Download APES FINAL

Chapter 6: Population Biology
EMMA DEBANY AND DERRIAN DURYEA
What’s Population?
Population: all members of a single
species living in specific area at same
time.
How Can We Express Population Growth?
 exponential growth: theoretical unrestricted increase
in populations, no limiting factors.
 Exponential growth can be expressed as an equation!
dN
 rN
dt
The Equation
dN
 rN
dt
dN: change in # of individuals
dt: change in time
r: rate of growth—a fraction representing the average individual
contribution to population growth.
If r is POSITIVE, population increasing.
r NEGATIVE population decreasing.
r is ZERO, no change, and dN/dt = 0
N: number of individuals in population
The Equation Continued
Previous equation also known as Biotic
potential:
the potential of a population to grow if
nothing is limiting its expansion.
How Many Years Will A
Population Take to Double?
 If population is growing exponentially (without
limits), this equation finds how many years a
population will take to double:
divide 70 by annual percentage growth = approximate
doubling time in years
in other words:
70 / x% = doubling time (years)
(for instance, a population growing at 35% doubles in 2 years)
What About When There are Limits to Growth?
Carrying capacity: max. population of any species that can
be supported by a particular ecosystem
Overshoots: when a population goes over the carrying
capacity of its environment--death rates begin to rise
past birth rates
Population crash: after an overshoot, population
decreases as fast OR faster than it grew
Growth and Decline of Populations
Populations normally cycle through growth and
decline
Regular Cycles: depend on simply factors (like seasonal algae
blooms that depend on light and temperature)
Irregular Cycles: depend on complex environmental
relationships (like outbreaks of migratory locusts in desert)
Irruptive growth: long periods of low population size then a
sudden population growth
Stable Populations
Remember exponential growth?
There’s also logistic growth!
logistic growth: species grow exponentially when
resources unlimited BUT pop. growth slows when
carrying capacity of environment is approached
(Population will decrease if it exceeds carrying
capacity)
Formula For Logistic Growth!
dN
N
rN(1 )
dt
K
dN/dt: change in numbers over time
r: exponential growth rate
N: population size
K: carrying capacity

Formula For Logistic Growth!
dN
N
rN(1 )
dt
K
(1-N/K): represents relationship between N (pop size) at any given
time step and K (# of individuals the environment can support)
If N is less than K, 1-N/K will be positive, and means the population
is growing (smaller numbers greater than 0 is slow growth, larger
numbers faster growth)
If N is more than K, 1-N/K will be negative and the population will
be decreasing.
Logistic vs Exponential Growth
Exponential: also known as J Curve
Logistic: also known as S curve
J curve: theoretical growth without restraint toward
biotic potential
S curve: stabilization in response to environmental
resistance
Factors that Limit Populations
Populations regulated by internal and external
factors
internal: maturity, body size, hormonal status
external: habitat, food availability, interactions
with other organisms
External Limits:
Density Dependent vs Independent
density-dependent: limits dependent on population
density
food and water
 disease, stress, exposure to predators or parasites

density-independent: limits not involved with
population/density of animals
Example: drought / early frost
 Habitat destruction: floods, fires, etc

What’s Environmental Resistance?
Environmental resistance:
environmental factors that tend to
reduce population growth rates.
Resistance is larger and rate of logistic
growth smaller as population
approaches carrying capacity
K-Adapted? R-Adapted? Whaaaat?
R adapted: species that persist by depending on high
rate of reproduction and growth
rapid reproduction
 High mortality of offspring
 Will vershoot carrying capacity and die back

