“Purple and red sea urchins 
Strongylocentroutus purpuratus and S. francisanus, regarded as enemies of abalones, are in many areas vigorous competitors. In most localities, abalones and sea urchins are not intermixed but the edge of one population will overlap that of the other. At these contact points there is competition for food and living space. Once sea urchins have gained a foothold, they seldom leave. Although both feed on algae, sea urchins are more intensive grazers than abalones and they crop the rocks almost completely bare. When this occurs, the abalones are forced to move to another are to find food.
“The largest sea urchin populations are in the deeper waters offshort where the algal growth is not as thick as it is inshore. However, sea urchins appear to move rather freely, both inshore and off. When a population moves inshore and enters etablished abalone habitat, it may completely replace the abalones in that area.
“Since sea urchins are probably one of the most numerous benthic marine organisms living along our coast (there are literally hundreds of acres of them), competition for food and living space is so intense they apparently are forced to move into any suitable territory that becomes available.”
From: “California Abalones, Family Haliotidae,” By Keith W. Cox, Dept of Fish and Gam