Population Size

Population sampling

Technique used to estimate species presence, abundance, and population size in a habitat

Purpose of sampling

To determine species present and estimate number of individuals in an area

Assumption of Sampling

Sample is representative of the whole habitat

Sampling types

Random, systematic, stratified

Random sampling

Samples taken at random locations to avoid bias

Use of random sampling

Uniform habitats, large areas, limited time

Method of random sampling

Grid system + random number table

Advantage of random sampling

Reduces observer bias

Limitation of random sampling

May not cover all habitat areas evenly

Systematic sampling

Sampling at fixed intervals across habitat

Tool used for systematic sampling

Transects

Use of systematic sampling

Studying environmental gradients and zonation

Stratified sampling

Sampling different habitat sections separately

Use of stratified sampling

When habitat has distinct zones or uneven distribution

Advantage of stratified sampling

Ensures all habitat types are represented

Sampling considerations

Organism mobility, habitat type, time available, and sampling method choice

Quadrat

Square frame of known area used to sample non-mobile organisms

Quadrat sizes

Typically 1 m² or 0.5 m²

Use of quadrats

Estimate abundance of sedentary organisms

Frame quadrat

Physical frame used to estimate density, frequency, and cover

Point qdadrat

Used to record presence/absence and percentage cover using pins

Density formula

Density = total individuals ÷ area sampled

Frequency

Proportion of quadrats where species occurs

Percentage frequency

(Quadrats with species ÷ total quadrats) × 100

Percentage cover

Estimated proportion of ground covered by species

Advantages of percentage frequency

Quick and uses exact counts

Disadvantages of percentage frequency

Does not reflect size or abundance within quadrat

Transect

Sampling line used to study distribution across environmental gradients

Line transect

Records presence/absence along a line; shows distribution pattern

Belt transect

Series of quadrats along a line; shows abundance and distribution

Use of transects

Study zonation and transitions between communities

Zonation

Gradual change in species across environmental gradient

Edge effect

Increased biodiversity at habitat boundaries

Sampling of moving organisms

Requires capture methods such as mark-release-recapture

Mark-release-recapture method

Estimate population size of mobile organisms using capture, marking, release, and recapture

Mark-release-recapture formula

N = (n1 × n2) ÷ m2

N

Estimated total population size

n1

Number marked in first sample

n2

Total number caught in second sample

m2

Number of marked individuals recaptured

Example calculation

(10 × 20) ÷ 4 = 50 individuals

Assumptions of mark-recapture

the proportion of marked individuals in the second sample will be the same as the proportion of marked individuals to unmarked individuals in the whole population organisms are captured randomly and there is no bias towards a particular group for both samples the mark is not lost between the period of release and recapture marking does not hinder the movement of organisms or harm them in any way or make them more or less likely to be preyed upon marked individuals mix randomly with unmarked individuals the likelihood of capture does not change with the age of the organism the organisms must not be trap-shy the population is a closed one in that during the study period, changes in population size due to immigration, emigration, death and birth are negligible.

Trap-shy organisms

Avoid traps after first capture

Trap-happy organisms

More likely to be recaptured

Limitations of mark-recapture

Violations of assumptions reduce accuracy

Abiotic factors

Non-living environmental conditions affecting organisms

Terrestrial abiotic factors

Temperature, rainfall, light intensity

Aquatic abiotic factors

DO, TSS, conductivity, turbidity, water current, nutrients

Dissolved oxygen (DO)

Oxygen available in water for aquatic organisms

Total suspended solids (TSS)

Particles in water reducing clarity and light penetration

Conductivity

Measure of dissolved ions in water

Turbidity

Cloudiness of water affecting photosynthesis

pH

Acidity or alkalinity affecting enzyme function

Light intensity

Controls photosynthesis rate and distribution

Importance of abiotic factors

Influence distribution, abundance, and activity of organisms

Microhabitat

Small specific area where organisms live; used for detailed ecological study

Importance of measuring abiotic factors

Links species distribution to environmental conditions

Sampling importance

Provides data for biodiversity, conservation, and ecosystem management