SPATIAL SCALING OF ALPHA, BETA, AND GAMMA: DIVERSITY PATTERNS OF ROCKY INTERTIDAL GASTROPODS ON SAN SALVADOR ISLAND, BAHAMAS
To investigate spatial distribution patterns, the rocky-shore fauna was surveyed along the shores of San Salvador Island. The survey was conducted over a four-week period in which a census of the intertidal gastropods was done along the coastline at 10 localities. While most studies center on the vertical movement and position of gastropods in relation to tides, here the focus was on the spatial distribution of gastropods across a given area. Four 50 meter transects were randomly placed parallel to the coastline at each of the 10 localities. Within each transect, 48 30 x 30 cm quadrats were randomly placed to census gastropods, substrate type, slope, associated fauna, algal cover, shelter, and energy. A total of 480 quadrats were sampled and over 17,000 gastropods were counted, including three species of Nerita and seven species of Nodolittorina, Cenchritis, and Littoraria.
A scale-controlled increase in diversity is evident when alpha, various levels of beta and gamma diversities are analyzed, with diversity increasing from quadrats (alpha) through transects and localities (various levels of beta) to the whole of the island (gamma). Regardless of the scale of analysis, the evenness of samples and relative abundance of each species were highly dependent on both biological and physical variables. Nodolittorina ziczac and Nerita versicolor are both widespread and abundant along the rocky shores of the island. However other species such as Nodolittorina mespilium and Nerita tesselata are variable and display small-scale patchiness. These analyses, conducted on multiple spatial scales using sampling standardization techniques, suggest that within a small island, variation in the distribution of species is governed by multiple factors.