The Transition Zone Hypothesis on biodiversity of the Indian River Lagoon System, Florida

The Indian River Lagoon System (IRL) is regarded as having a high level of biodiversity. The level of biodiversity is often attributed in part to the 2-degree latitudinal span (251 km) of the IRL and to its position in a transitional zone between tropical and temperate biotic provinces. This hypothesis was tested for submerged aquatic vegetation (SAV) based on a recent treatise (Littler, Littler, and Hanisak, 2008). Five of seven species of seagrass are tropical, and two have either a broad or very narrow latitudinal distribution; the transition-zone hypothesis does not hold for seagrasses. Most macroalgae (131 spp.) are tropical/subtropical; 73 species are widely distributed from temperate/boreal to tropical/subtropical latitudes and do not contribute to a Transition-Zone Hypothesis; and 8 species from higher latitudes range southward to the IRL region. Two-thirds of the 221 SAV species form a zone of overlap in the IRL, of which 61.5% are of tropical/subtropical origin. Several groups of IRL plants and animals reported in the literature are also primarily tropical/subtropical with smaller contributions from temperate regions, providing further support for the Transition Zone Hypothesis. But the possible retraction of temperate species with climate change might eliminate the IRL as a zone of transition.