Transcript File - Down the Rabbit Hole

Ecology
Population Biology
Population Ecology
Population – all of the individuals
of a particular species that live
together in one place at one time.
Population ecology is the study of
interactions within populations
(i.e., intra-specific interactions)
•Static population pattern – a population is
assessed with a measurement or estimate at a
single point in time (a “snapshot”)
•Dynamic population pattern –population that is
assessed with measurements or estimates taken
at two or more points in time
Population Ecology
• Principles of population ecology
may be used to:
– manage wildlife, fisheries, and
forests for sustainable yield
– reverse the decline of threatened or
endangered species
– reduce pest populations
– IPM = Integrated Pest Management
• Integrated pest management (IPM)
uses a combination of biological,
chemical, and cultural methods to
control agricultural pests
Population Demographics
• Demography is the study of the
characteristics of populations. It provides a
mathematical description of how those
characteristics change over time.
• Demographics can include any statistical
factors that influence population growth or
decline, but several parameters are
particularly important:
• Population size
• Population density
• Population distribution
• Sex ratio
• Age structure
• Birth and death rates
Population Size
… is the number of individuals present at a
given time.
The passenger pigeon was once
North America’s most numerous
bird, but it is now extinct.
Population Density
… is the number of individuals per unit area.
In the 19th century,
the flocks of
passenger pigeons
showed high
population density.
Population Dispersion
The dispersion pattern of spacing among
individuals within the boundaries of the population
Population Dispersion
• Clumped
– Individuals in patches or
groups
– Usually resource related
– Example: Bluestripe
snappers (Lutjanus
kasmira)
– Schooling of some fish is
a protective strategy
– Herd mentality –
protection of young
Population Dispersion
• Uniform
– Often the result of
antagonistic interactions
– Animals that defend
territories often show a
uniform patterns
– Example: Cape gannets
(Morus capensis)
– These birds space their
nests out evenly
– Plants - alleleopathy
Population Dispersion
• Random Dispersion
– Unpredictable
spacing
– Not usual in nature
because there is
usually a reason for
a pattern of spacing
Carrying Capacity
• refers to the size of a
population that can live
indefinitely in an
environment without
doing that environment
any harm. This applies to
all organisms.
• # of individuals in a
population that an
environment can support.
What limits the carrying capacity
of an ecosystem?
Limiting Factor: is any biotic or abiotic resource that limits a
populations size.
• Hunting
• Amount of space suitable for
•
•
•
•
breeding
Food availability - limited food
supply
Preditors
The buildup of toxic wastes
Increased disease
More Limiting Factors
• Terrestrial
–
–
–
–
–
–
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Sunlight
Temperature
Precipitation
Soil nutrients
Fire frequency
Wind
Latitude
Altitude
• Aquatic/Marine
– Light penetration
• Water clarity
– Water currents
– Dissolved nutrient
concentrations
• Esp. N, P, Fe
– Dissolved Oxygen
concentration
– Salinity
Limits on Population Growth:
Biotic Potential vs. Environmental Resistance
No population can increase its size indefinitely.
The intrinsic rate of increase (r) is the rate at which a
population would grow if it had unlimited resources.
Carrying capacity (K): the maximum population of a
given species that a particular habitat can sustain
indefinitely without degrading the habitat.
How are populations
measured?
• count all the individuals in a
•
•
population
estimate by sampling
mark-recapture method depends
on likelihood of recapturing the
same individual
Density Independent
• This is when a
population is controlled
by natural events other
than population
density.
• Natural disasters are
examples of density
independent factors
•
•
•
•
•
•
fires
Floods
Earthquakes
Hurricanes
Volcanoes
Drought
Density Dependent
• This occurs when the
density of the
population controls the
total population of
individuals in a species.
• Stress
• Disease
• Competition for
resources
• Lack of space
How do populations grow?
