Environmental change is a key feature of the rocky shore habitat through tides waves and exposure to the air. Breaking waves impose extreme mechanical forces on rocky-shore animals and plants (see Denny, 1995 for review). More than any fully marine ecosystem, therefore, the littoral zone endures constantly changing physical conditions. The extent of these changes is a major factor shaping the community of any shore. Rocky shore species have adaptations which allow them to survive the fluctuations they experience on a daily basis. However, intense levels of biological interaction can occur on rocky shores. In some cases, these interactions lead to instability in the community. In other cases, stable communities exist, structured by grazing, predation and competition. Even in stable communities, the abundance of many species can fluctuate significantly with the seasons. Shifts in physical conditions can restructure these communities by altering the relative abilities of different species to compete for space.

Many of the dominant species on rocky shores are adept at colonising empty space. Primary producers and filter or suspension feeders with motile larval phases can rapidly colonise bare rock after a disturbance. The process of recovery can begin rapidly and succession on rocky shores occurs over relatively short time scales, in the order of less than 5 to 10 years. The exception is very sheltered, shores dominated by Ascophyllum spp., where recovery may take tens of years. Frequent physical and biological disturbance from natural sources means that the abundance of all species shows some degree of fluctuation over time, although very sheltered shores can be quite stable.

Succession and community changes resulting from biological interactions are discussed elsewhere. The following sections will concentrate on changes in shore communities due to physical factors and begins with a discussion of the relative stability of various rocky shore communities under natural conditions.

The stability of some rocky shore communities under natural conditions

Effects of physical disturbance

Climate

Larval supply

References