green plants provide food for themselves / animals / consumers by performing photosynthesis & a source of something provides that something ->  green plants are a source of food for animals  green plants are a source of food for animals & a tree is a kind of plant ->  a tree is a source of food for animals  a tree is a source of food for animals & a tree is a source of shelter for animals in an ecosystem ->  a tree is a source of food and shelter for animals  a tree is a source of food and shelter for animals & an oak is a kind of tree -> an oak tree is a source of food and shelter for animals