In addition, some students believe that members of a food chain are only grouped together in predator-prey pairs, and most students have yet to grasp the concept of a producer and its place in the food chain. Organisms higher in a food web feed on some organisms lower in the food web. Available energy decreases as one progresses up a food web. The numbers of individuals in the populations of any species decreases with each step up the trophic levels, because the available energy decreases while the body size generally increases as one progresses up a food web. Traits appear as a result of random processes.
Traits are properties of individuals and vary within a population.
Natural selection influences the increasing or decreasing prevalence of traits in a population Populations exist in a state of dynamic equilibrium, fluctuating in numbers around an average population size. Biotic and abiotic factors in an ecosystem are limited and affect the carrying capacity for any given species. Varying the population of an organism affects an entire ecosystem. Varying the population of an organism affects other organisms in an ecosystem to varying degrees. Each species has unique needs from and effects on an ecosystem.