Once they, species, are gone, not only are they gone forever, but it takes millions of years for new species to evolve and cater for in their place.
The loss of even one species can ruin an entire, balanced ecosystem of plants and animals. The animals that depended on this vanished species as prey have now lost one of their few but major food sources; as the result, the animals that have been preyed on and fed on have lost a predator.
When summing up the events, it is clear that the prey of these species will inevitably undergo population explosions, which are devastating for the plants or animals that they feed on. Unless another species steps up to take the now-vacant seat, the entire ecosystem is in danger of collapsing.