There are also populations with small numbers of individuals that may be dense or very sparsely distributed in a local area. Population size can affect potential for adaptation because it affects the amount of genetic variation present in the population. Density can have effects on interactions within a population such as competition for food and the ability of individuals to find a mate. Smaller organisms tend to be more densely distributed than larger organisms [Figure 1].
As this graph shows, population density typically decreases with increasing body size. Why do you think this is the case? The most accurate way to determine population size is to count all of the individuals within the area. However, this method is usually not logistically or economically feasible, especially when studying large areas. Thus, scientists usually study populations by sampling a representative portion of each habitat and use this sample to make inferences about the population as a whole.
The methods used to sample populations to determine their size and density are typically tailored to the characteristics of the organism being studied. For immobile organisms such as plants, or for very small and slow-moving organisms, a quadrat may be used. A quadrat is a wood, plastic, or metal square that is randomly located on the ground and used to count the number of individuals that lie within its boundaries. To obtain an accurate count using this method, the square must be placed at random locations within the habitat enough times to produce an accurate estimate.
This counting method will provide an estimate of both population size and density. The number and size of quadrat samples depends on the type of organisms and the nature of their distribution. For smaller mobile organisms, such as mammals, a technique called mark and recapture is often used.
This method involves marking a sample of captured animals in some way and releasing them back into the environment to mix with the rest of the population; then, a new sample is captured and scientists determine how many of the marked animals are in the new sample.
This method assumes that the larger the population, the lower the percentage of marked organisms that will be recaptured since they will have mixed with more unmarked individuals. For example, if 80 field mice are captured, marked, and released into the forest, then a second trapping field mice are captured and 20 of them are marked, the population size N can be determined using the following equation:.
These results give us an estimate of total individuals in the original population. The true number usually will be a bit different from this because of chance errors and possible bias caused by the sampling methods. In addition to measuring density, further information about a population can be obtained by looking at the distribution of the individuals throughout their range. A species distribution pattern is the distribution of individuals within a habitat at a particular point in time—broad categories of patterns are used to describe them.
Individuals within a population can be distributed at random, in groups, or equally spaced apart more or less. These are known as random, clumped, and uniform distribution patterns, respectively [Figure 2]. Different distributions reflect important aspects of the biology of the species; they also affect the mathematical methods required to estimate population sizes.
An example of random distribution occurs with dandelion and other plants that have wind-dispersed seeds that germinate wherever they happen to fall in favorable environments. Biological features of the population also affect population changes over time. Birth rates will be higher in a population with the ratio of males to females biased towards females, or in a population composed of relatively more individuals of reproductive age.
The demographic characteristics of a population are the basic determinants of how the population changes over time. If birth and death rates are equal, the population remains stable. The population will increase if birth rates exceed death rates, but will decrease if birth rates are lower than death rates. Life expectancy, another important factor, is the length of time individuals remain in the population. It is impacted by local resources, reproduction, and the overall health of the population.
These demographic characteristics are often displayed in the form of a life table. Life tables, which provide important information about the life history of an organism, divide the population into age groups and often sexes; they show how long a member of that group will probably live.
The tables are modeled after actuarial tables used by the insurance industry for estimating human life expectancy. Life tables may include:. The life table shown is from a study of Dall mountain sheep, a species native to northwestern North America. The population is divided into age intervals, as seen in the leftmost column. The mortality rate per 1, individuals is calculated by dividing the number of individuals dying during an age interval by the number of individuals surviving at the beginning of the interval, multiplied by 1, Life table of Dall mountain sheep : This life table of Ovis dalli shows the number of deaths, number of survivors, mortality rate, and life expectancy at each age interval for the Dall mountain sheep.
For example, between ages three and four, 12 individuals die out of the that were remaining from the original 1, sheep. This number is then multiplied by 1, to get the mortality rate per thousand. As can be seen from the mortality rate data column D , a high death rate occurred when the sheep were between 6 and 12 months old, which then increased even more from 8 to 12 years old, after which there were few survivors.
The data indicate that if a sheep in this population were to survive to age one, it could be expected to live another 7. Another tool used by population ecologists is a survivorship curve, which is a graph of the number of individuals surviving at each age interval plotted versus time usually with data compiled from a life table. These curves allow comparison of life histories of different populations. Survivorship curves show the distribution of individuals in a population according to age : Humans and most mammals have a Type I survivorship curve because death primarily occurs in the older years.
Birds have a Type II survivorship curve, as death at any age is equally probable. Trees have a Type III survivorship curve because very few survive the younger years, but after a certain age, individuals are much more likely to survive.
Humans and most primates exhibit a Type I survivorship curve because a high percentage of offspring survive early and middle years; death occurs predominantly in older individuals.
These species have few offspring as they invest in parental care to increase survival. Birds show the Type II survivorship curve because equal numbers of birds tend to die at each age interval. These species may also have relatively-few offspring and provide significant parental care.
Trees, marine invertebrates, and most fishes exhibit a Type III survivorship curve. Very few individuals survive the younger years; however, those that live to old age are likely to survive for a relatively-long period. Organisms in this category usually have large numbers of offspring and provide little parental care. Privacy Policy. Skip to main content.
Population and Community Ecology. Search for:. Population Demography. Population Demography Demography is the study of population dynamics, using statistical and mathematical tools. Learning Objectives Explain the importance and function of population demography. Key Takeaways Key Points Demographic studies help scientists understand the population dynamics of species, such as invasive species like the Asian carp. Population fluctuations depend on the weather, food availability, natural disasters such as forest fires or volcanic eruptions, predation, and biological competition.
Researchers originally designed demographic tools to study human populations, but demographic approaches can be applied to all living populations. Key Terms demography : the study of populations and how they change population dynamics : the variation of populations due to birth and death rates, by immigration and emigration, and concerning topics such as aging populations or population decline statistics : a systematic collection of data on measurements or observations, often related to demographic information such as population counts, incomes, population counts at different ages, etc.
Population Size and Density Scientists study population size and density using a variety of field sampling methods, including quadrats and mark-recapture. Learning Objectives Choose the appropriate method to sample a population, given features of the organisms in that population. Population density is the number of individuals within a given area or volume. The quadrat method is used to sample sessile organisms, using a square within which all individuals are counted; extrapolation of the data to the entire habitat results in a population size estimate.
The mark and recapture technique is used for mobile organisms; it involves marking a sample of individuals and then estimating population size from the number of marked individuals in subsequent samples. Learning Objectives Differentiate among the ways in which species distribute themselves in space.
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