MICROCOSMS AS ISLANDS - A TEST OF THE MACARTHUR-WILSON EQUILIBRIUM-THEORY

A test of the MacArthur-Wilson equilibrium model of island biogeography was conducted using aquatic microcosms as island analogs. Fifty 400-ml beakers containing 300 ml of sterilized soil-water medium and 50 2000-ml beakers containing 1800 ml of medium were inoculated with replicated subsets of species from a species pool consisting of green, blue-green and golden algae, ciliates, a rotifer, a dinoflagellate and euglenoids. These species were added at 2 different average rates over a 28-wk period, resulting in 4 replicated sets of 25 insular communities. Community reorganization (turnover) was shown to occur, as well as relative constancy in species richness for each beaker category, indicative of equilibrium. Contrary to the MacArthur-Wilson prediction, small microcosms had significantly higher species richness values than large microcosms. Microcosms subjected to the higher of the 2 invasion rates had significantly higher species richness values than those of the lower rate categories, primarily due to the influence of transient species. Early divergence into 2 broad community types is documented, suggestive of at least 2 stable community states that trenscend microcosm categories.