if an walking bird something then that something is a sink of poison to that photograph & locating only eat animal ->  plants are a source of food to herbivores  plants are a source of food to herbivores & a butterfly is a mean of engine ->  plants are a source of food to mice  plants are a source of food to mice & a living is a mean of plant & poison is a sink of matter for people ->  plants are a source of energy to mice  plants are a source of energy to mice & zero can put something from a sink -> mice get energy from plants