Every food chain at its simplest contains three elements: a primary producer, a primary consumer, and a secondary consumer. Primary producers produce biomass which is consumed by the primary consumer, which are then consumed by the secondary consumer. Let's take an example from the African savannah. Plants are the primary producers, Zebras, gazelles and wildebeasts are examples of primary consumers, and Lions and Hyenas are secondary consumers. Each occuppies its niche in the food chain. If for instance zebras were to go extinct, there would be an opportunity for a forager to feed on what they used to eat (not all primary consumers snack on the same foods). A strain of wildebeast or gazelle could then move in and take over the forage that once sustained the zebras. In so doing they would have to adapt to the different food source, and those best fitted to utilize it would prosper, passing on their genes to future generations. When enough time passed, they would have mutated (adapted) enough to be a new species, separate and distinct from the ancestral species. In terms of evolution, organisms adapt to utilized available resources; sufficient adaptation results in new species being formed.
I could give an example from deep ocean vents, but most of you wouldn't understand it too well. It's easier to envision a world without zebras than a world without shrimps or giant tube worms.
