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Rocky Shore Trail
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An Introduction to Energy Flow on the Rocky Shore

an upper shore community on a sheltered shore

Close-up of an upper shore community on a sheltered shore: note the seaweeds, tar lichens on the rock with rough periwinkles and limpets.

The rocky shore demonstrates a good range of communities all with populations of producers, herbivores and carnivores. There are also scavengers and detritivores, all organisms that help in the recycling of dead matter. However, it is not quite as straightforward as, say, land habitats. For example, in a meadow grass (the producer) is eaten by rabbits (the herbivore) which in turn may be eaten by a fox (the carnivore). Superficially, it would appear a similar system on the shore with seaweeds as producers and grazing molluscs like periwinkles and topshells. Crabs and fish are the carnivores. However, a glance at the photo above shows the grazers are not actually on the seaweed. In fact there are very few animals on the shore that will feed directly on the visible seaweed (also known as macro-algae). Some exceptions will be the flat periwinkle which can be seen nibbling away at any large weed (Pepper Dulse in the picture below) . The Blue-rayed Limpet is another example, feeding on kelp .

Flat Periwinkle eating the tips of pepper dulse

Flat Periwinkle eating the tips of Pepper Dulse

If the grazers in the picture are not feeding on the macro-algae what are they doing? They are grazing the microscopic growths on the rock. This could be tar lichen that forms a fine layer next to the rock surface, just visible in the picture above. Secondly, micro-algae. These maybe microscopic algal species like the blue-green algae or cyanobacteria. More likely is that they are the very young stages of all the macro-algae. When seaweeds like bladderwrack reproduce they form spores that are dispersed in the seawater. These spores land in massive numbers on the shore and germinate quickly to develop a small thallus (a collection of undifferentiated cells). Initially just a small number of cells they later develop into the seaweed we recognise. It is this micro-algae that form the food for grazers like periwinkles, topshells and limpets. Also, within this microscopic mesh of cells other minute organisms and organic matter may become trapped and be consumed. This tiny material will not just be found on the rock. It will form on any substrate and so it is possible that grazers on large seaweeds are not actually eating it but the microscopic growths on the frond.

As time goes by the seaweeds can reach large sizes. Bladderwrack may just reach half a metre but egg wrack may be several metres or more. Kelps and Saccorhiza grow very large in the lower shore. The latter species is the largest seaweed in Europe and grows rapidly in a matter of months. All this biomass does not go to waste in the rocky shore ecosystem, even if there are no large grazers consuming it. During the life of seaweeds, fragments, some very small, will be lost to the surround sea. Some like the kelps lose large amounts in the autumn when they may shed the fronds completely. All of this organic matter degrades and decomposes into particles called detritus.

The sources of food and organic matter on rocky shores for animals to consume are therefore:

Animals feeding on detritus are called detritivores and the most common one is the barnacle.


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