Idealized models describe two kinds of
population growth
1. exponential growth - the rate of expansion of
a population under ideal conditions
2. logistic growth - idealized population growth
slowed by limiting factors as the population
size increases
Exponential Growth Curve
• A J-shaped growth curve
– G = the population growth rate
– r = the intrinsic rate of increase, or an organism's maximum capacity
to reproduce
– N = the population size
Logistic Growth Curve
• Logistic growth is slowed by population-limiting factors
– K = carrying capacity - maximum population size that an environment
can support
– (K - N)/K accounts for the leveling off of the curve
Logistic growth and exponential growth
compared
Exponential and Logistic Population
Growth: J-Curves and S-Curves
Populations grow
rapidly with ample
resources, but as
resources become
limited, its growth
rate slows and
levels off.
What does the logistic growth model suggest about
real populations in nature?
• A population’s growth rate will be small
•
•
•
•
when the population size is either small
or large
The growth rate will be highest when
the population is at an intermediate
level relative to the carrying capacity.
Limiting factors make the birth rate
decrease, the death rate increase or
both
Eventually the population will stabilize
at the carrying capacity when the birth
rate equals the death rate
These are mathematical models and no
population fits either perfectly
Exponential and Logistic Population
Growth: J-Curves and S-Curves
As a population
levels off, it often
fluctuates slightly
above and below
the carrying
capacity.
Types of Population Change
Curves in Nature
Population sizes may stay the same, increase,
decrease, vary in regular cycles, or change erratically.
Stable: exhibits dynamic equalibrium.
Irruptive: when populations explode and crash.
Cyclic: populations fluctuate up and down
Irregular: erratic changes.
Some factors that limit population growth
• As density of song sparrows
increase, the number of eggs
laid decreases because of food
shortages
• Plants grown under crowded
conditions tend to be smaller
and less likely to survive
• Disease transmission or
accumulation of toxic waste
products can increase mortality
Some factors that limit population growth
• A predator may capture more
•
•
of a particular kind of prey as
the prey becomes abundant
White-footed mice stop
reproducing at a colony size of
30-40 even when food and
shelter are provided. Stress?
The graph shows aphids which
feed on the phloem sap of
plants increase in population in
the summer and then die-off in
the fall and winter
Some factors that limit population growth
• Some populations remain fairly
•
•
stable in size close to carrying
capacity
Most populations fluctuate as
seen at the left
This graph shows song
sparrow populations, with
periodic catastrophic
reductions due to severe winter
weather
Boom and bust cycles
• Hare cycles may be
•
•
•
caused by increasing
food shortages during
winter caused by
overgrazing
They may be due to
predator-prey interactions
Cycles could be affected
by a combination of food
resource limitation and
excessive predation
Predators reproduce
more slowly than their
prey so they always lag
behind prey in population
growth.
Life History
• Life history traits are
products of natural
selection
• Traits that affect an
organism’s schedule of
reproduction and
survival make up its life
history
• Life history traits are
evolutionary outcomes,
not conscious
decisions by organisms
r- and K- Survivorship Strategies
r-selected species
• Many offspring
• Fast growing
• No parental care
K-selected species
• Few offspring
• Slow growing
• Parental care
Terms come from:
r = intrinsic rate of
population increase.
(Populations that can
potentially grow fast,
have high r.)
K = symbol for
carrying capacity.
(Populations tend to
stabilize near K.)
Survivorship Curves
• Type I curve – produce few
•
•
offspring and give them
good care (K type)
Type II curve -probability of
survival does not change
with age; no age bias
Type III curve – high death
rates for very young then a
period when death rates
are lower for those who
survive to a certain age
(generally r-type)
For example…
Dandelions and salmon produce many, tiny
offspring with low survivorship probabilities
For example…
Coconut palms and kiwis produce a few, big
offspring with high survivorship probabilities
Collapse of northern cod fishery –
• Renewable resource
•
management – harvesting
crops without damaging
the resource
Maximum sustainable yield
– harvest at a level that
produces a consistent
yield without forcing a
population into decline
Population growth crash
Some populations that rise too fast and deplete resources
may then crash, as with reindeer on St. Paul Island.
