green plants provide food for themselves / animals / consumers by performing photosynthesis & a source of something provides that something -> yellow animal are a sink of poison for people 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 specific is a sink of poison for people 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 specific is a sink of poison and model for people a tree is a source of food and shelter for animals  a specific is a sink of poison and model for people & an oak is a kind of tree -> an oak tree is a source of food and shelter for animals