measure could detect with a two-sample t test. The density measure was highly variable, and even a complete loss of snails failed to represent a statistically significant change for most sites. The precision improved somewhat when density was log-transformed, the number of replicate quadrats was increased, and larger sampling quadrat was used; however, statistical power to detect change remained low. In contrast, proportion measures were much more precise and could detect a 34% reduction in the proportion of quadrats with snails present. When the number of quadrats was increased to 30, a 24% change could be detected, and for 50 quadrats an 18% change. Proportion of quadrats with snails present was also highly correlated with the average density of snails (Pearson’s r = 0.91). In addition to being a more sensitive indicator, the proportion measure is more quickly observed for each sample, meaning that a larger area can be surveyed during the same amount of time.

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ELECTRONIC FILE - Zoology