St. Paul reindeer, Rangifer tarandus
Invasive
species are
also known as
“exotic,” “nonnative”, or
“introduced”
species
Invasive Species
second leading cause of
species extinction worldwide
• 4500 species have been
introduced to U.S.
• Annual costs - 100 billion
• 30% of endangered
species listings are due in
part to exotics
• Introduced species
–no natural enemies
–can outcompete natural
species
The Problem of Exotic
Species
• Species native to islands
•
are especially vulnerable.
The Mariana Fruit Dove,
Guam Flycatcher, Rufous
Fantail, Cardinal
Honeyeater, and other
native forest birds of
Guam have become
extinct since the late
1940s, when the brown
tree snake (Boiga
irregularis) is believed to
have first colonized the
island.
Brown Tree Snake
• Guam's unique
subspecies of
Micronesian kingfisher
has been eradicated from
the island by the brown
tree snake, however zoos
are currently breeding the
bird and hope to return it
to the island one day.
Brown Tree Snake
• By preying on native wildlife,
•
and competing with other
native predators, pythons
are seriously impacting the
natural order of south
Florida's ecological
communities.
The continued proliferation
of Burmese pythons-and the
continued introduction of
new foreign species-can
further threaten many of the
native endangered plants
and animals.
Pythons in the Everglades
The Spread of Shakespeare Starlings
• In 1890, a group of Shakespeare enthusiasts released
about 120 starlings in New York's Central Park
Today over 100 million starlings are spread over
North America
Current
Current
1955
1955
1945
1935
1925
1945
1905
1915
1935
1925
1925
1935
•The starling population in North
America has some features in common
with the global human population
–Both are expanding and are virtually uncontrolled
–Both are harming other species
The history of human population growth
• Throughout human history
•
•
parents had many children
but only two on average
survived to adulthood
Estimates that by 2025 the
world will have to double
food production, 2/3 of the
available fresh water on
earth will be in use, 60,000
plant species will be lost to
support the population
Issues: overgrazing, rivers
running dry, decrease in
groundwater, energy?
Demographic Transition
• Going from high birth
•
•
•
rates and high death
rates to low birth rates
and low death rates
May take 150 years to
complete
“Death rate falls due to
increased medical care
and sanitation
Falling birth rate takes
longer, thus delaying
transition.
Population pyramids for Canada and Madagascar.
Canada (a) shows a balanced age structure, with relatively even numbers of
individuals in various age classes. Madagascar (b) shows an age
distribution heavily weighted toward young people. Madagascar's population
growth rate is nine times that of Canada's. Data from U.N. Population
Division.
As China's population ages, older people will outnumber the young
Age pyramids show the predicted graying of the Chinese
population between 2005 (a) and 2030 (b). Data from U.N.
Population Division.
USA Baby Boom 2005
The "baby boom" is visible in the 2005 age pyramid for the United States,
in the age brackets between 40 and 50.
The nation is experiencing an aging population as baby-boomers grow
older. Data from U.N. Population Division.
The Ecological Footprint
(carrying capacity)
• Amount of productive
•
•
land and water needed to
support the people in a
population
Currently 1.7 hectares
per person is considered
suitable
A typical American has a
footprint of 10 hectares
What next?
How to achieve population stability?
• Zero population growth – when
•
•
•
birth rates equal death rates
Two ways to reach ZPG. High
birth and death rates or low
birth and death rates.
Demographic transition is
moving from the first to the
second. Most developed
countries have made the
transition
See the demographic transition
in Mexico at the left.
Conclusion
• Natural selection, speciation,
and extinction help determine
Earth’s biodiversity.
• Understanding ecological
processes at the population
level is crucial to protecting
biodiversity.
• Understanding population
ecology also helps us
understand human population
growth.