Exposure to the scouring effects of wave action and tidal currents means that rocky and sandy shores are the most resilient to the effects of a spill (Figure 7). This scouring also usually enables natural and rapid self cleaning to take place. A typical example of an impact on rocky shores in temperate climates is the temporary loss of the common limpet (Patella vulgata), a keystone species of marine snail. ’Keystone’ species are plants or animals that exert a controlling influence on the ecosystem that is disproportionate to their biomass, and their removal is likely to lead to a dramatic change to that ecosystem. Limpets grazing on micro-algae on rock surfaces limit algal growth and settlement of other fauna. Their loss typically leads to a rapid growth of opportunistic green algae (Figure 7 inset). Over time this algal growth is displaced by other algal species and as space becomes available for limpets to re-colonise the rock surface, the ecological balance is gradually restored. On tropical and sub-tropical sandy shores, ghost crabs (Ocypode sp.) occupy a similar environmental niche as limpets and high mortality rates are a common feature of shoreline oiling. Despite this, within weeks of shorelines becoming clean, the crabs often re-colonise the beaches