Download Population density - Zamora`s Science Zone

Instructor R. Zamora
AP Environmental Science
Edinburg North High School
Core Case Study
 Southern Sea Otters: Are
They Back From the
Brink of Extinction?
 Historical abundance
and distribution:
1 million along NA
Pacific Coast
 Habitat: Kelp Forests
 Habits: Use tools to eat
shellfish
 By early 1900s, hunted to near extinction
 Fur
 Viewed as competitors for shellfish
  kelp forests disappeared
 Keystone species
 Depredates herbivorous invertebrates (e.g., sea urchins)

Maintains ecologically and economically important kelps
 Recovery of otter populations
  recovery of kelp forests and overall diversity
  upsets commercial and recreational shellfishers
 Focus of this chapter, Population dynamics – study of
how populations change in their distribution, numbers,
age structure, and density.
Focus Questions
 What are the major characteristics of populations?
 How do populations respond to changes in
environmental conditions?
 How do species differ in their reproductive patterns?
Population Dynamics and Carrying
Capacity
 Population Distribution
 Three patterns of distribution or dispersion:



Clumping – most common
Uniform – when there is intense competition for resources
Random – least common
 Four reasons for clumping distributions




Resources vary from place to place
Living in groups provides protection against predators
Living in groups gives some predators a better chance at
getting food
Mating or caring for young
 Changes in Population Size: Entrances and Exits
 Population size is influenced by:




Births
Deaths
Immigration
Emigration
Change in Population  Births  Immigration  Deaths  Emigration
 Age structure: Young Populations Can Grow Fast
 Rate of population change depends on age structure –
proportion of individuals at various ages.

Usually described by three main categories:
 Pre-reproductive ages (juvenile or immature)
 Reproductive ages (adult)
 Post-reproductive (senescent)
Senescents
Adults
Immatures
Growing
Pop.
Stable
Pop.
Decreasing
Pop.
 Limits on Population Growth: Biotic Potential vs.
Environmental Resistance
 No pop. can grow indefinitely  limits to growth in
nature (lesson from one of nature’s four sustainability
principles)
 Pops. vary in their biotic potential – capacity for growth.


Intrinsic rate of increase (r) – rate at which a pop would grow
if it had unlimited resources.
Populations with high r:
 Reproduce early in life
 Have short generation times
 Can reproduce many times
 Have many offspring each time they reproduce
Example: House fly  5.6 x 106 descendants in 13-mo
Example: Bacteria w. generation time of 20-min  0.3-m deep
layer over the earth in 36-h
 There is a size limit to growth imposed by limiting
factors.

Limiting factors: water, light, living space, nutrients,
competition, predation, and disease.
 Environmental resistance – all factors that limit growth
of a pop

Negative, or corrective feedback
 Biotic potential and environmental resistance lead to
carry capacity (K) – the maximum population size that a
particular habitat can sustain indefinitely w/o degrading
the habitat.
 Exponential and Logistic Population Growth: J-curves
and S-curves
 With ample resources a pop can grow rapidly, but as
resources become limited, its growth rate slows and
levels off.
 With few, or no limitations populations grow
exponentially (exponential growth) at a fixed rate (e.g.,
2%).

N-t plot produces a J-shaped curve
 Logistic growth involves rapid growth followed by a
steady decline w/ time until pop size levels off.


Decrease occurs as pop experiences environmental resistance
N-t plot produces a J-shaped (or sigmoid) curve
Figure 8-3. No population can continue to increase in size indefinitely.
Figure 8-4. Logistic growth of sheep after being introduced to the
island of Tasmania
 Brown tree snake
 Multiplied exponentially
 Up to 5000 km-2
 Venomous
 Caused more than 2000
power outages
 Caused the extinction of 8
out of 11 of Guam’s forests
birds.
http://www.npswapa.org/gallery/album59/Brown_tree_snake_Boiga_irregularis_U
SGS_Photograph
Figure 8-5. Brown tree snake was
accidentally introduced to Guam
during WWII.
 What influence would a decline in population size of a
keystone species have on community composition?


Decrease in populations of species dependent on the keystone
species.
Increase in species that move in to occupy part or all of vacant
niches.
 Exceeding Carrying
Capacity: Move, Change
Habits, or Decline in
Size
 The transition from
exponential growth to
logistic growth may not
be smooth.