K adapted species: reproduce more slowly as they
approach the carrying capacity of the environment
R-Adapted Species
R Adapted Species grow exponentially
 Move quickly into disturbed environments
 grow rapidly
 mature quickly
 produce many offspring
 do little to care for offspring
 depend on sheer numbers and dispersal techniques
to ensure some survive
R –Adapted Graph
K-Adapted Species
 Larger
 Live longer
 Mature slower
 Produce fewer offspring in each generation
 Fewer natural predators
Factors that Increase/Decrease Populations
Natality: production of new individuals by
birth, hatching, germination, cloning
(sensitive to environmental conditions)
Successful reproduction tied to: nutritional
levels, climate, soil, water conditions, social
interactions
Fecundity vs Fertility
Fecundity: physical ability to reproduce
Fertility: measure of the actual number of offspring
produced
(Because of lack of opportunity to mate, fecund
individuals may not contribute to pop growth)
Immigration Additions to Populations
Methods of immigration:
• Wind (seeds, spores, small animals carried distances
• Fur/feathers/intestines
• Water
• Self-transportation (birds fly, fish swim, wolves
walk)
Immigration Continued
Some ecosystems can be maintained by
constant influx of immigrants
Mortality/Death Rate
How to calculate mortality/death rate:
divide the # of organisms that die in a
certain time period by the # alive at the
beginning of the period
X1 / X2
But What Is Mortality?
 Survivorship: percentage of a cohort that
survives to a certain age
 Life expectancy: the probable number of
years of survival for an individual
Life Expectancies in US
 Rose during 20th Century
 1900: 47.3 years expectany
 2003: 77.4 years expectancy
 Differences between sexes,
 races, economic class
Life Span
 Life span: longest period of life reached
by a given type of organism
 Most organisms don’t live anywhere
near the maximum life span for their
species
 Major factors in early mortality:
 Predation
 Parasitism
 Disease
 Accidents
 Fighting
 Environmental
influence (climate, nutrition)
Emigration
Emigration: the movement of the members out of a
population size.
When a group/individual immigrates to a new area,
they emigrate out of an old area
Same techniques used for immigration are used for
emigration
Can help protect a species if area is overpopulated
Population Growth Factors
What are the factors that regulate
population growth?
These factors primarily affect natality
and mortality
Types of Factors:
 Intrinsic: operating within individual
organisms or between organisms in the
same species
 Extrinsic: imposed from outside the
population
More Types of Factors
 Biotic: caused by living organisms (tend
to be density-dependent)
 Abiotic: caused by nonliving
components of the environment (tend
to be density-independent)
Which Factor is More Important in
Regulating Population Dynamics?
 Has been much debate
 In general, depends on:
 the particular species involved
 that species’ tolerance levels
 The stage of growth and development of organisms involved
 Ecosystem where the organisms live
 The way combination of factors interact
Abiotic Generally Density-Independent
 Weather or climate are most important factors
 Extreme cold, high heat, drought, excess rain, severe
storms also important
 Factors don’t always diminish population:
 After a rainstorm (an abiotic factor) the desert will flourish
 Some forests need fires to bloom
Density-Dependent Factors
Density-dependent factors reduce population size
Decrease natality
Increase mortality
Result of not only interactions between populations of
a community, but also interactions within a
population
Interspecific Interactions
 These interactions occur between species
 Predator vs prey
 Prey species can also benefit:
 Moose are killed by wolves
 Old/sick moose are killed off
 This strengthens herd of moose as a whole
 Also benefits wolves
Intraspecific Interactions
 These occur within species
 Animals in species compete for resources
 Population density is low = resources plentiful
 Pop. Density high = resources low
Stress and Crowding
 Stress related diseases: when pop. density is high,
organisms have symptoms of this
 Too much competition/too close proximity to other
organisms
 Can affect reproduction, thus lowering population
density once more—population fixes itself
Case Study 2: THE LOCUSTS
 Locust plagues have been tragically destructive
throughout history
 Ever few decades rain comes to the desert and
locusts flourish
 This high population density for some reason causes
them to stop reproducing, grow longer wings, and
swarm desert
 The swarms devour hundreds of thousands of plants
and all die within a few weeks.
Case Study 2: THE LOCUSTING
 Locust swarms can affect livelihood of 1/10 of the




earth’s population with all the plants/crops they
destroy
In 2004 heavy rains in the Sahara cause locust
swarm
28 countries in Africa and Mediterranean were
affected
Crop losses reached 100% in some places
This study illustrates power of exponential growth
and danger of boom and bust life cycles
Conservation Biology
Small, isolated populations can undergo declines due
to environmental change, genetic problems, or
random events
Island Biogeography
 Island Biogeography: a theory that MacArthur and
Wilson came up with in 1967 to explain why islands
have fewer species than the mainland
 Theory explains that diversity in isolated habitats =
balance between colonization and extinction rates
Biogeography Continued
 Islands have low colonization rates because islands
are hard to reach
 Limited habitat forces population to be small
 Therefore larger islands closer to mainland are more
populated and more diverse
 Genetics = important in survival/extinction of small,
isolated populations
 Hardy-Weinberg equilibrium: in large populations.
If mating is random, no mutations occur, the
distribution of gene types will preserve genetic
diversity
Genetic Drift
 Genetic Drift: the gradual changes
in gene frequencies (occurs in small
populations due to fewer
individuals with slight genetic
variation being involved in mating)
Founder effect/demographic bottleneck: a few
members of the species survive a disaster, or colonize
a new habitat isolated from other members of species
Results in loss of genetic diversity
Seals and Cheetahs
 Elephant seals were nearly hunted to extinction, but
their numbers are now normal again. They are also
now all almost nearly identical genetically
 All male cheetahs are nearly identical genetically,
suggesting they all came from one common male
ancestor

This lack of diversity responsible for a low fertility rate an low
survival rate of offspring