Sensitivity to natural events
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