mutual relationship (when both organisms's benafit)
Common descent best demonstrates the unity of among all living organisms, not natural selection.
you can find the answer herehttp://www.takdangaralin.com/kinds-of-relationships-between-organisms/
foodweb
a web for relationships among organisms in a ecosystem
Food web
I don't really know that one I will have to look it up and put it up here
Competition (the competition between organisms for a limited amount of resources), predation (where one thing kills and eats another), and symbiosis (a close relationship between two organisms where one is benefited.
natural selection
It is a symbiotic relationship, as both benefit.
So if there is a same amount of populations of different types of organisms, and they are consumers and decomposers, it balances the populations of organisms because they eat other plants and animals. Wheras if you take a population of organisms out that changes organisms
Ecology is the study of interactions between organisms and their environment. THis includes interactions organisms have with each other, as well as their abiotic environment. Ecologists often cover diversity, distribution, population of organisms, as well as the competition between them and among ecosystems.
Diversity in large numbers help in large scale interaction among organisms such as in the food web In the nitrogen cycle, bacteria, plants have a crucial relationship, earthworms contribute to soil fertility.