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 might you explain this result?
a) Both limpets and urchins eat seaweed, so both species have to be removed in order for seaweed cover to increase.
b) When only urchins were removed, limpets increased in abundance and reduced seaweed cover.
c) Both limpets and urchins eat seaweed, so removing either species has the same effect on seaweed cover.
d) If only limpets are removed, more time is needed for seaweed cover to increase.