Population biology
Population biology is a field of study that explores populations and how they interact with their environment. Scientists observe all factors influencing a population within an ecosystem when gathering data about specific populations of interest. Often these observations are vital to decisions made about how to protect a species.
Murchison Mountains
This environment sustains a rich diversity of life, including the native takahē, once thought to be extinct but rediscovered here in 1948.
Environmental factors
Environmental factors that influence populations are divided into two categories – abiotic and biotic factors. Abiotic factors refer to the non-living physical and chemical elements found in an ecosystem such as rainfall, temperature, pH, sunlight, shelter and day length. Biotic factors refer to the living or once-living organisms in an ecosystem and their impacts such as predation, competition, food supply, human impacts and parasites.
Environmental factors such as rainfall, climate, predators, shelter and food availability can change. Often, these factors play an important role in the survival of populations. Some factors change from day to day or season to season. Some, like food availability and predation, may vary over several years. A species that successfully survives in an environment has adapted to tolerate any minimal or seasonal fluctuations in these factors.
For each factor, there is an optimum range where a species will thrive. If conditions change, organisms that can will move to live within the optimal conditions for survival.
On either side of the optimum range, conditions can become difficult. This is referred to as a stress zone. Beyond the stress zone is the zone of intolerance. In this zone, individuals – and entire populations – may die.
Most environments have one factor that determines the distribution of a species. This is known as Liebig’s law of the minimum. This critical factor is called the limiting factor. For aquatic species, it may be water temperature or tidal exposure. For birds like the takahē, it could be food availability.
The functioning of an organism is limited by the essential environmental factor that is present in the least favourable amount.
Liebig’s law of the minimumn
How the distribution of a species is affected by limiting factors
This graph is a representation of a population’s common distribution for most influencing biotic and abiotic factors.
Within any population, most individuals will prefer to stay within the optimum range. While some may tolerate less-optimum ranges, no individuals will survive outside this range.
You can download a pdf version here.
Interspecific relationships are a biotic factor that describe the interactions between organisms within their environment. These interactions may have negative, positive or neutral effects on either species’ ability to survive and reproduce. The major types of species interactions are predation, competition, parasitism, mutualism, commensalism and amensalism.
Populations dynamics
All the organisms of the same species found in a particular region are called a population. The distribution of a population is determined by limiting factors. A population can vary in density (number of individuals in a population), distribution (the size of the population’s area and how the population is spread out in this area) and age structure.
Populations change over time. Natality (birth rate) and mortality (death rate) are two aspects that control the age structure of a population. The relationship between the two aspects can be plotted on a survivorship curve.
Individuals can also migrate to other groups or start new groups, and this can lead to changes in the genetic make-up of populations. Genetic drift can cause big losses of genetic variation for small populations. Reduced genetic variation can impact the ability for the population to be able to adapt to new selection pressures such as changes in available resources or other abiotic and biotic factors.
Changes to allele frequencies in populations can have dramatic effects such as bottleneck and founder effects. For takahē, the rapid decline of the population meant that the gene pool is limited to the few individuals that survived and reestablished the population.
Measuring population sizes can be done using a number of different methods such as direct counts or sampling using quadrats or transect lines. Population density is calculated as the mean number of individuals per quadrat divided by the area of the quadrat, and the population size can be calculated by multiplying the density by the area of distribution
