1. Lecture 12 Populations

2. ‘members of a particular species that inhabit a particular area’
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)

3. Population growth rate: Discrete-time
Geometric growth
Species which have discrete breeding seasons
Continuous time
Exponential growth

4. Exponential Population growth logeλ = r
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 over time
Intrinsic rate 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

5. Geometric Growth – with discrete reproductive seasons
Estimate population at same time in each year
Mortality of young

6. Slope (at any point) = dN/dt = rN
Log population size increasing exponential against time produces straight line
Slope (at any point) = dN/dt = rN

7. Logistic Population Growth

8. 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

9. As resources are depleted, population growth rate slows and eventually stops: logistic population growth.
Sigmoid (S-shaped) population growth curve.

10. Growth slows as N approaches value of K or as (1-N/K) approaches 0
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

11. 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

12. Reproductive success decreases as population size increases
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 success decreases as population size increases
Song sparrow
These effects have an increasing effect as population size increases

13. 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

14. Size of Populations
Abundance: number of individuals within a specified area
Abundance/area = Density
How do we determine how many individuals there are?

15. 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
Examples of techniques
Capture-mark-recapture
Christmas Bird Counts
Aerial surveys
Acoustic monitoring

17. Survivorship and Age structure
Age structure: Proportion of individuals in various age classes
Survivorship is the percentage of an original population that survives to a given age
Involves study development of life table
Cohort
Example: Cactus finch
Static
Example: Dall sheep

18. Age Structure Diagrams: Visualization of future population growth

20. 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

21. 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

22. Example: Range of Canyon Wren Distribution:
‘confined to areas with rock faces’, canyons, bluffs

24. Fundamental niche: Realized 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

26. 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

29. 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:

31. Sub-populations adapted to particular local environments
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