Species zonation

Abstract

Species zonation is a common feature within estuaries where the combined and interactive effects of steep environmental gradients and food availability and to interspe-cies competition, herbivory, and predation result in compositional changes in biotic assemblages across these gradients. For vegetation and for rock-inhabiting biota, it is often manifest as visually distinct communities occurring in bands. For infauna, it is expressed as compositional and abundance changes in the communities across the habitat. In areas of less steep environmental gradients, species often occur in less differentiated formations, such as mottled mosaics or with diffuse zonation. Species zonation and changes in biotic assemblages can be expressed across the whole of the estuary responding to an along-estuarine environmental gradient of salinity, from freshwater to marine, and a gradient in substrate grain size. For zonation within a given habitat, while there may be a range of factors that influence species zonation that include wave energy, tidal currents, extent of water turbidity, water depth, temperature, degree of light penetration, pH, and nutrients, some of the main environmental determinants for forcing species zonation are pore-water salinity, open-water salinity, substrate type, and inundation. These affect the survivorship of a given species, determine the suitability of a habitat for an organism, and also affect the microbiota in the environment that influence the occurrence of macrofauna and macroflora. One of the best examples of species zonation in estuaries is afforded by mangroves in tropical regions and saltmarsh on tidal flats in temperate regions in response to tidal flat environmental gradients. For example, in mangroves with strong environmental gradients of salinity and inundation, there is zonation of the species in terms of composition, vegetation structure, and plant physiognomy.