rainfall is the amount of rain an area receives & rain is a source of water ->  as the amount of rainfall increases in an area , the amount of available water in that area will increase  as the amount of rainfall increases in an area , the amount of available water in that area will increase & as the available water in an environment increases , the populations of organisms in that environment will increase ->  as the amount of rainfall in an environment increases, the populations of organisms in that environment will increase  as the amount of rainfall in an environment increases, the populations of organisms in that environment will increase & a plant is a kind of organism -> as the amount of rainfall in an environment increases, the populations of plants in that environment will increase