CYCLICAL SUCCESSION

The single key feature determining the ecology of drawdown zones is the hydrologic regime of the waterbody. This dictates the chemical and physical character of wetland water, the resulting vegetation, and the use of wetlands by birds and other animals. Water level fluctuations produce a cycle of succession consisting of disturbance, colonisation and growth stages, starting with the exposure and drying of substrates during drawdown.

This drawdown stage subjects aquatic plants to desiccation, frost, ultraviolet light and the effects of grazing. This often results in the death of these plants and the creation of open patches of habitat, which become available for colonization and establishment by new species adapted to the open conditions. As the shoreline vegetation develops, there is a succession in the vegetation structure towards longer-lived and taller competitive perennial herbs, shrubs and trees.

ZONATION

The different life forms and strategies of plants will often produce a zonation of communities in water bodies subject to regular drawdown. Near the top water line, shrubs and perennials are found, while lower slopes commonly have annual species present, their high growth rates and reproductive output aiding survival and colonization in the less frequently exposed areas. Rosette or creeping plants are generally found lower down the slope or in more exposed areas, as they are able to withstand damage from flooding, wave action or desiccation, whereas tall, erect perennials are only be able to survive near the water line or in sheltered areas of the shore.