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Ecology:
Populations
Characteristics of Populations
1.
2.
3.
4.
Geographic distribution
Density
Growth Rate
Age Structure
Geographic Distribution
• Also called Range
• Describes the area inhabited by a
population
Population Density
• The number of individuals per unit area;
can vary tremendously depending on the
species & its ecosystem
How many Cacti are there per square kilometer in this desert?
Population Growth
Three factors that effect population growth
1. Number of births
2. Number of deaths
3. Number of individuals that enter or
leave the population
Examples
• What happens if the number of births are
greater than deaths?
• What happens if the number of deaths
are greater than the number of births?
• Which situation will allow for population
growth?
Movement within populations
• Immigration - The movement of individuals
INTO an area
• Emigration - The movement of individuals
OUT OF an area
Which of these will allow a population to
grow?
Exponential Growth
• Occurs when the individuals in a
population reproduce at a constant rate
• Under ideal conditions with unlimited
resources, a population will grow
exponentially
J-shaped
curve
Logistic Growth
• Occurs when a population’s growth slows
or stops following a period of exponential
growth
• This happens as resources become less
available causing the growth of a
population to slow or stop.
S-shaped
curve
Carrying Capacity - The largest number of
individuals that a given environment can
support
K
X – number of individuals
t – time
K – carrying capacity
Limiting Factors
• A factor that causes population growth to
decrease such as predation, competition,
climate extremes or human disturbances
Two Types of Limiting Factors
1. Density-dependent
2. Density-independent
Density-dependent
• Limiting only when the population
density reaches a certain level
• Biotic factors include competition,
predation, parasitism, disease
• Operates most strongly when a
population becomes very large and
dense
Competion
• When organisms are crowded this causes
more competition
• Examples: space, food, water, sunlight
• This competition can be between
different species which may lead to
evolution to occupy different niches
Predation
• Predator-prey relationships are one of
the best mechanisms of population
control
Parasitism and disease
• Parasites take nourishment at the
expense of the host
• Usually weakens but can cause death
• Similar to predation
Density-Independent
• Affect all populations in similar ways
regardless of the population size
• Abiotic factors include natural disasters,
seasonal cycles, unusual weather
• Can lead to a characteristic crash
• Most populations can adapt to some change
• Some changes can effect entire populations
with major upsets leading to long-term
declines
Human Population Growth
• Size of human population tends to
increase with time
• Human population growth was very slow
due to harsh conditions and limiting
factors
• High death rates for years
Why did population growth
increase?
• Life got easier with improvements in
industry and agriculture
• Safer food supply
• Improved sanitation
• Improved health care
• Decrease in death rate while birth rate
had no change
• Exponential Growth
Demography
• Scientific study of human populations
• Birthrates, death rates, and the age
structure help predict countries growth
rates
Demographic Transition
• The hypothesis as to why population
growth has slowed dramatically in
countries such as US, Japan, and Europe
• A dramatic change in birth and death
rates
• Historically human societies have had
both high birth and death rates
Age Structure Diagrams
• Help predict future population growth
• Bar graph of the number of people in
each age group in the population
• United States has a slow but steady
growth rate
• What is the shape of Rwanda’s age
structure diagram?
• Indicates that there more young children
than teenagers and more teenagers than
adults.
• Predicts a population that will double in
approx. 30 years
Future Population Growth
• Estimated human population by 2050 will
be more than 9 billion.
• Human population is still growing
exponentially.
• Will it level out to logistic growth?
• How might that happen?