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• Theory • Population Size • Growth Rate
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 too 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 do not always agree on who, or what, should be included.
Within ecosystems, however, it is possible to recognize simpler levels or organization. Communities consist of the members of two or more species of organisms, living in the same geographical area and somehow overlapping with one another. For example, squirrels live near trees. In a northern hardwood forests, squirrels live in and on oak tress where they build 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 are within the study of communities.
But squirrels do not live alone. A population is a group of interacting individuals of a species which live and interact with one another within a defined geographical 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.
Theory
Natural populations must live within certain limits. Squirrels cannot live where it gets too hot or too cold, and obviously they cannot live in the sea. Physical factors such as temperature, availability of water, or mineral nutrients can limit where a particular species can live. There are also geographic boundaries. The Grand Canyon prevents the Kabab squirrel from crossing over to the other side and greeting its cousins on the opposite rim. Many other species are limited to a certain geographic range by barriers such as seas, mountains, deserts, and rivers.
Within their own range, a population of organisms may spread itself out and become randomly distributed, or members may clump together (such as beside a river or stream) where local conditions are most favorable or because they appreciate each other's company. But wherever they live they are vulnerable to a series of forces that control and limit their numbers. Understanding these forces and how they control the size of populations is the basis of this exercise.
A number of factors control the size of a population. Some of these factors (such as the number of births) will increase the size of a population, while other factors (such as emigration) will cause the size of a population to decrease. When these opposing forces cancel one another out, the size of the population will not change, but when ever there is an imbalance, the population will either increase or decrease accordingly. Many studies of population dynamics, therefore, concentrate on the nature of these forces and their combined effects.
The rate of change in the number of a population is called the growth rate and is an important number in studying population dynamics.
We have seen how the growth rate is influenced by factors such as births and deaths, but it will also be influenced by factors such as immigration, emigration, predation, availability of food, and many more. In each case another symbol can be included in the mathematical formula, and rate adjusted accordingly.
Computers are very good at manipulating these mathematical formulae. In this, and the following simulations, a mathematical formula built into the software performs a series of calculations based on the numbers you provide. In this way it is possible to study the forces and influences that determine how populations grow (or do not grow!).