Crabs eat krill. Despite its name, crabeater seals don't eat crabs. Crabeater seals actually eat krill as well! Thus, the krill are prey to the crabs and crabeater seals. Likewise, the crabs and crabeater seals are predators to the krill.
When the size of a lower trophic level decreases in a biological community, the population size of all higher trophic levels usually decrease as well.
Therefore, if the number of individuals in the krill decreased, then the crab and crabeater seal populations would decrease because there would not be enough food for all of the individuals of their populations to eat. However, if the population of an alternative food source of the crabs or seals increased, then the seal and crab populations may be sustained. Likewise, if the crabeater seal and crab populations learn how to eat an alternative food source, then the size of their populations may not be affected.
The opposite is also true. If the krill population increased, then more crabs and crabeater seals could be sustained. Thus, the population size of the crabs and crabeater seals would also increase.
Such predator-prey cycles are common in biology.