Population ecology is the study of the structure and dynamics of populations. The term population has many definitions. We will use it here to mean a group of individuals of the same species inhabiting the same area (Mackenzie et al. 1998). Population characteristics are based on the individual characteristics of the individuals within the population. We describe populations in terms of their size, age, and structure. Along with the populations physical attributes, we also must consider the factors which affect the population such as mortality from competition and predation (Mackenzie et al. 1998). We call the study of changes in populations through time population dynamics.

Population size is a very straightforward concept. It is simply the number of individuals present in a given area. In contrast, population growth is a bit more complicated as it depends on the mortality, natality, survivorship and migration into and from populations.

Mortality or the death rate is the number of individuals within a population which die during a given time interval divided by the average population size over that time (Mackenzie et al. 1998). For example, if we have a deer population of 10,000 at the beginning of the time period and a population of 6,000 at the end of the period, we have an average population size of 8,000. The given mortality rate is therefore 4,000/8,000 which gives us a mortality rate of 0.5 or 50% overall. If this change occurred over 10 years, the mortality rate can be expressed as an average of 5% per year. We can conduct this calculation for either the population as a whole or for individual age classes to produce an age specific mortality rate.

Natality is the birth of new individuals over a period of time. The natality rate is a reflection of the seasonality of reproduction, the number of individuals capable of reproduction, the length of time reproduction takes etc. Often, natality rates are density-dependent, for example the 3/2 power law of self thinning in forested stands. We will talk more about density-dependent populations on the next page. We can also report natality rates for specific age classes.

Survivorship is the opposite of mortality. Survivorship is often expressed as a life expectancy or the average number of years that an individual of the population will live (Mackenzie et al. 1998). Just as with mortality and natality, life expectancy can be reported for the whole population or for a given age class within the population. There are three general patterns of survivorship. The first has a high survival rate of young and high death rates during old age. The second has a steady death rate over time (a negative linear pattern). The third has a high mortality rate in the young with less mortality as age increases.

Populations grow due to an increase in the number of individuals within the population. Individuals are added to a population through birth and immigration and are removed from the population due to mortality and emigration. If the population gains more individuals than it loses, the population will grow. Population growth can only continue as long as enough resources are available to support the growing population. Thus, population growth depends upon the size of the population and how close it is to the carrying capacity of the habitat. The carrying capacity is the maximum number of individuals that can be sustained for a given area under a given set of conditions (Mackenzie et al 1998). The population density is defined here as the number of individuals per unit area.

Populations which are density dependent can often be explained by an “s” shaped curve. In the early stages of population growth when there are few individuals, nutrients are readily available and the death rate is minimal. This allows for reproduction and population growth. Growth will continue until the population size approaches the carrying capacity. At this point the growth rate declines to zero and the population stabilizes at the maximum carrying capacity. In the real world, examples have shown fluctuations around the carrying capacity as a result of environmental fluctuations.

It is also possible to have a population which is not density dependent. In this case, the population is not limited by resources. A density independent population will tend to grow exponentially with no limit to the population size. In other words, the mortality and birth rates will not be affected by the density of the population.

Density is just one factor which limits population growth. Other factors such as lack of food or nutrients, lack of water, lack of suitable habitat, adverse weather conditions, predators, disease, parasites and competitors also can influence the population density. For example within a given area there ma be only a small area which is suitable habitat for a given species, or in other cases the amount of suitable habitat may be plentiful but one species may be out competed by another to occupy that habitat.

Density dependence can be classified into three categories depending upon the effect of an increased starting population competing for resources.

Exact compensation means that the number of survivors will be constant, or the process itself will compensate for any variation in the starting population (Mackenzie et al. 1998). In contrast, in an over-compensating population, the number of survivors falls as the starting population is increased. An under-compensating process allows the number of survivors to increase as the starting population is increased (Mackenzie et al. 1998).

A population may be overcompensating at low population levels and exact compensating at higher density levels.

Real populations are not typically found at their carrying capacity for very long because they are dynamic and changing all the time. Populations may be increasing or contracting in time due to changes in the environmental conditions or in the biotic environment.

There have been many reasons proposed to explain a population’s flux around the carrying capacity. Some common explanations are due to a timelag between the population size and its effects on population density or due to the stochastic nature of the environment.