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In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation. 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. Environmental factorsEnvironmental 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.
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 dynamicsAll 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
Red Fox The Red Fox (above) is a typical predator. Factors that influence populations include competition, predation, parasitism and symbiosis. 1. Competition Competition is the struggle between organisms for the same resource e.g. grass, dandelion, buttercup and daisy compete for space, light, water, minerals; fox, thrush and hedgehog compete for earthworms. Competition reduces population numbers. Intra-specific competition occurs between members of the same species. Inter-specific competition involves competition between different species e.g. blackbirds and thrushes competing for snails and insects. Two types of competition are:
Effects of competition
Adaptations to survive competition
Some effects of biotic factors on organisms
2. Predation Predation refers to the catching, killing and eating of prey by its predator.
A fox has reddish fur to camouflage and avoid detection by rabbits. It also has developed long canine teeth to kill prey and tear flesh, and can travel at speed to outrun prey to capture it.
A rabbit digs narrow underground burrows which prevents large predators from entering. It also has long ears that provide good hearing to detect a predator. It also has a white tail to act as a warning signal to other rabbits. Plants can develop thorns, spines and stings e.g. holly and cacti to deter predation. Some plants also have a bad taste to deter predators e.g. giant hogweed. Animals can often swim, fly and run faster than predators. They can also mimick animals the predator would normally avoid e.g. hoverfly is a harmless insect but protects itself by mimicking the colouration of wasps. Animals often stay in large groups to avoid predation e.g. in flocks and herds like deer. They can camouflage to enable them to blend with surroundings e.g. greenfly, stick insects, frogs. Some animals are poisonous to predators e.g. caterpillar of large white butterfly, ladybirds contain large amounts of formic acid, which is unpalatable to predators. Mice also flee and hide to avoid being eaten.
Predation maintains the prey species at a sustainable level and is a major factor in the evolution of prey species.
Population dynamics describes the factors that cause changes in population numbers. Predator-prey numbers interact due to:
Human population continues to increase despite famine, disease, war and contraception. Rapid population growth in the 20th century was due to reduced infant mortality and people living longer because of improved sanitation and medicines and disease eradication programmes.
3. Parasitism Parasitism occurs when two organisms of different species live in close proximity and one organism (parasite) obtains its food from, and to the disadvantage of, the second organism (host). A parasite is an organism that feeds from and harms another organism. Parasites sometimes reduce the numbers in a population e.g. potato blight or disease-causing bacteria, but often have little effect on host numbers. Endoparasites feed in a living host e.g. liverfluke, potato blight fungus, bacteria of disease in human body. Ectoparasites feed on the outside of the host e.g. fleas, athletes foot fungus, mosquitoes on human skin, greenfly on a rosebush, blood-sucking leeches on human skin, lice on hawks Parasites are often thought of as predators but they differ by:
4. Symbiosis Symbiosis is a close association between two organisms of different species in which at least one of them benefits. Symbiosis also increases the numbers of both species. In one type of symbiosis, mutualism, both organisms benefit.
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