a field is a kind of environment & as the supply of food in an environment decreases , the population of animals in that environment will decrease ->  as the supply of food in a field decreases, the population of animals in that field will decrease  a mouse is a kind of animal & mice live in in holes under the ground in fields / in forests ->  a mouse is a kind of animal that lives in fields  as the supply of food in a field decreases, the population of animals in that field will decrease & a mouse is a kind of animal that lives in fields -> as the supply of food in a field decreases, the population of mice in that field will decrease