Why do concentration gradients exist?

Which is the best way to define a common in environmental science a. a worthless resource b. shared resource c. a rare resource d. a typical resource

Out of the seven main animal groups (fish, mammals, birds, insects, reptiles, amphibians, and arachnids), how many contain members with internal backbones? a. 5 b. 3 c. 7 d. 1

An easy way to calculate the amount of energy available at each level is to what?