Describing Populations
What are the factors of population demographics?
A populationglossary term (opens in a new window) is all the organisms of a given speciesglossary term (opens in a new window) in a given area:
- The size of a population is the number of individuals of the same species that live in an area.
- Changes in population size can be used to determine the health of a population.
- Population densityglossary term (opens in a new window) is the number of individuals living in a specific unit area or volume.
- Population density can be estimated using sampling techniques.
- The distribution of a population describes how the individuals are spread throughout the given area.
- Clumped distribution is due to unequal distribution of resources or positive interactions between individuals.
- Uniform distribution is due to fairly equal distribution or resources and negative interactions between individuals.
- Population demographicsglossary term (opens in a new window) is the study of changes in size, density, and distribution of populations.
What is the difference between density-dependent and density independent factors?
Density-dependent factorsglossary term (opens in a new window) are related to population density. Density-independent factorsglossary term (opens in a new window) are not related to population density:
- Density-dependent factors can include resource availability, predation, and infectious disease.
- Density-dependent factors are generally biotic.
- Density-independent factors can include severe weather and other natural disasters, pollution, and non-infectious disease.
- Density-independent factors are generally abiotic.
What are the differences between exponential and logistical growth?
Exponential growthglossary term (opens in a new window) and logistical growth are two types of population growth:
- Exponential population growth is seen when a population's growth rate is density independent and constant. This causes the population size to increase more quickly over time.
- Logistical population growth is seen when a population's growth rate slow as the population's size nears the ecosystemglossary term (opens in a new window)'s carrying capacityglossary term (opens in a new window). Logistical growth is density dependent.
- R-selected organisms use exponential growth as their main life strategy. These organisms tend be small, grow quickly, produce many offspring, and are adapted to unstable environments.
- K-selected organisms use logistical growth as their main life strategy. These organisms tend to be larger, grow slowly, produce fewer offspring, and are adapted to stable environments.
What is carrying capacity (K)?
The carrying capacity is the largest population size that can be supported in an ecosystem:
- Available resources determine a population’s carrying capacity.
How are age structures created?
An age structure is a diagram, often pyramid-shaped, that shows the age distribution of a population:
- An age structure diagram, or an age pyramidglossary term (opens in a new window), can be used to determine whether a population’s size is growing, shrinking, or remaining relatively constant.
What are the differences between type I, type II, and type III survivorship curves?
A survivorship curveglossary term (opens in a new window) is a graph that shows survivorship rates at various ages for individuals survive in a population:
- A type I survivorship curve describes a population in which long life spans are typical.
- Humans and other large mammals are examples of organisms with a type I survivorship curve.
- A type II survivorship curve is similar to a straight line, as death rates are fairly consistent among individuals of all age groups.
- Some invertebrates, rodents, and most birds are some examples of organisms with a type II survivorship curve.
- A type III survivorship curve depicts populations in which mortality rates are significantly higher for very young individuals than for individuals in other age groups.
- Most insects and annual plants are examples of organisms with a type III survivorship curve.
