W. J. Fletcher, of the University of Sydney, Australia, reasoned that if sea urchins are a limiting biotic factor in a particular ecosystem, then more seaweeds should invade an area from which sea urchins have been removed. To isolate the effect of sea urchins from that of a seaweed-eating mollusc, the limpet, he removed only urchins, only limpets, or both from study areas adjacent to a control site.
Fletcher observed a large difference in seaweed growth between areas with and without sea urchins.
Removing both limpets and urchins resulted in the greatest increase in seaweed cover, indicating that both species have some influence on seaweed distribution. But since removing only urchins greatly increased seaweed growth whereas removing only limpets had little effect, Fletcher concluded that sea urchins have a much greater effect than limpets in limiting seaweed distribution.
Compared to the control (red line), in which both sea urchins and limpets are present, removing limpets alone (blue line) did not have much effect. This might suggest that limpets have little effect on seaweed distribution. However, seaweed cover increased much more when both urchins and limpets were removed (purple line) than when only urchins were removed (green line).
How can the result of removing both sea urchins and limpets be explained?