Lecture –
Populations
Properties
Estimation of Size
Population Growth
What is a population?
• ‘members of a particular species that
inhabit a particular area’
• Various aspects:
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Range and distribution
Size
Density
Age structure
Growth
Genetic uniqueness  subpopulations
(ecotype)
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Size of Populations
Abundance: number of individuals within a
specified area
Abundance/area = Density
How do we determine how many
individuals there are?
Two primary techniques:
– Capture-mark-recapture
• estimate of total population = (total number
captured (second time) x number marked) / (total
number recaptured with mark)
– sampling
• Estimation of population sizes
• Choice of technique depends on
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motility of target species
Nature of habitat
Resources
Resolution required
Generally rely on statistical sampling /various
assumptions
http://www.pwrc.usgs.gov/monmanual/approaches/popsize.htm
Population growth rate:
• Discrete-time
– Geometric growth
• Species which have discrete breeding seasons
• Continuous time
– Exponential growth
• Geometric Growth:
• N(t+1) = N(t) λ : at each interval of time population grows
by the multiple λ
• Exponential Population growth
• logeλ = r
Change
Intrinsic rate
over time
of increase
• Growth rate = dN/dt = rN
No. of individuals
in a population
• The actual rate of population increase is
Birthrate
Deathrate
Net immigration
• r = (b – d) + (i – e)
Net emigration
• Geometric Growth – with
discrete reproductive
seasons
• Estimate population at
same time in each year
– Mortality of young
Log population size
increasing exponential
against time produces
straight line
Slope (at any point) =
dN/dt = rN
Logistic Population Growth
Carrying Capacity
• No matter how fast populations grow, they
eventually reach a limit
– This is imposed by shortages of important
environmental factors
• Nutrients, water, space, light
• The carrying capacity is the maximum number of
individuals that an area can support
– It is symbolized by k
• As resources are depleted, population
growth rate slows and eventually stops:
logistic population growth.
– Sigmoid (S-shaped) population growth
curve.
• dN/dt = rN ( K – N
K
)
= rN(1-N/K)
Growth slows as N
approaches value of
K or as (1-N/K)
approaches 0
Limits to Population Growth
• Environment limits population growth by
altering birth and death rates.
– Density-dependent factors
• Disease, Resource competition
– Density-independent factors
• Natural disasters
Density-dependent effects
• Competition for resources
– food
– Suitable habitat – example: nesting sites
– Effects that are dependent on population size and act to
regulate growth
Reproductive
• These effects
have an
increasing
effect as
population
size
increases
success decreases
as population size
increases
Song
sparrow
• Density-independent effects
– Effects that are independent of population size
but still regulate growth
• Most are aspects of the external environment
–Weather
»Droughts, storms, floods
–Physical disruptions
»Fire, road construction
Where is a species found?
• Range: Geographical boundaries a
species occupies
– Determined by basic ecological parameters
– No indication of distribution or abundance
• Fundamental niche:
– Indication of parts of habitat in which a
species may be found
– Typically patchy locally aggregated) w/i range
• Realized niche:
– Portion of fundamental niche in which species
is actually found
Factors which impact range:
• Physiological adaptations
• Available food, nesting sites, etc. – factors which
define suitable habitat
• Predators
• Competition – competitive exclusion principle –
to be discussed later
• Chance – past climatic and physiological events
– Species could/does survive elsewhere, has not been
introduced
• Current and past climate influences all these
things
Example:
• Range of Canyon Wren
• Distribution:
• ‘confined to areas with rock
faces’, canyons, bluffs
• Fundamental niche:
– Indication of parts of habitat in which a
species may be found
– Typically patchy locally aggregated) w/i range
• Realized niche:
– Portion of fundamental niche in which species
is actually found
Patchiness and Subpopulations
• Metapopulations – Local Populations (demes) in
suitable habitat isolated in matrix of unsuitable
habitat
• Source/Sink Populations – source population
over-reproduces, sink absorbs population
• Landscape – Metapopulations linked in matrix of
varied quality
• Marmots on Vancouver Island
• Unique species – isolated populations in cleared
areas – impacted by fire/forestry practices
• Loss of local populations results in fewer
‘stepping stones’ – genetically isolated
metapopulations
– Loss in genetic diversity
– Movement between populations maintains variability
within species
– Important to continued viability of species
• From: http://www.marmots.org/notes_vim.html
Ecotypes
• Sub-populations adapted to particular local
environments
– Unique genetic make-up?
– Same species
• Common Garden Experiment
– Seed collected from plants of same species
growing in different environments grow in
same location(s) (p 282)
– Isolation may lead to differentiation into
different species – uniquely adapted to
specific environments –( see p 200) 
restricted range