Population Dynamics

Chapter 4.1

How fast do populations grow?

• Populations of organisms do not experience linear growth– They start out slowly– It begins to resemble a J-shape curve

• The initial increase in the number of organisms is slow because the number of reproducing individuals is small

• The rate of population growth soon increases because the total number of individuals that are able to reproduce has increased

Human Population Growth

• Human population growth is different from bacteria population growth in that we can consciously change our environment– ex: we eradicated smallpox– ex: developed methods for producing more food– ex: infant mortality rate has decreased– ex: technological developments improved water

quality

• Result = live longer and produce more offspring that live long enough to produce more offspring

Is growth unlimited?

• Exponential Growth: As a population gets larger, it grows at a faster rate

What can limit growth?

• Population growth has limits

• Limiting factors: food, disease, predators, or lack of space – All cause population growth to slow

• Under these pressures, the population may stabilize in an S-shaped growth curve

Carrying Capacity

• Carrying Capacity: The number of organisms of one species that an environment can support indefinitely

• When a population is developing in an environment with resources…– There are more births than deaths – And the population increases until the

carrying capacity is reached or passed

Carrying Capacity Cont.

• When a population overshoots the carrying capacity– Then limiting factors may come into effect– Deaths being to exceed births– And the population falls below carrying

capacity

• Many organisms in nature show a graph that has a line below the carrying capacity

Reproduction Patterns

• Life-history pattern: an organisms reproductive pattern

• Example #1 = mosquitoes (rapid life history pattern)– Reproduce very rapidly – Produce many offspring in a short period of time

• Example #2 = elephants (slow life history pattern)– Slow rate of reproduction – Produce relatively few young over their lifetime

Rapid life-history patterns

– Common among organisms that live in changeable or unpredictable environments

– Characteristics:• small body size• mature rapidly• reproduce early• and have a short life span• Populations increase rapidly then decline when

environmental changes occur suddenly– The small surviving population will reproduce

exponentially

Slow life history patterns

– Large species that live in a more stable environment

– Ex: bears, elephants, whales, humans, and trees

– Characteristics:• reproduce and mature slowly• long lived• maintain populations at or near carrying capacity

Density factors and population growth

– Limiting factors are both biotic and abiotic• They determine whether or not an organism can

live in a particular environment• Ex: limited food supply, space, chemicals

(produced by plants themselves), extreme temperature, storms

– Organisms disperse in 3 patterns• random• clumped• uniform

Density factors and population growth continued

– 2 kinds of limiting factors that are related to dispersal = density-dependent and density-independent.

• Density-dependent factor– ex: disease, competition, predators, parasites, food– These factors have an increasing effect as the

population increases

• Density-independent factor– Affects populations, regardless of their density– Often abiotic factors– ex: volcanic eruptions, temperature, storms, flood,

drought, chemical pesticides

Organisms Interactions Limit Population Size

Predation affects population size

• Predation can affect population sizes in both minor and major ways– ex: owl kills and eats a mouse– ex: swarm of locusts eats and destroys acres

of lettuce– ex: brown snake introduced to Guam (no

native predators) drastic effect on prey population

Predation affects population size continued

• Populations of predators and their prey are known to experience cycles or changes in their numbers over period of time

• Graph: the number of hair increases then the number of lynx increases

• Predation increases the chance that resources will be available for the remaining individuals in a prey population

Competition within a population

• When a population is small, resources can build up and become plentiful Then as these resources are used, the population

increases in size Competition for resources (food, water, and

territory) increase significantly• Competition is a density dependent factor• If few individuals are present Then there is

little competition for resources• When a population increases to the point where

demand exceeds supply the population size decreases

The effects of crowding and stress

• When populations = crowded, individuals exhibit stress– Stress is shown via

• aggression• decrease in parental care• decreased fertility• decreased resistance to disease

– All have negative effects on populations

• They become limiting factors for growth and keep populations below carrying capacity