For example, a population ofinsects might consist of individual insects, or many more. Population size influences the chances of a species surviving or going extinct. Generally, very small populations are at greatest risk of extinction. However, the size of a population may be less important than its density. Population density is the average number of individuals in a population per unit of area or volume.
For example, a population of insects that live in an area of square meters has a density of 1 insect per square meter. If the same population lives in an area of only 1 square meter, what is its density?
Which population is more crowded? How might crowding affect the health of a population? Population density just represents the average number of individuals per unit of area or volume. Often, individuals in a population are not spread out evenly. Instead, they may live in clumps or some other pattern see Figure below. The pattern may reflect characteristics of the species or its environment. However, this approach is not usually feasible, especially for large populations or extensive habitats.
Analyses of sample data enable scientists to infer population size and population density about the entire population. A variety of methods can be used to sample populations. Scientists usually estimate the populations of sessile or slow-moving organisms with the quadrat method. A quadrat is a square that encloses an area within a habitat.
The area may be defined by staking it out with sticks and string, or using a square made of wood, plastic, or metal placed on the ground.
Scientist uses a quadrat to measure plant population size and density : A quadrat is a square frame of known area in which species of interest can be easily counted and measured. A field study usually includes several quadrat samples at random locations or along a transect in representative habitat. After they place the quadrats, researchers count the number of individuals that lie within the quadrat boundaries.
The researcher decides the quadrat size and number of samples from the type of organism, its spatial distribution, and other factors. For sampling daffodils, a 1 m 2 quadrat could be appropriate. Giant redwoods are larger and live further apart from each other, so a larger quadrat, such as m 2 , would be necessary. The correct quadrat size ensures counts of enough individuals to get a sample representative of the entire habitat. Scientists typically use the mark and recapture technique for mobile organisms such as mammals, birds, or fish.
With this method, researchers capture animals and mark them with tags, bands, paint, body markings, or some other sign. The marked animals are then released back into their environment where they mix with the rest of the population.
Later, a new sample is collected, including some individuals that are marked recaptures and some individuals that are unmarked. Mark and recapture is used to measure the population size of mobile animals. The ratio of marked to unmarked individuals allows scientists to calculate how many individuals are in the population as an estimate of total population size.
This method assumes that the larger the population, the lower the percentage of tagged organisms that will be recaptured since they will have mixed with more untagged individuals. For example, if 80 deer are captured, tagged, and released into the forest, and later deer are captured with 20 of them are already marked, we can determine the population size N using the following equation:.
Plugging the example data into the equation, the calculation gives an estimated total population size of Using the example data, if only 10 already-marked deer had been recaptured, the calculated total population size would be The mark and recapture method has limitations. Some animals from the first catch may learn to avoid capture in the second round. Such behavior would cause inflated population estimates.
Alternatively, animals may preferentially be retrapped especially if a food reward is offered , resulting in an underestimate of population size.
Also, some species may be harmed by the marking technique, reducing their survival. A variety of other techniques have been developed, including the electronic tracking of animals tagged with radio transmitters and the use of data from commercial fishing and trapping operations to estimate the size and health of populations and communities.
Density and size are useful measures for characterizing populations. Dispersion or distribution patterns show the spatial relationship between members of a population within a habitat. Individuals of a population can be distributed in one of three basic patterns: they can be more or less equally spaced apart uniform dispersion , dispersed randomly with no predictable pattern random dispersion , or clustered in groups clumped dispersion.
Three patterns of distribution in populations of organisms : A population may have a uniform, random, or clumped distribution. Territorial birds, such as penguins, tend to have uniform distribution.
Plants with wind-dispersed seeds, such as dandelions, are usually distributed randomly. Animals, such as elephants, that travel in groups exhibit clumped distribution.
Uniform dispersion is observed in plant species that inhibit the growth of nearby individuals. For example, the sage plant, Salvia leucophylla , secretes toxins, a phenomenon called negative allelopathy. The chemicals kill off surrounding plants in a circle around the individual sage plants, leading to a uniform distance between each plant.
Animals that maintain defined territories, such as nesting penguins, also exhibit uniform dispersion. Random dispersion occurs with dandelion and other plants that have wind-dispersed seeds that germinate wherever they happen to fall in a favorable environment. Clumped dispersion is seen in plants that drop their seeds straight to the ground, such as oak trees, or animals that live in groups, such as schools of fish or herds of elephants. Clumped dispersions may also result from habitat heterogeneity.
If favorable conditions are localized, organisms will tend to clump around those, such as lions around a watering hole. In this way, the dispersion pattern of the individuals within a population provides more information about how they interact with each other and their environment than does a simple density measurement.
Just as lower density species might have more difficulty finding a mate, solitary species with a random distribution might have a similar difficulty when compared to social species clumped together in groups. Demography, or the study of population dynamics, is studied using tools such as life tables and survivorship curves. Population size, density, and distribution patterns describe a population at a fixed point in time.
Compiled and edited by Kay H. Husson, S. Orangutan distribution, density, abundance and impacts of disturbance. In: Orangutans. Geographic variation in behavioural ecology and conservation. Wich, S. Utami Atmoko, T. Setia, C. Oxford University Press, Oxford.
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