Population Dynamics
Chapter 8
Sea Otter – the other, other white
meat
Why are sea otters considered keystone
species?
They control urchin populations which feed on
kelp, hence they keep the kelp forests healthy
Why did their populations decline?
Originally due to hunting, now chemical pollution
is suspected
Characteristics of a population
Size – number of organisms
Density – number /space
Dispersion – spatial distribution
Age distribution – pre-breeding, breeding, or
post breeding age
Population dynamics – how these factors
change due to environmental stresses
Population growth
Population change (growth) = (births + immigration) –
(deaths + emigration)
ZPG – zero population growth is when incoming equals
outgoing
Biotic potential – max growth for that particular
population
Intrinsic rate of increase – rate of growth with unlimited
resources
High intrinsic growth rates
Reproduce early in life
Have short time between generations
Reproduce many times
Have many offspring each time
Roaches, mice, fish, flies
Environmental Resistance
These are the vast assortment of environmental
factors which help keep populations from
growing out of control
This is a way a population finds an equilibrium
point
POPULATION SIZE
Growth factors
(biotic potential)
Abiotic
Favorable light
Favorable temperature
Favorable chemical environment
(optimal level of critical nutrients)
Biotic
High reproductive rate
Generalized niche
Adequate food supply
Suitable habitat
Ability to compete for resources
Ability to hide from or defend
against predators
Ability to resist diseases and parasites
Ability to migrate and live in other
habitats
Ability to adapt to environmental
change
Decrease factors
(environmental resistance)
Abiotic
Too much or too little light
Temperature too high or too low
Unfavorable chemical environment
(too much or too little of critical
nutrients)
Biotic
Low reproductive rate
Specialized niche
Inadequate food supply
Unsuitable or destroyed habitat
Too many competitors
Insufficient ability to hide from or defend
against predators
Inability to resist diseases and parasites
Inability to migrate and live in other
habitats
Inability to adapt to environmental
change
Fig. 9.3, p. 200
Carrying capacity
Biotic potential and environmental resistance will
determine the population a given area can hold
and sustain indefinitely
A population must not drop below the minimum
viable population or lowest number needed to
keep population from disappearing due to
environmental resistance
Number of sheep (millions)
2.0
1.5
1.0
.5
1800
1825
1850
1875
Year
1900
1925
Fig. 9.5, p. 201
Logistic growth
Exponential growth (J curve) is not possible
forever because resources and space eventually
run out. When a population reaches a certain
point, environmental resistance increases
causing the population size to stabilize. This is
known as logistic growth (s curve) and this
generally happens to all populations
Population size (N)
Population size (N)
K
Time (t)
Exponential Growth
Time (t)
Logistic Growth
Fig. 9.4, p. 201
Can you overshoot your carrying
capacity?
Absolutely, it happens all the time
When you have too many individuals for the area
to support you will have a population crash
If the overshoot was not too drastic, and the
crash was small the population re-stabilizes
Types of population curves
Stable – nearly flat line
Irregular – widely fluctuating pattern with no
periodicity
Cyclic – regular growth and crash at set
intervals, usually seasonal
Irruptive – normally stable, but with a random
spike or crash
Irregular
Number of individuals
Stable
Cyclic
Irruptive
Time
Fig. 9.7, p. 202
Top-down or bottom-up?
Evidence seem to show both happening
Top-down – predators hunt and kill prey keeping
their population stable
Bottom-up – prey are the food source that allow
predators to keep the populations up
Types of reproduction
Asexual – cloning, single parent donates both
parts of DNA (bacteria)
Sexual – two parents donate DNA
Females have to give birth more (males do not as in
asexual)
More genetic errors from combining
Mating is more damaging, and energy intensive
Does provide more genetic diversity, hence a
stronger species
R-selected species
Also known as r-strategists and fill generalist niche
Have many offspring
Reach reproductive age early
Short time between generations
Little to no parental care and adapted to unstable
climate (low survivorship)
Short life span (usually under a year)
Algae, rodents, bacteria, annual plants and insects
r-Selected Species
cockroach
dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching
reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
Fig. 9.10a, p. 205
K-selected species
K-strategists or competitors, specialist niche
Fewer, larger offspring (usually develop inside)
Mature slowly (often protected while vulnerable)
Lower population growth rate
Long lived with stable population near carrying capacity
Depend heavily upon suitable habitat
Large mammals, birds of prey, long lived plants such as
oaks, redwoods, some cacti
K-Selected Species
elephant
saguaro
Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close
to carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
Fig. 9.10b, p. 205
Survivorship curve
Late loss - typical for k-strategists
Early loss – typical for r-strategists
Constant loss – for species in the gray area inbetween k and r strategists with intermediate
reproductive patterns
Song birds, lizards, and small mammals
Percentage surviving (log scale)
100
10
1
0
Age
Fig. 9.11, p. 206
Conservation biology
Sensible use of natural resources
Originated in 1970’s – uses current science
Investigate human impact on the biodiversity
Develop practical approaches to maintain
biodiversity
Maintain – endangered species, wildlife
reserves, ecological restoration, ecological
economics, environmental ethics
Assumptions of conservation bio
Biodiversity is necessary
Humans should not affect extinction or vital
environmental processes
Protecting ecosystems is the best way to protect
Based on Aldo Leopold’s ethical principle, that if
we maintain the earth’s life-support system it is
appropriate
Human impact on ecosystems
Fragmentation – breaking up large tracts with
roads, fences, towns, etc.
Habitat loss/degradation – pollution, lumber,
mining, etc.
Simplifying ecosystems – lower biodiversity
through habitat change (monocultures)
Strengthening species – pesticide use,
antibiotics
Human impact continued
Predator elimination – wolves, coyotes, bear,
etc.
Introduce alien species
Overharvest potentially renewable resources –
trees, soil, other biomass (grasses, nuts, etc)
Interfere with natural chemical cycling – clear
cutting, monocultures, pesticides (we kill and
simplify a system)
Way to go humans!! You’re the best!
Goals for the future (if we want to be a part of it)
Maintain balance between human impacted
simple ecosystems and natural rich ecosystems
Slow down rates at which we alter nature for our
own purpose
Realize that we never do merely one thing,
everything is interdependent and unpredictable
How can you help
Use consumer power – buy products that are friendly to
the environment
Use voting power – elect officials that will strive to
protect the environment
Educate – most people have no idea about the
consequences of their actions
Identify “mother culture” that says spend, buy, consume
and learn to tune it out
Exploit nature for its aesthetics and renewable
resources
That’s all folks
Have a nice day