There are two types of aliens on Kepler-20e (a distant earth-like planet) both aliens eat from the same extremely tall trees in the local forests. The alien A is able to eat the leaves, bark, and fruit that are produced by these trees. Alien B is only able to digest the fruit from the trees as they lack an important enzyme that breaks down the tough material in the leaves and bark. One year a new disease prevents all of the trees from growing any fruits. Both of these alien species have babies year round but each year some of each species die due to larger alien predators