back to ecology

C4.1 - populations and communities

How do interactions between organisms regulate sizes of population in a community?

What interactions within a community make its populations interdependent?

• Factors limit populations

  • food/water availability

  • Predators

  • Disease

  • Chemical competition

• No species lives in isolated bubble

C4.1.1 - populations

Understand members of a population normally breed and that reproductive isolation used to distinguish one population from another

• Population - group of individuals of same species living in same geographical area at same time and interbreedable

  • ex) emperor penguins in antarctica

  • Bush-crickets in grassland 

  • Rainbow trout in northwest coast

  • Dandelions - grassland w/ crickets

• Emperor penguins example

  • How do we draw the line between different populations?

    • Interbreedable?

    • Not likely if separated geographically

• Measurable aspects:

  • Size - most obvious

    • Change in population size over time

    • Consider factors affecting size (immigration, death)

  • Population density

  • Geographical distribution

  • Maximum number of individuals supported by available resources

• Need to understand population to observe interactions in environment and other species

C4.1.2 - estimating population size

Understand reasons for estimating population size, rather than counting every individual, and that the need for randomness in sampling procedures

• Cannot just count every member of a species

• Rely on estimates for population count

  • Count sample

  • Use sample to estimate overall population

• Two types of sampling

  • Systematic sampling

  • Random sampling

Systematic sampling

• Line or grid set up and measurements carried out only specified, regular intervals

• Ex: 50 m measuring tape on rocky shore, seaweed and snails counted in 1 m² area every 5m

Random sampling

• Arbitrary zones of population’s geographic distribution sampled

• Random directions

• Random distances

• Overcome bias for particular area

• Quadrants and method of mark

C4.1.3 - sampling sessile organisms

Both sessile animals and plants, where the numbers of individuals can be counted, are suitable

• Random sampling used to estimate population size for organisms that stay in one place

• Ex: lichen and choral

  • Sessile organisms - much of their lives, do not change location

• Use quadrant for random sampling

• Quadrant - square of a particular dimension that can be made of a rigid material

  • Surface area of sampling size ios same for each count you take 

C4.1.4 - sampling motile organisms

• Capture-mark-release-recapture method - sampling technique to estimate number of animals in ecosystem

  • Quadrant substitute for motile organisms

  • Motile - organism that moves around

• Steps 

  • Catch some of population

  • Mark 

  • Release animals back into ecosystem after suitable period of time to remix with others in population

  • Second sample captured

    • Some will be marked, others unmarked

• Marked vs unmarked proportion 

• Lincoln index:

  • 
    

• Can be rewritten as:

  • 
    

  • M = number of individuals originally caught and marked

  • N = total number of individuals recaptured

  • R = number of individuals recaptured 

• Limitations of method

  • Capturing and marking animals injure them

  • Mark may make animal visible to predators

    • If eaten bc of mark, sample is not reliable

  • Assumes population is closed

    • No immigration or emigration

    • Only few populations are closed

C4.1.5: carrying capacity and competition for limited resources

Simple definition of carrying capacity is sufficient with some examples of resources that may limit carrying capacity

Carrying capacity

• Carrying capacity - largest population of a species that an environment can support over time 

• Factors affecting carrying capacity

  • Food availability

  • Water availability

  • Space

  • Shelter disease predators

  • Climate

C4.1.6: negative feedback control of population size by density-dependent factor

Numbers of individuals in a population may fluctuate due to density-independent factors, but density dependent factors tend to push the population back towards the carrying capacity

Density independent factors

• Density-independent factors limit population growth regardless of population size

  • Usually abiotic factors

• Examples

  • Climate events and natural disasters (e.g. extreme weather) increases death rates

  • Habitat destruction reduced available resources

  • Seasonal changes affect resources availability 

Density dependent factors

• Density-dependent factors affect a population’s growth based on its size

  • Factors are usually biotic

• As population increased, density-dependent factors help bring the population back to its carrying capacity

  • Negative feedback

• Examples of density-dependent factors:

  • Competition for resources (e.g. food, water, space)

  • Risk prediction 

  • Disease 

Competition for resources

• Members of same species need the same resources to survive and reproduce

• As population grows, there is more competition for resources, making them less available for each individual

• Better-adapted individuals are more likely to survive and reproduce 

Risk of predation

• Predators target large prey populations

• When prey populations are high, predators population increase due to more food available 

• High predator populations decrease prey populations

• Predators help control the growth of prey populations

Disease 

• High population densities help pests and diseases spread more easily through contact and vectors

• Spread pathogens can increase death rates and reduce population 

C4.1.7: population growth curves

One case study in an ecosystem

Understand reasons for exponential growth in initial phases. Lag phase is not expected as part of sigmoid population growth 

Factors affecting population growth 

• Four factors determining whether a population size increases or decreases

  • Natality

  • Mortality

  • Immigration

  • Emigration 

Natality, mortality, immigration and immigration

• Four factors affecting size of population

  • Natality (N) - birth rate for population

  • Morality (M) - death rate

  • Immigration (I) - individuals entering population

  • Immigration (E) - individuals leaving a population 

  • Change in population size = (N+I) - (M + E)