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Population Dynamics AP Environmental Science. Population Dynamics Outline. Characteristics of a Population How do populations change in size,density and make up in response to environmental stress? Carrying Capacity What is the role of predators in controlling populations?

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Population Dynamics AP Environmental Science

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population dynamics outline
Population Dynamics Outline
  • Characteristics of a Population
  • How do populations change in size,density and make up in response to environmental stress?Carrying Capacity
  • What is the role of predators in controlling populations?
  • What different reproductive patterns enhance survival of different species?
  • What is conservation biology?
  • What are the major impacts of human activities on populations, communities , and ecosystems?
  • How can we live more sustainably?
the plight of the southern sea otter
The Plight of the Southern Sea Otter
  • Live in kelp forest in shallow waters of Pacific coast; keystone species, as they consume sea urchins, which left unchecked, destroy kelp forests.
  • Kelp forests allow for great diversity of animals and plants in these waters. competing for food; have dropped off due to Killer whales eating more otters because they have less Stellar sea lions and harbor seal to consume;
  • otters may be poisoned by toxic shellfish, less resistant to disease. Dynamics of population help us to understand ecological importance of this species.
population dynamics and carrying capacity
Population Dynamics and Carrying Capacity

Population– group of individual organisms living in he the same communities inhabiting the same area at the same time


  • Populations are dynamic and change in response to environmental stress or change in conditions.
  • Changes occur in
    • Size (number of individuals)
    • Density (number of individuals in a given space)
    • Dispersion (spatial pattern such as clumping, uniform or random
    • Age distribution (proportion of individuals of each age in a population
population density
Population Density
  • Population density
    • The number of individuals of a species per unit area or volume at a given time
    • Ex: minnows per Liter of pond water
  • Ovals below have same population, and different densities

Examples of Dispersion




(creosote bush)



population growth
Population Growth
  • Four variables that govern or limit changes in population size
    • Births
    • Deaths
    • Immigration
    • Emigration
  • Changes in population size are expressed by:Population change = (Births + Immigration) – (Deaths + Emigration)
  • Zero population growth is expressed by: Births + Immigration = Deaths + Emigration
  • Depend on change in resource availability or environmental changes.
biotic potential intrinsic rate of increase
Biotic Potential & Intrinsic Rate of Increase
  • Biotic Potential
    • A population varies in it’s capacity for growth;its maximum reproductive rate
  • Intrinsic Rate of Increase (r):
    • Rate of growth if unlimited resources
  • Individuals in populations with high biotic potential:
    • Reproduce early in life
    • Have short generation times (times between successive generations)
    • Can reproduce many times (have long reproductive life)
    • Have many offspring each time they reproduce(ex. Potentially/Housefly: 5.6 trill./13mo)
  • Can populations grow indefinitely?
    • NO! due to environmental resistance or limits
carrying capacity k
Carrying Capacity (K)
  • What is it?
    • Number of individuals that can be sustained indefinitely in a certain area(area or volume).
  • Biotic potential and environmental resistance interact to create this limit.
  • MVP (minimum viable population)
    • Below this minimum population level it is difficult to locate mates and maintain genetic diversity needed for adaptation.
    • r( intrinsic rate) is dependant on MVP

Carrying Capacity : determined by these Limiting Factors (growth vs. environmental resistance)

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


© 2004 Brooks/Cole – Thomson Learning

population growth1
Population Growth
  • Types of growth
    • Exponential (J-curve); few resource limitations
    • Logistic (S-curve or sigmoid curve); exponential growth with decline when encountering environmental resistance
  • Exceeding the carrying capacity: no smooth transition between exponential growth and log. growth
    • Overshoot; overconsumption(due to reprod. Lag time)
    • Reproductive time lag; time need for birth to fall and death rates to rise in response to overconsumption RESULT!!
    • Dieback/ crash; must switch or move for survival
  • Carrying Capacity is also dynamic, varying with factors:
    • Competition forces (interspecific and intraspecific)
    • Immigration and emigration
    • Natural and Human-caused catastrophic events
    • Seasonal fluctuations
population growth2
Population Growth

© 2004 Brooks/Cole – Thomson Learning


Population size (N)

Population size (N)

Time (t)

Time (t)

Exponential Growth

Logistic Growth

  • Types of growth
    • Exponential (J-curve) unlimited resources (r)
    • Logistic (S-curve or sigmoid curve) growth rate decreases but population gets larger then stablizes







Time Lag


Number of sheep (millions)











Exponential growth, overshoot, and population crash of reindeer introduced to a small island off the coast of Alaska in 1910



Number of reindeer









factors controlling population growth how does density affect growth
Factors controlling population growth: how does density affect growth?
  • Density-independent controls – population’s size is affected regardless of its density:
    • Flood, fire, hurricane, weather, habitat destruction, pesticides
  • Density-dependent controls – population’s density affects the impact of control on its size
    • Competition, predation, parasitism, disease(ex. Bubonic plague)
kinds of population change curves fluctuations
Kinds of Population Change Curves: Fluctuations
  • Stable : a species whose population size fluctuates slightly above and below its carrying capacity. (undisturbed tropical rain forests)
  • Irruptive: fairly stable then explodes and crashes to more stable lower level(fav. weather, more food, fewer predators)
  • Irregular or chaotic (no reoccurring pattern): due to chaos in species system. Not completely understood
  • Cyclic fluctuations: over reg period of time ex. Lemming, grouse, lynx. Rise/fall every 10yrs

