Lecture –

Populations

Properties

Estimation of Size

Population Growth

What is a population?• ‘members of a particular species that

inhabit a particular area’• Various aspects:

• Range and distribution• Size• Density• Age structure• Growth• Genetic uniqueness subpopulations

(ecotype)

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

– 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

• Growth rate = dN/dt = rN No. of individuals in a population

Intrinsic rate of increase

Change over time

• The actual rate of population increase is

• r = (b – d) + (i – e)

Birthrate Deathrate Net immigration

Net emigration

• Geometric Growth:• N(t+1) = N(t) λ : at each interval of time population grows

by the multiple λ

• Exponential Population growth

• logeλ = r

• Geometric Growth – with discrete reproductive seasons

• Estimate population at same time in each year– Mortality of young

Slope (at any point) = dN/dt = rN

Log population size increasing exponential against time produces

straight line

Logistic Population Growth

• 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

Carrying Capacity

• As resources are depleted, population growth rate slows and eventually stops: logistic population growth.

– Sigmoid (S-shaped) population growth curve.

= rN(1-N/K)

Growth slows as N approaches value of

K or as (1-N/K) approaches 0

• dN/dt = rN K – NK

( )

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

• These effects have an increasing effect as population size increases

Song sparrow

Reproductive success decreases as population size

increases

• 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