The answers would be:
increase in interspecific competition for resources.
decrease the population size of black-lipped abalone
or both the sea urchin and black-lipped abalone could evolve
If you'd like to know more read on:
In this situation, the sea urchin and the abalone feed on the same resources and live in the same habitat. This would result in a competitive relationship between the two species. If the sea urchins will continue to expand, then the black lipped abalone would more likely lose the competition for resources and their population would slowly decrease.
As a result of this continued competition, both species could evolve to adapt to the limited available resources by changing their diet or resources. If they do not evolve however, the abalone's population would continue to decrease.