Creation of new space for colonisation

The Intermediate Disturbance Hypothesis

Physical effects on individual performance - wave action

Space is the major resource for which rocky shore species compete. Removal of space-occupying individuals provides room for new individuals to colonise. In contrast to predation and grazing, the removal of individuals by physical processes is not species selective. Sessile prey such as barnacles become increasingly unstable as they grow older and larger. The action of waves can dislodge mats of these animals. The action of waves can be more damaging when they carry hard materials which smash into or scour the shore. In heavily forested areas, such as Canada and the northern USA, water borne logs are the most common cause of disturbance. The force of a log smashing against the shore is sufficient to kill and dislodge most barnacle and mussel species (Shanks and Wright, 1986). Disturbance due to drift wood and other debris probably has a relatively minor effect on most UK bedrock shores. However, waves can also move rocks and stones (Shanks and Wright, 1986). When stones on boulder shores are turned over or crash into each other, the organisms on the affected side can be wiped out (Sousa, 1979). Connell (1961) reports significant mortality of barnacles due to water borne debris on a Clyde Sea shore during gales in 1955. Similar effects are caused when shores are scoured by sand or gravel in suspension.

Caswell’s (1978) intermediate disturbance hypothesis suggests that at low levels of disturbance, a few competitively superior species will dominate communities resulting in low levels of diversity. At intermediate disturbance levels, frequent removal of these species will provide space for others, resulting in high diversity. At very high levels of disturbance only a few very tolerant or opportunistic species will occur. This hypothesis has been tested in boulder fields where it seems to hold true (Sousa, 1979). Physical disturbance appears to play a role in structuring communities with limited disturbance promoting biodiversity.

Rough seas can affect the extent of biological interactions on the shore. During periods of rough weather, dogwhelks are more likely to remain sheltered in crevices than to forage for prey on exposed rock surfaces (Burrows and Hughes, 1989). During prolonged periods of harsh weather they will shelter in crevices low on the shore. As conditions abate, for example in spring, the predators emerge as a front, depleting barnacle populations close to the crevice. High amplitude waves and heavy swells may also reduce predation in the littoral zone by mobile predators such as crabs and fish. The effects of predation intensity on rocky shore communities are discussed elsewhere.

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