Ecological equilibrium

A critic might object that creationism (be it old-earth or young-earth) is ad hoc in this respect: if a particular ability confers a survival advantage, why does an organism ever lose that ability? Conversely, if an organism has an adaptive potential that confers a survival advantage, why does that ever remain undeveloped? 

Of course, if an organism is in an environment where the ability ceases to be beneficial (e.g. eyesight in caves), then it's understandable why it might become vestigial. But what about situations where it will still be advantageous, yet that ability is lost? 

The problem with that line of objection is that it treats species in isolation. But the frame of reference is what is good for the ecosystem, and not what's optimal for any particular species. The goal is to maintain the equilibrium of the ecosystem. 

Predators should succeed often enough to main replacement rate. Prey should elude predators and propagate often enough to maintain replacement rate. Likewise, if herbivores are too competitive, they will overgraze and thereby damage the ecosystem. It's not just a relationship between predators and prey, but fauna and flora. Plants and herbivores. 

So we're dealing with a dynamic system that has to be flexible. Adjust to changing variables. At one time or place, predators may need to be more proficient, while at another time or place they may need to be less proficient, to maintain the balance of nature. It's important to have potential abilities. Sometimes those need to be developed. At other times they need to atrophy. 

I'm not a biologist or zoologist, but that's my layman's explanation.