Population Dynamics
Evolution, Natural Selection, and
Human Impacts for Introduction to
Environmental Science
Population Levels
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Population Density – is the number of
individuals of a population at a given
time.
Ecological Population Density – the
number of individuals (n) per unit of
habitat area
Population dispersion – spatial values of
population including clumping, uniform,
and random (see figure in text)
Individual to Population
Age Structure
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This is a breakdown of the major
contributors to populations in
ecosystems
Prereproductive
 Reproductive
 Postreproductive
The population growth rate swings based
upon biotic and abiotic factors leading
to growth or decline
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Response to stress
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Populations change =
(births + immigration) –
(deaths + emigration)
A balance is considered Zero
Population Growth (ZPG)
What are the implications of ZPG?
Courtesy: www.globalchange.umich.edu
Courtesy: www.globalchange.umich.edu
Data from J. Kimbell, 2006, Biology
Population Stresses
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Carrying capacity (K) is defined as the
number of individuals of a given species
that can be sustained indefinitely –
spatially.
Population could grow exponentially with
unlimited resources considered its
maximum rate ( r) with unlimited biotic
or reproductive potential
How Much
Can we Hold?
This shows the growth of Japan and approach
to meeting its carrying capacity.
*Carrying Capacity:A Model with Logistically Varying Limits by
PERRIN S. MEYER1 and JESSE H. AUSUBEL2 in Technological
Forecasting and Social Change 61(3):209-214, 1999.
Population Stresses
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Any population growing
exponentially goes through a “J”
shaped growth, but most of the
time environmental influences
create an “S” shaped pattern in
growth
Carrying Capacity and J shaped growth
Curve demonstrating the population of rabbits
*MIT System Dynamics in Education Project Under the
Supervision of Dr. Jay W. Forrester by Leslie A. Martin
This shows exponential growth leading to overshoot and
Population dieback of species like the reindeer in Alaska
Population Density
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Density – Dependent population
controls include competition for
resources including predation,
parasitism, and disease. Some more
drastic controls include diseases like
the Bubonic Plague in Europe in
fourteenth century.
Density-Independent factors affect
populations regardless of the size like
natural disasters and use of chemicals
(like pesticides causing cancers, etc…)
Population Curves
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Populations could be stable, cyclic
or irruptive due to various factors
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Predation, like the classic Lynx and
Snowshoe Hare demonstrate a classic
predator-prey population curve in
nature.
Reproductive Strategies
“r” strategists (Type III)
- numerous offspring with a low
survival rate to adulthood
- include many invertebrates and other
animals that produce enough
offspring to preserve their genetic
variability
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Reproductive Strategies
“K” strategists (Type I)
- few offspring with high survivorship
includes a high degree of parental
investment to insure offspring
success
- examples include many mammals
(i.e., humans, marine mammals, river
otter, etc…)
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r- and K- selected Organisms
r
K
Unstable environment,
density independent
Stable environment, density
dependent interactions
small size of organism
large size of organism
energy used to make each
individual is low
energy used to make each
individual is high
many offspring are produced
few offspring are produced
early maturity
late maturity, often after a
prolonged period of parental
care
short life expectancy
long life expectancy
each individual reproduces
only once
individuals can reproduce more
than once in their lifetime
most of the individuals die
within a short time
but a few live much
longer
most individuals live to near
the maximum life span
Courtesy of www.bio.indiana.edu
Survivorship Curves
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These curves demonstrate
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Late Loss Populations (K – strategists)
Constant Loss Populations (K –
strategists)
Early Loss Populations (usually rstrategists)
These could be done regionally for
human age population studies
Classic survivorship where I is a curve representing a
Late loss population; II is a Constant loss like songbirds; and
III is an Early loss curve like those that are r-strategists
posted by Bruce W. Grant, Department of Biology,
Widener University, Chester, PA 19013
The Emergence of Life
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Taxonomically, species have been
identified and the process by which they
have adapted (changes over a long period
of time) are due to genetic variations and
Natural Selection. Charles Darwin
observed many ecological similarities and
differences verses geographical location,
which led to the Natural Selection theory.
Selection is a process, complex system of
stresses, that lead to adaptations.
Evolution, Adaptation, and Natural
Selection
http://www.mhhe.com/biosci/genbio/virtual_labs/BL_12/BL_12.html
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Heritable Changes in population’s genetic
makeup through successive generations is
Evolution
The sum all genes in a population is called
the gene pool
A gene with two or more molecular forms is
called an Allele
New alleles are referred to as mutations
A genetic trait that is that leads to survival
in environmental conditions is called an
Adaptation
http://www.pbs.org/wgbh/evolution/library/01/5/quicktime/l_0
15_03.html
Natural Selection
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This is a process in which a population has
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Variation among individuals in some attribute or
trait
Fitness differences a consistent relationship
between that trait and some measure of
reproductive success
Inheritance (consistent relationship) for that
trait between parents and their offspring (at least
partially independent of the environment)
http://www.pbs.org/wgbh/evolution/librar
y/11/2/quicktime/e_s_4.html
Courtesy of http://facstaff.uww.edu/wentzl/modes.html
Three Types of Selection
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Stabilizing tends to eliminate
individuals on the ends of a
population without shifting the
mean population
Directional tends to shift allele
frequency so that the mean genetic
outcome changes
Disruptive (Diversifying) favors
individuals at the extremes and
reduces the norm, but does not
shift the mean genetic outcome
Coevolution
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If two different species interact over
a long period of time, changes in
the gene pool of one species will
lead to changes in the gene pool of
the second species.
www.indiana.edu/~curtweb/Research
Evolution
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Microevolution – works on a species
levels with changes over time
Macroevolution goes beyond the
species level and longer geological
time periods.
There are misconceptions about this
theory… Fitness versus Fittest
Species
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Geologically speaking the start of the
Cenozoic Era (65 million years),
started with 99% of all species on
earth being extinct!
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Large scale continental movement
Gradual climate caused by shifting
continents
Rapid climate change caused by
catastrophic events (5 great ones)
Major extinctions: 65, 180, 250, 345
and 500 million years ago
Mass Extinction Epochs in Millions of Years Ago
Human Impacts
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Simplifying ecosystems
Altering species control
Eliminating predators
Introducing new species
Overharvesting resources
Interfering with geochemical cycles
Gentically modifying organisms,
thus affecting Selection pressures
Bobbi Low
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“We haven’t evolved to be
environmental altruists – but we
can solve environmental problems”
We must work with, rather than
against, our evolved tendencies.
Eliminate selfish genes
Species Interactions
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Symbiotic Relationships
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Mutualism
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Commensalism
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Parasitism
SEM of lichen: the linear fungal hyphae and the roundball-like
algal groupings.
Island Biogeography
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MacArthur and Wilson’s theory
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Larger Islands support more biodiversity
Islands closer to the mainland also
support more biodiversity
Islands farther from the mainland
support less biodiversity and sometimes
smaller-sized species
Island Biogeography
Island Biogeography
www.geog.ucsb.edu
Island Biogeography