Each individual planktonic plant or animal is exposed to the hazards of starvation and
risk of predation, and each plank tonic population is under the control of resource
limitation from the bottom up (growth and reproduction) and by predation from the top
down (mortality). While the bottom-up and top-down impacts are traditio nally conceived
as compatible with each other, field population-density data on two coexisting Daphnia
species suggest that the nature of the two impacts is differen t. Rates of change, such as the
rate of individual body growth, rate of reproduction, and each species’ population growth
rate, are controlled from the bottom up. State variabl es, such as biomass, individual body
size and population density, are controlled from the top down and are fixed at a specific
level regardless of the rate at which they are prod uced.