Limpet experiment

PROCESSES AFFECTING PATIERNS OF DISTRIBUTION ON ROCKY SHORES

Competition for food

Grazing gastropods are so effective at removing microscopic algae that they are often short of food. Wherever the grazers are present in large numbers, or the rate of production of micro-algae is small, the animals will consume all of the food as soon as it becomes available.

Prediction:

That at higher levels on the shore and in summer, when food supplies are small, limpets should compete more intensely than is the case at low levels on the shore and in winter, when food is more readily available.

Method:

In experiments, I created a range of densities of limpets and snails at different heights on the shore and in the two seasons of the year.

The rates of decline of weight and, for the limpets, the rates of mortality, were entirely consistent with the predictions. Competition was more intense where food was in smaller amounts.

Such competitive inter­ actions help to explain why there are different mixtures and different relative abundances of snails and limpets in different parts of the shore and on different shores.

These types of competitive interactions are a 'scramble', in that all of the grazers, whatever the species, are going to suffer to some extent.

There can be clear 'winners' and 'losers'. For example, the snail Nerita atramentosa survives and maintains its body-weight better than do limpets Cellan tramoserica when the two are together in areas of insufficient food. Nevertheless, the Nerita are themselves suffering from intraspecific competition for the food supply and eventually lose weight and die before all the limpets have died. Every grazer has problems when the supplies of micro-algae are insufficient to provide for all. It is therefore quite unusual for competition to lead to the local disappearance of any species.

Limpets, when grazing on the surface of the rocks, kill newly-settled barnacles, by crushing or eating them. As-a result, where there are large numbers of limpets, it is not possible for barnacles such as Tesseropora rosea to invade the shore, unless the barnacles happen to arrive in such large numbers that they are not all killed by the limpets. In contrast, where there are no limpets, foliose algae will grow and prevent the barnacles invading. In this complicated example, the barnacles will settle and thrive best where there are some limpets, enough to prevent the algae from taking over all of the surfaces of the rocks. The limpets do this by eating the plants while they are in their early, microscopic stages of life-history. In contrast, if there are too many limpets, they kill the young barnacles, preventing them from becoming established on the shore. The chances of barnacles arriving and surviving on any particular patch of shore are somewhat dependent on the rates of colonisation and growth by algae and the numbers of grazing limpets present.

This demonstrates the complexity of some interactions among species in assemblages on rocky shores. It also demonstrates why it is necessary to prevent too much human interference with the populations that interact in these habitats. Removal of the limpets will locally result in increased coverage by foliose plants, with the result that there can be fewer sessile animals such as barnacles.

Source:

Coastal Marine Ecology (1995), M. G. Chapman and A. Underwood