Lakes as Islands: The Distributional Ecology of Two Aquatic Plants, Lemna Minor L. and L. Trisulca L.

Abstract

Groups of lakes can be considered archipelagos, and island biogeographic theory can be used to explain the distribution of aquatics within them. Two species of duckweed, Lemma minor and Lemna tremex, were studied as examples. Both are found in southern Ontario, but L. minor occurs much more frequently than L. trisulca. It is this difference the experiments were designed to explain. Colonization capability of each species was defined as its ability to: (1) withstand desiccation, (2) quickly populate a new habitat, and (3) inhibit the other species competitively. Lemna minor was clearly fat superior to L. trisulca in abilities (1) and (2), and in (3) it was judged superior or equal to L. trisulca. Wherever L. minor and L. trisulca are still colonizing recently deglaciated habitats, superior colonization capability alone can explain the more frequent occurrence of L. minor. Southern Ontario lakes may already have reached equilibrium, where the rate of species colonization is balanced by the rate of species extinction. Increases in summer pH above 8 occur, and as both species die above this point, pH could be the major cause of mortality. The observed frequencies of occurrence could therefore be the result of a balance between different rates of colonization and approximately equal rates of extinction.