Increasing precision in randomised field experiments:: barnacle microtopography as a predictor of Littorina abundance

We investigated whether measurements of barnacle microtopography could be used as covariates to increase precision in randomised field experiments with intertidal Littorina populations. We used image analysis to quantify the microtopography of the barnacles within 100 cm(2) quadrats in July 1994 using as variables: number of barnacles, number of dead empty barnacles, mean distance to nearest neighbouring barnacle, total area covered by barnacles, and number of pockets of different sits-classes [extra-small (< 0.15 cm(2)), small (greater than or equal to 0.15 and < 0.30 cm(2)), medium (greater than or equal to 0.30 and < 0.45 cm(2)), large (less than or equal to 0.45 and < 0.60 cm(2)), and extra-large (greater than or equal to 0.60 cm(2))] at least 75% surrounded by barnacles. We then used these variables to predict the abundance of Littorina spp. in the same quadrat. Two variables: the number of extra-small pockets and the number of small pockets accounted for 50% of the variation in total littorinid snail abundance among quadrats for January 1995 and for 47% of the variation for August 1994 but for only 16% of the variation for June 1994. However the single variable, the number of barnacles, accounted for 29-36% of the variation in snail abundance on these dates and can be measured without special equipment. The best combination of covariates was the number of extra-small pockets alone. The number of extra-small Dockets between barnacles may be influencing the snail abundance by providing a refuge from wave shock and heat stress/desiccation. We suggest that the availability of these refuges among the barnacles could act as a density-dependent regulator of the total population size of a particular species of Littorina.