Population Ecology

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Population Ecology

• Population ecology is the study of populations in
  relation to environment
• Including environmental influences on population
  density and distribution, age structure, and
  variations in population size

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• Levels of Organization
• Organism
• Population
• Community
• Ecosystem
• Biome

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• Describing populations

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Describing populations

• Size total of individuals in a pop (N)
• Density / unit area

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Estimation of Population Size
Describing populations

• Sampling technique
• Mark and recapture

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Mark and Recapture
Describing populations

• Orgs are captured, tagged, and then released.
• Some time later, the same process is repeated and
  the following is used

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Describing populations

• Ex
• Suppose that 50 zebra mussels are captured,
  marked, and released. One week later, 100 zebra
  mussels are captured and 10 are found to have
  markings already. The estimated population would
  then be
• N (50100) / (10) 5000/10 500
• N500

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Dispersion
Describing populations

• Describes how individuals in a population are
  distributed
• Clumped
• Uniform
• Random

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• Clumped dispersion (most common)
• Individuals aggregate in patches
• May be influenced by resource availability
  behavior

Describing populations
Fish travel in schools safety in numbers
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• Uniform dispersion
• -Individuals are evenly distributed
• -May be influenced by social interactions such as
  territoriality

Describing populations
Some plants secrete toxins that keep others away
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• Random dispersion
• The position of each individual is independent of
  other individuals

Describing populations
Dandelions grow from windblown seeds that land at
random and later germinate.
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Describing populations
Changes in Population Size

• Biotic potential of a population
• max rate at which a population could increase
  under ideal conditions
• Influenced by several factors
• Age at which reproduction begins
• Life span during which the organisms are capable
  of reproducing
• Number of reproductive periods in the lifetime
• Number of offspring the organism is capable of
  having

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Age Structure
Describing populations

• Description of the abundance of individuals of
  each age in a population.

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Age Structure Diagrams
Describing populations
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2004
China
India
United States
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Zero population growth
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Describing populations
1954
Animation
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Describing populations
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Survivorship Curves
Describing populations

• Describes how the mortality of individuals in a
  species varies during their lifetime

• The survivorship curve for Beldings ground
  squirrels
• Shows that the death rate is relatively constant

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• Survivorship curves can be classified into three
  general types Type I, Type II, and Type III

• Type I
• A species in which most survive to middle age
• Parenting
• Type II
• Length of survivorship is random. Death rate is
  constant.

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• Type III
• Most individuals die young.
• Few reach reproductive maturity.
• Fish release 1000s of eggs.
• No parenting

Animation
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• Factors Affecting Population

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Limiting Factors
Factors Affecting Population

• Things that prevent a population from attaining
  its biotic potential
• 2 categories
• Density-dependent
• Density-independent

Density Dependent
Density Independent
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Density Dependent
Factors Affecting Population

• Populations regulated by density-dependent
  factors are affected by the number of organisms
  present.
• Predation
• Parasitism
• Disease
• Competition
• -the more organisms crowd together, the more
  damaging are food shortages, parasites, and
  predators.

Intraspecific Competition the struggle between
members of a population for scarce resources
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Density Independent
Factors Affecting Population

• The number of organisms present does not affect
  the influence of the factor.
• Fire, earthquakes, storms, floods
• Density-independent factors show no correlation
  with the size of the population.

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Describing Population Growth
Population ecologists describe two general
patterns of populations growth

1. Exponential
2. Logistic

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• The growth of a population can be described by
  the following equation
• r reproductive rate (or growth rate)
• N population size at the beginning of the
  interval for which the births and deaths are
  counted.
• r
• .

Types of Population Growth
Net increase of individuals
births-deaths N
Can r be negative? Zero?
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r
Types of Population Growth
births-deaths N

• Ex
• A population of 1000 had 60 births and 10 deaths
  over a one year period. What is the growth rate?
• 0.05 per year

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Exponential growth
rgt0
Types of Population Growth
In a population showing exponential growth the
individuals are not limited by food, disease,
predation or competition.
If the rate of reproduction per individual
remains constant through
time, then the
rate at which
the population
increases is a
multiple of the
number of
individuals in
the population.
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• Exponential population growth
• Results in a J-shaped curve
• Usually short lived in nature- not sustainable

rgt0
Types of Population Growth
Population size (N)
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Carrying Capacity (K)
the real world
Types of Population Growth

• Populations usually reach a carrying capacity.
• Upper limit to the number of individuals the
  environment can support.

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Logistic growth
Types of Population Growth
In most real populations both food and disease
become important as conditions become crowded.

• A population can grow exponentially only for
  short periods of time, because it would very
  quickly deplete all the resources necessary for
  its survival.
• Over long periods of time, populations tend to
  attain an equilibrium population size which is
  determined by the available resources.

Animation
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Types of Population Growth

• Zero population growth
• Occurs when the birth rate equals the death rate
• The population growth equation can be expressed as

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Life-history strategies

• Logistic and exponential growth are associated
  with 2 kinds of life-history strategies
• R-selected species (strategists)
• organisms are opportunistic they reproduce
  rapidly when the environment is uncrowned and
  resources are vast
• K-selected species (strategists)
• operate at a density near K

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R-selected species
Life-history strategies

• a. Opportunistic species, which tend to be
  colonizers.
• b. Grasses, many insects
• -Usually quickly invade, reproduce many
  offspring and die
• -Offspring mature quickly and require very
  little parental care
• c. Strategies for continued existence is based
  on individuals having the following traits
                  1) small size                
  2) short life span                 3) mature
  fast                 4) produce many offspring
                  5) engage in little care of
  offspring

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K-selected species
Life-history strategies

• sensitive to population density

Such populations are equilibrium species, tend to
be specialists rather than colonizers, and may
become extinct when their evolved way of life is
disrupted (e.g., the grizzly bear, Florida
panther, etc.). Overall strategy for continued
existence is based on having the following
traits                 1) large size
                2) long life span
                3) slow to mature
                4) produce few offspring
                5) expend considerable energy in
care
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Human Population Growth

• The following made exponential growth possible
• Increases food supply
• Reduction in disease
• Reduction in human waste
• Expansion of habitat

As of 28 April 2010, the human population of the
world is estimated by the United States Census
Bureau to be 6,817,500,000.
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Countries Ranked by Population
2010 Source U.S. Census Bureau,
International Data Base.
6,817,500,000
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Map of countries by population for the year 2007
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