Populations

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Populations
What is a population? a group of organisms of
the same species occupying a defined area during
a specific time. How do we study populations?
Estimating abundance, density, and demographic
characteristics
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Definitions

• Abundance - number of animals
• Density- number of animals per unit of area
  (/acre, /mi2)

1 mi
1 mi
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Definitions
Age structure-- distribution of numbers of
individuals of various ages

Age Class (yrs) No. in Population 0-1 10 1-2
18 2-3 26 3-4 25 4-5 19 5-6 1
1
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Definitions
Population regulation-- the tendency of a
population to maintain densities within a limited
range.
No. of animals
time
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Population change

Population
Births - Deaths
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Population change
Closed System r (b - d) where, r
growth rate of population b birth rate or
natality d death rate or mortality
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Population change

Immigration
Population
Births - Deaths
Emigration
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Population change
Closed System r (b - d) where, r
growth rate of population b birth rate d
death rate Open System r (b - d) (i -
e) where, i immigration rate e
emigration rate
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Population change

Immigration
Population
Births - Deaths
Emigration
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Seven characteristics of birth rates (natality)

• 1. Age at first reproduction
• 2. Length of gestation period
• 3. Sex ratios
• 4. Breeding system (monogamous or polygamous)
• 5. of females that breed at each age
• 6. of young per female of various ages
• 7. Influence of density on reproduction

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Seven characteristics of birth rates

• 1. Age at first reproduction
• 2. Length of gestation period
• 3. Sex ratios (M F)
• 4. Breeding system (monogamous or polygamous)
• 5. of females that breed at each age
• 6. of young per female of various ages
• 7. Influence of density on reproduction

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Sex Ratios and Mating Systems

• 1. Monogamy
• Seasonal (same mate only for 1 year)
• Lifetime (same mate for ever)

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Sex Ratios and Mating Systems

• 2. Polygamy
• polyandry (one female mates
• with several males)
• polygyny (one male mates with several females)

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Seven characteristics of birth rates

• 1. Age at first reproduction
• 2. Length of gestation period
• 3. Sex ratios
• 4. Breeding system (monogamous or polygamous)
• 5. of females that breed at each age
• 6. of young per female of various ages
• 7. Influence of density on reproduction

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Age-specific Birth Rates
A fecundity schedule for chamois from New
Zealand. _________________________________________
___
Age (yrs) N No. Female births
per pregnant female ___________________________
__________________ 0 - - 0.00 1 60
2 0.02 2 36 14 0.20 3 70 52 0.37 4 48 45 0.
47 5 26 19 0.37 6 19 16 0.42 7 6
5 0.42 gt7 10 7 0.35 _________________________
__________________
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Seven characteristics of birth rates

• 1. Age at first reproduction
• 2. Length of gestation period
• 3. Sex ratios
• 4. Breeding system (monogamous or polygamous)
• 5. of females that breed at each age
• 6. of young per female of various ages
• 7. Influence of density on reproduction

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Density-dependent effect
Refers to an inverse relationship between
reproduction and density Ex As population
density increases, birth rate decreases
Birth rate
Population density
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Density-dependence
Reindeer
No. young produced
Sheep
Population size/density
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Population change

Immigration
Population
Births - Deaths
Emigration
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Survivorship curves show pattern of mortality for
different types of animals
Surviving
Time
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Actual survivorship curves for male female
moose on Isle Royale
females
Survivors
males
2 4 6 8 10 12 14 16 18 20
Age at Death (years)
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Age Structure

• Every population is composed of animals of
  various ages.
• We can learn much from looking at age structure
  health of population, future growth

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Age Structure

• Every population is composed of animals of
  various ages
• Age pyramids summarize structure

Growing
Stable
Declining
Old
Young
Number of individuals
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Population change

Immigration
Population
Births - Deaths
Emigration
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Immigration and Emigration

• Both usually due to dispersal movement of young
  away from their birth place
• Birds More often, female is disperser
• Mammals More often, male is disperser

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Immigration and Emigration

• Dispersal can be important in colonization of new
  habitat

B
Population A
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Immigration and Emigration

• Immigrants can rescue local populations that
  would otherwise decline to extinction

Population
Deaths gtgt Births
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Patterns of population growth

Immigration
Population
Births - Deaths
Emigration
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Population growth
Exponential growth
of animals
time
Growth curve with unlimited resources
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Exponential growth in Crows?
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Population growth
Examples of exponential growth Ring-necked
Pheasants on Protection Isl. Washington 1937 2
males, 6 females introduced 1941 1500 pheasants
present!! (188x increase) 2000 40-50 pheasants
still there (change in land use from farm to
fallow fields seabird colonies present too)
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Population growth

• Examples of exponential growth
• Australian rabbit (European hare)
• 1859 24 hares introduced (for human food?)
• 1865 over 20,000 hares were harvested, actual
  population much greater.
• Mid-1800s to mid-1900s major problem with too
  many hares caused habitat destruction and
  reduction in native mammals
• 2000 still present, local problems

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Population Growth Carrying capacity (K)
K
N
N
time
time
Growth curve with limited resources
Growth curve with unlimited resources
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Carrying Capacity (K)
The maximum population size that can be supported
indefinitely in a given environment
of animals
K
time
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Population Regulation
Population growth affected by 1. Density
dependent factors (intensity driven
by population density) food supply presence
of competitors territoriality contagious
disease 2. Density-independent factors (intensity
not driven by population density) weather la
rge disturbances (fire, floods)
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Population Regulation weather and disturbances
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Population Regulation experiment with added food
High food addition
Low food addition
Townsends vole
No food added
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Population Regulation Competition
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(No Transcript)
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Will arrival of West Nile Virus limit population
growth in crows?
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Aldo Leopold, 1933 Game Management
Regulating factors
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Population Cycles
Some species exhibit variable cycles in
numbers Difficult to determine specific factors
that regulate populations - Lynx and hare famous
example
Snowshoe hare
Lynx
Cycling populations
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Some species that often show cycling populations
Some lagomorphs (rabbits hares)
Microtine rodents (voles, lemmings)
Some grouse
Lynx
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Take Home Message

• Most wildlife populations do not grow
  exponentially (unlimited growth) for very long
• Wildlife populations are regulated by various
  factors
• Can be density-dependent or density-independent
  regulation

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Populations

• Population concepts

What is a metapopulation? -a geographical pattern
of semi-isolated subpopulations, some of which
disappear and others appear through time.
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Metapopulation
Local populations linked by dispersal
Range of Total Population
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Metapopulation
Over time, some populations may go locally extinct
Range of Total Population
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Metapopulation
But area may be re-colonized again
Range of Total Population