There are some animals which actually cultivate their plant food. I have travelled from the Arctic, through the tropics to Cape Town, and I have seen many strange sights, but the most amazing animals I have yet seen are the gardening limpets of the South African coast. There are several species which tend a garden of their algal food, but the most obvious seaweed farmers are Patella cochlear and Patella longicosta.

The pear limpet, Patella cochlear, occurs at incredibly high densities on the low shore of the south and west coasts, where it forms a distinct zone, the cochlear zone. These limpets are usually found in the midst of a garden of small red seaweeds. All other algae except the small red seaweeds are grazed away by the limpets, but these red seaweeds are only cropped. George Branch of the Zoology Department at the University of Cape Town has shown that these algae actually grow faster when they are continuously cropped by grazing limpets. Two of my students, Aldridge Groener and Paul Wilton, have removed Patella cochlear from their gardens, and we have seen that the small red seaweeds develop into a lush patch initially, but soon disappear as the garden is overgrown by sea lettuce (Ulva) and other algae. Thus the limpets benefit from their gardening by having a continuous supply of food, and the seaweeds benefit by having other algae removed and their growth stimulated.

Patella cochlear (the pear limpet) maintains a garden of the small red algae along the South and West coasts of South Africa.

The effect of grazing by pear limpets is clearly evident in this photograph. The area inside the painted square has been kept clear of limpets for two months, while outside of the painted square limpets graze normally.

Another farming limpet, Patella longicosta, maintains a garden of an encrusting brown seaweed called Ralfsia. According to George Branch, this limpet cuts regular paths across the Ralfsia crust, creating numerous edges from which the Ralfsia can regrow quickly. This limpet is territorial, and each farmer fiercely defends its Ralfsia garden against limpets and other grazers. In the absence of limpets, the Ralfsia is overgrown by other, larger algae.

Patella longicosta maintains a garden of the crustose brown alga, Ralfsia, along the South and West coasts of South Africa. In true agricultural fashion, its farming activities actually increas the productivity of the crust that makes up its garden.

Both of these limpets affect the composition of the community by maintaining, in their gardens, patches of seaweed species which would not compete successfully with larger seaweeds in the absence of limpets. Additionally, the limpets are territorial, and this keeps other herbivores from grazing the seaweed patch.