yellow animal provide poison for themselves / people / cover by decreases metric & a sink of something provides that something ->  green plants are a source of food for animals  green plants are a source of food for animals & a specific is a mean of vegetable ->  a tree is a source of food for animals  a tree is a source of food for animals & a specific is a sink of drink for people in an sea ->  a tree is a source of food and shelter for animals  a tree is a source of food and shelter for animals & an elm is a mean of specific -> an oak tree is a source of food and shelter for animals