Physical forces of winds waves and currents

In nature there are many forces of the environment that have their effect by virtue of the force of physical movement - wind and water are prime examples.

In streams and rivers, both plants and animals face the continual hazard of being washed away. The average velocity of flow generally increases in a downstream direction, but the greatest danger of members of the benthic (bottom-dwelling) community being washed away is in upstream regions, because the water here is turbulent and shallow. The only plants to be found in the most extreme flows are literally 'low profile' species like encrusting and filamentous algae, mosses and liverworts. Where the flow is slightly less extreme there are plants like the water crowfoot (Ranunculus fluitans), which is streamlined, offering little resistance to flow and which anchors itself around an immovable object by means of a dense development of adventitious roots. Plants such as the free-floating duckweed (Lemna spp.) are usually only found where there is negligible flow.

The conditions of exposure on sea shores place severe limits on the life forms and habits of species that can tolerate repeated pounding and the suction of wave action. Seaweeds anchored on rocks survive the repeated pull and push of wave action by a combination of powerful attachment by holdfasts and extreme flexibility of their thallus structure. Animals in the same environment either move with the mass of water or, like the algae, rely on subtle mechanisms of firm adhesion such as the powerful organic glues of barnacles and the muscular feet of limpets. A comparable diversity of morphological specializations is to be found amongst the invertebrates that tolerate the hazards of turbulent, freshwater streams.

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