Ecology is a branch of scientific study that investigates the complex series of actions and interactions that take place between organisms and their environment. These interactions may be between the organisms themselves; biotic (an owl catching and eating a mouse) or between organisms and their non-living physical environment; abiotic (sunlight, rain, snow or even the minerals in its diet).
Even with modern high speed super computers, and a simple ecosystem, it is almost impossible to unravel all the millions of possible interactions between each individual creature, each other individual creature and each element in their environment. Ecosystems contain to many entities and interactions for easy study. Just working out how many entities (living and non living) there are in a typical ecosystem is a topic of controversy, and scientists don't always agree on who, or what, should be included.
Within ecosystems, however, it is possible to recognize simpler levels of organization. Communities consist of the members of two or more species of organisms, living in the same geographic area and somehow overlapping with one another. For example, squirrels live near trees. In a northern hardwood forest squirrels live in and on oak trees where they build their nests and collect acorns. The squirrel and the oak tree have a relationship. The nature and mutual benefits (or harm) that each species gets from this relationship is within the study of communities.
But squirrels don't live alone. A population is a group of interacting individuals of a species which live and interact with one another within a defined geographic area. Big squirrels chase smaller squirrels; large females get the best nesting trees; young squirrels need to learn from their mothers and also who is boss, and so on. While any one squirrel in the forest may live for a time and then die, the population of squirrels usually continues on through time, independent of the comings and goings of individual members. The dynamics of populations is the subject of this investigation.
