Populations have many
Characteristics
• Populations go through 3 Stages
– Scientists study how populations or organisms change
as they interact with each other & the environment
– POPULATION DYNAMICS = study of why
populations change & what causes them to change.
• 4 Characteristics define a Population:
– Population size; population density; population
spacing & age structure
• Scientists can predict a Population Change
Growth, Stability & Decline
• All populations go through growth, stability & decline.
• CARRYING CAPACITY = maximum number of individuals
an ecosystem can support(with its available resources); if a
habitat has enough resources, the population grows fast, then
it stabilizes and population stays about the same for a while.
This stability stage is usually the longest time period. Often,
it is followed by a sharp decrease in population size.
• During growth phase it can occur very quickly (as in bacteria
reproducing, or gradually as in a population of seals).
Darwin’s Observations of
Population Growth
• Charles Darwin was a naturalist scientist who wrote
On the Origin of Species based on his observations
in the Galapagos Islands. He noted the following
about population growth:
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All populations are able to grow rapidly
Populations tend to remain constant in size
There are limits to natural resources
Within a given population there is genetic variation
• He knew that if there were no limitations to growth,
populations would grow rapidly. But also realized,
there are natural limits to growth in the real world.
If birth rate doesn’t match the death rate, a
population can decline until it becomes extinct.
Population Size
• Population size= number of individuals in a
population at a given time. (varies from habitat to
habitat, & even within a habitat). In our area,
mosquito population size changes during different
times of the year.
• Many deserts experience blooming & growing
during our spring, but by summer (with less water &
higher temperatures) there is a rapid decline in plant
population size.
Population Density
• Population density= measure of the number of individuals in
a certain space at a particular time. Related to population
size; if size increases and they all stay in same area, density
increases too. Some species live in high density (bees) while
others live in low density populations (blue herons).
• RANGE = the distribution of a population across a large
geographic area. The population density within that range
may vary (more grasshoppers in middle of prairie than its
edge). Habitats located in the middle of a population range
usually have a greater density than its edges (more resources)
Population Spacing
• Population spacing= the pattern in which the individuals of a
population are found. 3 main types: clumped, uniform and
random.
• Clumped spacing: individuals form small groups within a
habitat (elephants do this due to social nature, salamanders
due to habitat-moist rotten logs found in 1 area)
• Uniformly spaced: individuals living at an equal distance
from each other; plants often due this due to competing
resources (if too close, some die out)
• Random spacing: dandelions/weeds grow where ever.
Age Structure
• Scientists divide a population into 3 groups based on
age:
– Postreproductive: organisms can no longer reproduce
– Reproductive: organisms capable of reproduction
– Prereproductive: organisms not yet able to reproduce
• The age structure of a population affects how much
it can grow. On graph below, postreproductive age
range is over 45, while prereproductive range is 0-14
Predicting Population Change
• Scientists use size, density, spacing & age structure to
describe a population & predict how it may change over time.
• A population may change due to its surroundings(frogs living
in pond that gets saltier will probably experience a population
decrease).
• Scientists can also predict change by looking at the
distribution of a population. (New York City released a
population of 60 starlings in 1890; they now number over
200 million spread across the U.S., Canada & Mexico. The
population rose as they moved into new habitats with
available resources.