Population dynamics
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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?

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Sea otter the other other white meat
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
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 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
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
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 dynamics


Growth factors

(biotic potential)

Decrease factors

(environmental resistance)



Too much or too little light

Temperature too high or too low

Unfavorable chemical environment

(too much or too little of critical


Favorable light

Favorable temperature

Favorable chemical environment

(optimal level of critical nutrients)



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


Ability to adapt to environmental


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


Inability to adapt to environmental


Fig. 9.3, p. 200

Carrying capacity
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

Population dynamics



Number of sheep (millions)









Fig. 9.5, p. 201


Logistic growth
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 dynamics


Population size (N)

Population size (N)

Time (t)

Time (t)

Exponential Growth

Logistic Growth

Fig. 9.4, p. 201

Can you overshoot your carrying capacity
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
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

Population dynamics



Number of individuals




Fig. 9.7, p. 202

Top down or bottom up
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
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
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

Population dynamics

r-Selected Species



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


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-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

Population dynamics

K-Selected Species



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


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
Survivorship curve

  • Late loss - typical for k-strategists

  • Early loss – typical for r-strategists

  • Constant loss – for species in the gray area in-between k and r strategists with intermediate reproductive patterns

    • Song birds, lizards, and small mammals

Population dynamics



Percentage surviving (log scale)




Fig. 9.11, p. 206

Conservation biology
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
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
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
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
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
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
That’s all folks

Have a nice day