B. They can often include and affect other factors which can be difficult to find a solution that doesn’t leave behind a negative affect.
I would say B-growth of a villages into cites or C
They could adapt to live together so that for example one starts eating only seaweed while the other starts eating only organic matter so they don't compete for food. They could also adapt like when squirrels are separated into those who eat on trees and those who eat on the ground. The 2 urchin types could split into those who eat of the ground and those that eat what floats and drifts.
