Jovaro -- still the same, aren't you? As an egg, the frog is food for some. As a tadpole, the frog eats vegetation and is still food for some. As a frog, the frog eats insects and still is food for some. In other words, at its different stages, the frog is part of every level of the food chain.
You asked how a creationist explains this. We reply that God did it. You do not accept this as any different from 'evolution did it.' However there is a great difference. We know that God created frogs as an act of special creation, and so there is no need to explain how, as we are not God and cannot do as He has done. On the other hand, evolution, by definition, works by entirely natural means, and therefore the 'how' is necessary for evolutionists to deal with. Without the 'how', you're reply that 'evolution did it' is merely a statement without any basis in known reality. It is like "Once upon a time" in a fairy tale.
Girolamo -- which one are you this time? Your example of gravity and a unicorn makes absolutely no sense at all and has nothing to do with this discussion. The fact of the matter is that evolutionary biology has no explanation whatever for the concept of metamorphosis in some animals. It is a giant question mark. That is not proof one way or the other, true, but it does give just a wee bit more credence to the biblical response that God created ex nihilo by kind and not via evolution. Personally, I think perhaps metamorphosis is one of God's ways of getting us to wake up to that fact.