C4.1 Populations and communities

Population

A group of organisms of the same species living in an area and interbreeding

True or False? Members of different populations do not interbreed

True. Separate populations of a species live in different areas

Why is sampling used in population studies?

Because it's not practical to count every individual

Why use random sampling?

To avoid bias

Sampling error

Difference between estimated and true population size

True or False? Random sampling always leads to sampling error

True. Assumes even distribution which is unlikely

Quadrat

Square frame used to study sessile organisms

Organisms sampled by quadrats

Plants and non-motile animals

True or False? Quadrats estimate size for uncountable species

False. They count individuals for population estimates

Using quadrats for population estimates

Count individuals in random quadrats, calculate mean, multiply for total area

Standard deviation

Measures data spread around mean (not the average)

Small standard deviation

Data points cluster close to mean

Capture-mark-release-recapture purpose

Estimate population size of motile organisms

Marking method requirement

Must not affect survival to avoid inaccurate estimates

Lincoln Index equation

Population = (M × N)/R (M=marked, N=total captured, R=recaptured)

Lincoln Index assumptions

Marked individuals mix fully, marks don't affect survival/visibility, no births/deaths/migration

Carrying capacity

Maximum population an ecosystem can support

Abiotic factors affecting capacity

Light, water, temperature, soil minerals, O₂, CO₂

Biotic factors affecting capacity

Competition, predation, disease

Population density

Number of individuals per unit area

Density-dependent factors

Affect populations differently at different densities (e.g. disease spreads faster in dense populations)

True or False? Density-dependent factors maintain populations at/below capacity

True via negative feedback

Negative feedback in populations

Above capacity: reduced survival/reproduction. Below capacity: increased survival/reproduction

Predator-prey cycle

More prey→more predators→less prey→less predators→more prey

Famous predator-prey example

Canada lynx and snowshoe hare

Top-down control

Population limited by predators

Bottom-up control

Population limited by resources

True or False? One control type dominates per ecosystem

True

Allelopathy vs antibiotics

Both secrete harmful chemicals to outcompete others

Sigmoid curve phases

A=exponential, B=transition, C=plateau

Exponential phase growth

No limiting factors→rapid increase

True or False? Limiting factors start in plateau phase

False. They begin in transition phase

Plateau phase

Death rate = birth rate at carrying capacity

Model use in biology

Study complex/long-term systems through simulation

True or False? Models are perfect representations

False. Useful but imperfect

Testing exponential growth

Plot log(population) vs time→straight line indicates exponential growth

True or False? Can't model sigmoid growth in lab

False. Use yeast/duckweed

Intraspecific relationships

Interactions between same-species individuals

Intraspecific cooperation

Same-species individuals work together

Intraspecific competition

Same-species individuals compete for resources

Plant competition

Light, minerals, water, space

Animal competition

Food, mates, territory

Community

All interacting populations in an area

True or False? Communities only include plants/animals

False. Includes all organisms

Interspecific relationships

Interactions between different species

Herbivory

Organism feeds on plants

Predation

Organism consumes animals (live or recently dead)

Interspecific competition

Different species compete for same resources

Mutualism

Different species benefit each other

True or False? Parasitism benefits both

False. Benefits parasite, harms host

Pathogenicity

Pathogens cause host disease

Mutualism examples

Legume root nodules, mycorrhizae, coral-zooxanthellae

Legume mutualism benefits

Bacteria: get carbon. Plant: get nitrates

Invasive species

Harmful non-native species

Invasive species success

No natural predators/competitors→rapid population growth

Invasive species effects

Outcompete, displace, transmit disease, cause extinctions

Detecting interspecific competition

Remove one species→observe other's success

True or False? Competitive exclusion proves competition

False. Suggests possible competition

Lab competition test

Culture species alone/together→measure population effects

Field competition tests

Random sampling or species removal experiments

Hypothesis

Testable proposed explanation

Testing hypotheses

Through experiments/observations

True or False? Chi-squared tests species associations

True

No significant association

Species distributions are independent

Significant association

Indicates important interaction (symbiosis/competition)

Null hypothesis

States no association between variables

Rejecting null hypothesis

Significant difference/association exists

True or False? Non-significant=independent distributions

True

Contingency table purpose

Record quadrat data (both/neither/one species present)

p-value 0.05

95% certainty association isn't random

Chi-squared > critical value

Significant association

Chi-squared ≤ critical value

No significant association, accept null