Different Types of Population Curves

© 2004 Brooks/Cole – Thomson Learning



Number of individuals




do predators control population size top down vs bottom up population controls







Population size (thousands)

















Do Predators control population size?Top Down vs. Bottom Up population controls
  • Bottom-up control hypothesis:
  • -No lynx population present
    • Herbivore vs. producer
    • Short food supply
    • .
  • Top-down control hypothesis:
    • Lynx (predator) control hare (prey)by reduction of lynx in pop.
    • Cyclic pattern
how do species reproduce sexual vs asexual reproduction
How do Species Reproduce: Sexual vs. Asexual reproduction

Reproductive patterns

  • Asexual:
    • Exact genetic copies of a single parent. Ex single cell bacteria
  • Sexual:
    • Organisms reproduce by combining gametes from two parents. 97% of organisms reproduce this way.
    • Pro:
      • Genetic diversity
      • Division of labor. Males gather food and protect and train
    • Con:
      • Males do not produce offspring
      • Chances for genetic error are high
types of reproductive patterns r selected and k selected patterns
Types of Reproductive Patterns: R-selected and K selected patterns
  • r-selected: reproduce early and often, algae, bacteria, rodents, annual plants, most insects…many offspring each pregnancy, reach reproductive age rapidly, short generation times, no care from parent, short (intrinsic rate of growth)lived…OPPORTUNISTS…strive in disturbed conditions when new habitats or niche is open. Changing environmental conditions.
  • Most go through irregular or boom and bust cycles
  • Must invade new areas to compensate for lose.
k selected
  • The other extreme from r-selected: little energy into reproduction, fewer, larger offspring, high parental involvement, later reproductive age, young develop within mother, most energy into child rearing, rather than reproduction…COMPETITIVE …thrive best in stable conditions when population reaches near carrying capacity. (K)
  • K or r: must have space in habitat.
  • Elephants, whales, humans, oak trees, tropical rain forest trees
  • The availability of suitable habitat for individuals of a population in an area is what determines it’s size
r vs k
“r” vs. “K”

Reproductive Patterns


Carrying capacity

K species;


K selection


Number of individuals






survivorship curves
Survivorship curves
  • Different species have different reproductive strategies and therefore different life expectancies. Represent with a survivorship curve.(# of survivors of each age group for a particular species)

1.Late loss curve: typical for K-species, longer life expectancy, limited births

2.Early loss curve: r-selected, many offspring, high juvenile mortality, high survival rate once young have reached a certain age

3.Constant loss curve: intermediate between r- and K- species… songbirds, lizards, small mammals.

survivorship curves life expectancies



Percentage surviving (log scale)




Survivorship Curves: Life Expectancies
  • Type I: Late loss: K select
  • Type II: Constant loss: intermediate reproductive pattern; constant rate of death in all age classes(ex. Songbirds)
  • Type III: Early loss: r select; many young, high juvenile death rate
Conservation biology: take action to preserve species and ecosystems.Conservation Biologists seek answers to these questions
  • What species are in danger of extinction?
  • What is the status and value of ecosystems and organisms?
  • What can we do to sustain ecosystems and populations?
conservation biology principles
Conservation biology:Principles
  • Biodiversity is necessary & should not be reduced by human actions.
  • Humans should not cause extinctions or disrupt vital ecological processes.
  • The best way to preserve biodiversity is to protect intact ecosystems.

Do you agree?

“Something is right when it tends to support earth’s life support systems for us and other species, and wrong when it does not”~ Aldo Leopold

9 5 human impact on ecosystems
9-5 Human Impact on ecosystems

We have used technology to alter nature by

  • 1. fragmenting and degrading habitat
  • 2. simplifying natural ecosystems.
  • 3.using, wasting, or destroying an increasing percentage of earth’s primary productivity.
  • 4.strengthening some populations of pest species and disease causing bacteria.
  • 5. Eliminating some predators
  • 6. introducing new native or non native species
  • 7. overharvesting renewable resources
  • 8.simplified some ecosystems, consumed resources, reduced numbers of predators, introduced exotics, interfered with natural cycles
  • 9.Challenge is to maintain the balance and slow the rates of change in nature
  • 10. interfering with normal chemical cycling and energy flows in an ecosystem
principles of sustainability
Principles of Sustainability
  • We are totally dependent on the earth and the sun. Earth does not need us.
  • Human actions do not occur in a vacuum. There are always side effects.
  • We should reduce and minimize the damage we do to nature and help heal some of the ecological wounds we have inflicted.
  • We should use care, restraint, humility, and cooperation with nature as we alter the biosphere to meet our needs and wants.