Occurs because of a
reproductive lag time.
Dieback, or crash ensues
(Fig. 8-6)
Figure 8-6. Exponential growth,
overshoot, and population crash after
introduction to St. Paul Island in Bearing
Sea in 1910.
 Carrying capacity if an area or volume is not fixed.
 Habitat may be degraded by the population that exceeded K.
 Also, K varies temporally increasing or decreasing seasonally
or year to year.
 Weather
 Climate
 Other factors
 K for a population man increase by developing adaptive
traits through natural selection.
 Population may migrate when K has been exceeded.
 Humans are not exempt from population overshoot and
dieback.



Ireland, 1845, 1 million died, 3 million migrated
Polynesians on Eater Island, pop crashed after using up most
of island trees
Earth’s carrying capacity for humans has been extended by
technological, social, and cultural changes.
 Population Density and Population Change: Effects of
Crowding
 Population density – the number of individuals in a
population found in a particular area or volume.


Pop density can affect how rapidly it can grow or decline.
Some control factors are not affected by population density.
 Density-dependent factors can control population size
increase as the density increases.


Competition, predation, parasitism, and diseases (e.g.,
bubonic plague in the 14th century)
Tend to regulate a pop at a fairly constant size, often near K
 Density independent factors control independently of
pop density.

Mostly abiotic
Examples: freezes, floods, hurricanes, fire, pollution, and
habitat destruction
 Types of Population Change Curves in Nature
 Four general patterns:

Stable – size fluctuates slightly above and below K


Irruptive – explosive growth to a high peak and then crash.



Characteristic of short-lived, rapidly reproducing species
Linked to seasonal changes in weather and nutrient availability
Cyclic – regular cycles of increase and decrease



Characteristic of species in stable environments
Rise and fall of lemmings every 3-4 years
Lynx and snowshoe hare, 10-yr
(Fig 8-7)
 Top-down pop regulation
 Bottom-up regulation
Irregular – no pattern in change
of population size
Figure 8-7
 Cases Study: Exploding White-tailed Deer Population
in the US
 Since the 1930s the white-tailed deer population in the
US has exploded.



By 1900, reduced to 500 000
1920s and 30s laws passed to protect deer, and wolves and
mountain lions nearly eliminated
Today there are 25-30 million
 Problem with the rebound
 Encroachment
 Suburbanization
 Vector for Lyme disease
 Solutions
 Change hunting regulations
 Trap and relocate
 Birth control
Reproductive Patterns
 Ways to reproduce: Sexual Partners Not Always
Needed
 Asexual Reproduction


Produces clones
Common in taxa such as bacteria, plants and some animals
such as corals.
 Sexual Reproduction

Mixes genetic material of two parents producing offspring w/
genetic traits of each parent.


Disadvantages of sexual
reproduction
 First, males don’t give birth;
female has to produce twice as
many offspring to break even.
 Second, increased change of
genetic errors separation and
recombination of
chromosomes.
 Third, courtship and mating is
expensive (time and energy
budgets), can cause disease,
and injury may be inflicted in
males that combat for mates.
Advantages
 Provides genetic diversity in
offspring
 Males of some species can
help raise young
Figure 8-8. Courtship display
 Reproductive Patterns: Opportunists and Competitors
 Species differ in reproductive strategies to help ensure
survival.
(instead opportunists)
(good competitors)
 Most species have
Figure 8-9. Positions of r-selected and Kselected species on the sigmoid population
growth curve.
reproductive patterns
between extreme r- and Kselected species.
 Reproductive patterns may
give a species a temporary
advantage, but the
ultimate population
regulator is available
habitat.
 Survivorship Curves
 A representation of age
structure that shows the
percentage of members
surviving at different ages
(Fig. 8-11)

There are three generalized
curves: late loss, early loss,
and constant loss.
 A life table shows
projected life expectancy
and probability of death
for individuals at each age
in a survivorship curve.
Figure 8-11. Survivorship curves for
populations of different species.
The problems to be faced are vast and complex, but come down to this: 6.7 billion
people are breeding exponentially. The process of fulfilling their wants and needs is
stripping earth of its biotic capacity to support life; a climactic burst of
consumption by a single species is overwhelming the skies, earth, waters, and
fauna.
-Paul Hawken
 The next chapter applies the principles of population
dynamics discussed in this chapter to the growth of
human population and its environmental impact.
 The principle of population dynamics are also used to
help us harvest fish and wildlife resources more
sustainably.
Figure 9-1. Crowded
street in China.