45.1 Population Demography

By the end of this section, you will be able to describe how ecologists measure population size and density, describe three different patterns of population distribution, and use life tables to calculate mortality rates.

Natural disasters such as forest fires and volcanic eruptions, as well as seasonal and yearly changes in the environment, are some of the factors that affect populations.
The tools were originally designed to study human populations.
The term "demographics" is often used when discussing humans, but all living populations can be studied using this approach.

The study of any population usually begins with determining how many individuals of a particular species exist and how closely associated they are with each other.

Population size and density are two characteristics used to describe populations.
Populations with more individuals may be more stable than smaller ones because of their genetic variability.
A member of a population with low population density might have a harder time finding a mate than a population with higher density.

The inverse relationship between population density and body size is shown by Australian mammals.

The easiest way to determine population size is to count all of the people in the area.

This method is not logistically or economically feasible when studying large habitats.
Scientists usually study populations by sampling a representative portion of each habitat and using this data to make inferences about the habitat as a whole.
A variety of methods can be used to sample populations.
A quadrat is a way of marking off square areas by using sticks and string, or by placing a wood, plastic, or metal square on the ground.
Researchers count the number of people that lie within their boundaries after setting the quadrats.
The population size and density of the entire habitat is estimated using multiple quadrat samples.
The number and size of quadrat samples depends on a number of factors.
If sampling daffodils, a 1 m2 quadrat might be used.
A quadrat of 100 m2 could be used with giant redwoods.
This ensures that enough individuals of the species are counted to get an accurate sample that matches the habitat.

This method involves marking a sample of captured animals and then releasing them back into the environment to allow them to mix with the rest of the population.

Some individuals that are marked and others that are not are collected later by researchers.

The population size of mobile animals such as bighorn sheep, California condor, and salmon are measured using Mark and Recapture.

Scientists use the ratio of marked and unmarked individuals to figure out how many people are in the sample.

The total population size is estimated from this.
The method assumes that the larger the population, the lower the percentage of tagged organisms that will be captured.

Population estimates may be inflated due to animals from the first catch avoiding capture in the second round.
If a food reward is offered, some animals may prefer to be retrapped, resulting in an underestimate of population size.
Some species may be harmed by the marking technique.
The use of data from commercial fishing and trapping operations to estimate the size and health of populations and communities is one of the techniques developed.

Further information about a population can be obtained by looking at the distribution of individuals.

There is uniform dispersion in plants that release substances that affect the growth of nearby individuals and in animals that maintain a defined territory.
An example of random dispersion is when dandelion and other plants have wind-dispersed seeds that fall in a favorable environment.
A clumped dispersion can be seen in plants that drop their seeds straight to the ground, or in animals that live in groups.
Habitat heterogeneity may be a factor in clumped dispersions.
The dispersion of individuals within a population gives more information about how they interact with each other than a simple density measurement.
When compared to social species clumped together in groups, solitary species with a random distribution might have a similar difficulty in finding a mate.

Population characteristics may affect each of these measures.
A large population size results in a higher birth rate because more people are present.
A large population size can result in a higher death rate because of disease and competition.
A higher population density can result in more potential reproductive encounters between individuals, which can increase birth rate.

The demographic characteristics of a population can affect the population's growth or decline.

The population is stable if birth and death rates are the same.
The population will decrease if birth rates are less than death rates.
Local resources, reproduction, and the overall health of the population are impacted by the length of time individuals remain in the population.
A life table displays demographic characteristics.

Life tables show how long a group of people are likely to live.
They are modeled after actuarial tables used by the insurance industry.
An example of a life table can be found in Table 45.1 from a study of the Dall mountain sheep.
The population is divided into age intervals.
The mortality rate is based on the number of individuals dying during the age interval divided by the number of individuals surviving at the beginning of the interval.

Between the ages of three and four, 12 people die out of the 776 remaining from the original 1000 sheep.

The mortality rate per thousand is obtained by taking the number and dividing it by 1000.

A high death rate occurred when the sheep were between 6 and 12 months old, and then increased even more from 8 to 12 years old, after which there were few survivors.

The data shows that a sheep in this population could live another 7.7 years on average if it survived to age one.

The life table of Ovis dalli shows the number of deaths, survivors, mortality rate, and life expectancy.

The table shows the number of deaths, number of survivors, mortality rate, and life expectancy at each age interval.

A high percentage of offspring survive their early and middle years--death occurs mostly in older individuals.
These types of species usually have small numbers of offspring at one time, and they give a high amount of parental care to ensure their survival.
Birds die more or less equally at each age interval, which is an example of an intermediate or Type II survivorship curve.
These organisms provide parental care and have relatively few offspring.
The survivorship curve of trees, marine invertebrates, and most fishes is a result of how few of these organisms survive their younger years, but those that make it to an old age are more likely to survive for a long period of time.
Organisms in this category usually have a lot of offspring, but little parental care is provided once they are born.
The sheer numbers of these offspring assure the survival of enough individuals to perpetuate the species.