RES are likely opportunistic feeders rather than starve. They could eat whatever is most abundant. The more competition the RES have the more they are forced to get less food. If snails are abundant, few other fish are well adapted to eat snails and small clams, thus the RES can thrive in mixed fish habitats that have a lot of snails (mollusks). I suspect RES will eat fish fry and insect larvae when those things are abundant, but so do a lot of other sunfish eat fish fry and insect larvae thus there is a feeding competition which could crowd out the RES.
