What is Ecology

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What is Ecology?

• Definition The scientific study of the
  interactions between organisms and their
  environment

Biotic vs. Abiotic factors
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Populations

• A population is a group of individuals of the
  same species inhabiting the same area at the same
  time.
• Important characteristics
• Population size, density, and dispersion
• Birth and death rates
• Growth rates
• Age structure
• Genetic Diversity

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Communities

• Communities an assemblage of populations of many
  species living together in the same location at
  the same time.
• Community structure and functioning
• Community Biodiversity
• Number and types of species
• Relative abundance of species
• Interactions among species
• Community Development
• Community resilience to disturbance
• Nutrient and energy flow

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Examples of Communities

• Chesapeake Bay shallow water community
• Piedmont forest
• Salt marsh
• Alpine community
• Dune community

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Ecosystems

• Ecosystems are composed of all the communities
  and their physical, chemical, and biological
  processes.
• Ecosystems sustain themselves entirely through
  energy flow through food chains, and nutrient
  recycling. Examples
• Watersheds
• The Chesapeake Bay

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Population Ecology

• A population is a group of plants, animals, or
  other organisms, all of the same species, that
  live together and reproduce.
• Numbers of individuals in a population
• Population dynamics how and why those numbers
  increase or decrease over time
• Population ecologists try to determine the
  processes common to all populations

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Population Ecology in Action

• Biologists in applied disciplines such as
• Forestry
• Agronomy (crop science)
• Wildlife management
• Must manage populations of economic importance
• Prevent threatened or endangered species from
  extinction

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Characteristics of Populations

• Population size, density, and dispersion
• Birth and death rates
• Growth rates
• Age structure
• Genetic Diversity (covered in Unit 1
  Microevolution)

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Density and Dispersion

• The number of individuals within a population is
  meaningless without regard to space.
• Population Density (Size) the number of
  individuals of a species per unit of area or
  volume at a given time
• Different environments can support different
  population densities
• Density can vary seasonally within a habitat
• Population Dispersion The spacing of individuals
  relative to each other

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Quantifying population density

• By simple visual count
• By sampling methods to extrapolate captured
  organism number to size of population
• Mark-recapture method

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Hectors Dolphins

• Biologists at the University of Otago mark and
  recapture to a group of endangered dolphins near
  Banks Peninsula in New Zealand.
• The biologists identified 180 dolphins by
  photographing their distinctive dorsal fins from
  boats.
• After waiting a few weeks for the marked
  dolphins to mix back into the population, they
  set out to capture a second set of dolphins. The
  team of biologists captured 44 dolphins in their
  second sampling, 7 of which had been photographed
  before.
• What is the population size of endangered
  dolphins near Banks peninsula?

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Population Dispersion (Spacing)

• Random dispersion
• unpredictably spaced
• Clumped dispersion
• clustered in specific parts of the habitat
• Uniform dispersion
• evenly spaced

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Clumped Dispersion

• Also referred to as patchiness
• Often results from the
• Patchy distribution of resources
• Family groups or pairs among animals
• Limited seed dispersal or asexual reproduction
  among plants
• Advantages
• protection against predation
• Social animals need to be in close proximity to
  each other

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Uniform Distribution

• Occurs when individuals are more evenly spaced
  than would be expected from a random distribution
• Occurs as a result of
• Territoriality
• Competition
• Allelopathy among plants
• Antibiotic production in bacteria
• Streptomyces are the white, chalky colonies

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Allelopathy

• Chaparral plant communities lack a defined ground
  cover layer. This is caused by chemical
  inhibitors called allelopathic agents. These
  volatile or water-soluble chemicals are exuded by
  the shrubs and carried by the heat of the day or
  by water to the soil. The allelopathic agents may
  also leach out of the leaves or leaf litter to
  accumulate in the soil beneath. These compounds
  effectively stunt the growth of plants and reduce
  or eliminate seed germination.
• Allelopathy is a plant defensive mechanism. It
  ensures the limited moisture and nutrients
  available in the soil are only capable of being
  used by the plant producing the allelopathic
  chemicals.

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Factors That Affect Population Size

• Mathematical models help to understand general
  processes shared by all populations
• The models are used as starting hypotheses which
  are ultimately tested in the field.
• Population size can be estimated from the growth
  rate

dN/dt rN
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Population Size and Population Growth

• The primary factors that affect population size
  are
• Birth and death rates (natality vs. mortality)
• Immigration and emmigration
• Birth and death rates are themselves influenced
  by many factors such as
• Reproductive strategy of the species
• Competition for resources (at the population and
  community levels)
• Predation
• Stochastic events (hurricanes and floods)
• Survivorship/ Age Structure

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Intrinsic Growth Rate

• Imagine an ideal population, where NONE of the
  factors mentioned previously exist.
• Population growth rate the change in population
  size N/ over time t can be modeled with a
  continuous differential equation (dN/dt )
• Let B the birth rate ( births/unit time)
• Birth rates are dependent on population size (N)
• A population of 1000 robins will produce more
  eggs than a population of 25 robins
• But, if each individual produces the same number
  of offspring, the birth rate is the same
• Calculate the per capita birth rate
• Births per individual/ unit time b
• B bN

