In the great oceans, sometimes animals live one atop the other in great heaps, with those above eating the ones beneath them. Other animals are born in the dark regions that lie between continents. After taking birth, a child and its mother can no longer see one another. These animals prey on each other, eating whatever comes within reach of their mouths. The situation is similar in the human realm.
Hawks eat small birds, while small birds in turn eat worms and the like.
Beasts of prey and other wild animals kill one another. Hunting dogs also chase wild animals and kill them. But we should not look at the suffering of these beings as though we were watching a spectacle that only affects others. Rather, through meditation, we must evoke a vivid sense of what it would be like for us to be born as these beings and to undergo the same sufferings ourselves. So we should realize that there is no guarantee that we won't be born as such a worm.