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• Let D the death rate/unit time
• Death rates are dependent on population size (N)
• The per capita death rate Deaths per
  individual/ unit time d
• D dN
• The change in population size over time is
  expressed as
• dN/dt (b-d) N
• Ecologists often use r to represent the per
  capita growth rate
• let (b-d) equal the rate of increase (r)
• Then dN/dt rN

dN/dt rN
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Intrinsic Growth Rate Exponential Population
Growth

• J-shaped curve
• an accelerated pattern of growth in a population
  for limited periods of time
• Eventually the growth rate decreases to around
  zero or becomes negative

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Growth Rate (r)
dN/dt rN

• The value of r determines whether a population
• Increases exponentially (r gt 0)
• when the average per capita birth rate (natality)
  is greater than average per capita death rate
  (mortality)
• Remains constant in size (r0)
• when natality equals mortality
• Declines to extinction (r lt 0)
• when mortality is greater than natality

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The assumption of the model for intrinsic growth
rate

• No immigration or emmigration
• Constant birth and death rates
• No genetic structure
• No age or size structure
• Continuous growth with no time lags

dN/dt rN
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Logistic Population Growth

• In real populations there are limiting resources,
  
• Availability of food, water, shelter, sunlight
• Which leads to competition
• Also limitation imposed by disease and predators
• Therefore, most populations exhibit logistic
  population growth

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Logistic Population Growth

• S-shaped curve
• As the environment puts limits on population
  growth, (natural selection), d gt b and r
  approaches zero
• The carrying capacity of the environment is
  reached when r zero

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Logistic Population Growth

• Carrying capacity (K)
• largest population a particular environment can
  maintain for an indefinite time
• The S-shaped curve has three parts
• Lag phase
• Exponential growth (Log phase)
• Until it reaches K stationary phase

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Logistic Population Growth

• The logistic population growth equation takes
  into account the carrying capacity (K)
• Note the new part added to the original equation
  K-N/K
• K-N/K Reflects a decline in growth as the
  population approaches carrying capacity
• K-N/K Reflects the proportion of unused
  resources remaining.
• When N is small K-N/K approaches 1
• When N is large K-N/K approaches zero

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Carrying Capacity

• The larger the population size N, the closer it
  comes to its carrying capacity.
• If K1000, N900, and r 0.1
• At medium values of N500
• At low values of N 100

dN/dt ( 0.1)(900) (1-900)/1000 9
dN/dt ( 0.1)(500) (1-500)/1000 25
dN/dt ( 0.1)(100) (1-100)/1000 9
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Carrying Capacity

• A wildlife biologist is responsible for
  maintaining the crab population in the Chesapeake
  Bay.
• Experimentally determine r
• Experimentally Estimate K for the Bay
• Could also be determined from catch history
• Determine current populations size
• When is population growth rate at its greatest?
• Growth is greatest at medium values of N.
• r max occurs when N K/2

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Carrying capacity in the field

• K is determined in the lab and the field
• Amount of resources (food) required by the
  population
• Amount of resources available
• N is determined by field census of population
• r would be calculated from the birth and death
  rates

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The Logistic Growth ModelDensity Dependent Growth

• Therefore, we can say that dN/dt is dependent on
  K (carrying capacity) and N
• Density Dependent Growth
• Natural populations seldom follow logistic growth
  curve very closely
• Populations rarely stabilize at carrying capacity
• Instead, N fluctuates, rising higher than K, and
  then falling, sometimes crashing

dN/dt rN K-N/K
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Density-Dependent Factors

• Regulate population growth
• by affecting a larger proportion of population as
  population density rises
• Examples
• Predation
• Disease
• Competition
• All are biotic factors

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Density Dependent Factors

• Predation
• As the density of a population increases,
  predators are more likely to find individuals of
  a given prey species
• Disease
• As the density of a population increases,
  individuals encounter one another more frequently
  increasing the probability of spreading parasites
  and disease

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• Density dependent
• Mortality is affected by population size
• Density independent
• Population size is not a factor in the mortality
  rate
• Inverse density dependent
• Mortality decreases with increasing population
  size
• Example A lion pride always consumes the same
  amount of prey regardless of herd size

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Density-Independent Factors

• Limit population growth
• but are not influenced by changes in population
  density (N)
• Density Independent factors tend to be abiotic
  factors
• Examples
• weather
• hurricanes and blizzards
• Example Mosquito populations in the Arctic

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Mosquito populations in the Arctic

• Mosquitoes produce several generations in the
  summer
• Achieve a high population density by the end of
  the season
• Not affected by a shortage of food nor breeding
  habitat (plenty of ponds in which to breed)
• Winter brings an end to the mosquitoes.

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• Although short-tailed shrew populations display
  density dependence
• Density independent factors also play a role
• The population increases as precipitation
  increases
• Soil moisture increases
• Drinking water availability
• Greater numbers of invertebrate prey

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Life history strategies

• Continuum of life history strategies used by
  species
• r-selected species high rate of per capita
  population growth, r, but poor competitive
  ability (weeds)
• K-selected species more or less stable
  populations adapted to exist at or near carrying
  capacity, K
• Lower reproductive rate but better competitors
  (trees)

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