9.2 - Population Ecology

Introduction to Population Ecology

• Population ecology: A branch of ecology that studies the interaction of a population with its surroundings.
• Population distribution: The way organisms of the same species are distributed in a habitat.

Key Parameters in Population Ecology

• Population size: The number of organisms present in a population.
• Population density: The number of individuals of a species per unit area of a habitat.

Factors Affecting Population Distribution

Factors Affecting Plant Distribution

• Temperature
  • Optimum plant growth occurs between $25^{\circ }\text{C}$ and $30^{\circ }\text{C}$.
  • High temperatures denature enzymes, disrupt biochemical processes, inhibit growth, increase transpiration and decrease photosynthesis.
  • Low temperatures reduce enzyme activity and slow biochemical reactions.
• Water: Required for enzyme activity, photosynthesis, transport and support in herbaceous plants. Dry deserts and tundra have low plant population distribution and density.
• Light: Essential for photosynthesis; areas receiving less light have fewer plants.
• Soil pH: Important for root nutrient absorption. Excessively acidic or alkaline soil reduces mineral nutrient absorption and impairs growth.
• Salt content: Affects water absorption by osmosis; high soil salt causes water loss by osmosis. Minerals produce proteins, enzymes, nucleotides, vitamins and other compounds; phosphorus forms phospholipids in cell membranes.

Factors Affecting Animal Distribution

• Temperature: High-temperature habitats are less occupied by animals such as worms and snails.
• Water: Animals concentrate where sufficient water is available for drinking and cooling.
• Breeding site: A suitable, safe site is needed to give birth and raise offspring.
• Food supply: Animals are heterotrophs and depend on plants and other animals for survival.

Patterns of Population Distribution

Clumped, Random and Uniform Patterns

• Clumped: Organisms form separate groups because resources are unevenly distributed.
• Random: Organisms are freely distributed; individuals interact little with one another.
• Uniform: Organisms are spread throughout the occupied area; individuals compete for limited resources.

Estimating Organism Population Size

• Random sampling estimates population size when the study area is large and complete data are difficult to obtain.
• A representative sample estimates organism abundance and distribution pattern.

Quadrat Sampling Technique

• Estimates terrestrial plants or animals that are inactive or move very slowly.
• Organisms counted in randomly placed quadrats represent the whole study area.
• A quadrat is a square frame made of wood, iron or plastic, normally divided into equal sections.
• Size depends on organism type and size, study-area size, distribution and density. A $1{\text{ m}}^2$ quadrat suits a school field.
• Frequency: Probability of obtaining an individual of a plant species in each quadrat.

$\text{Frequency}=\frac{\text{Number of quadrats containing the studied species}}{\text{Total number of quadrats used}}\times 100\%$

• Density: Average number of individuals of a species per unit area.

$\text{Density}=\frac{\text{Total individuals of the studied species in all quadrats}}{\text{Total quadrats used}\times \text{Area of one quadrat}}$

• Coverage: Soil surface area covered by the shoots of a plant species.

$\text{Percentage coverage}=\frac{\text{Area covered by the studied species in all quadrats}}{\text{Total quadrats used}\times \text{Area of one quadrat}}\times 100\%$

Capture-Mark-Release-Recapture Technique

• Estimates freely moving organisms such as civets, sharks, snails, millipedes, insects and butterflies.
• Capture animals randomly in their natural habitat and record the number caught.
• Mark each animal with nail polish, paint or Indian ink; release them into the same area.
• After a period, capture a second random sample in the same area.
• Record the total second sample and its marked animals.

$\text{Population size}=\frac{P\times Q}{R}$

• $P$: Number of animals in the first capture.
• $Q$: Number of animals in the second capture.
• $R$: Number of marked animals in the second capture.

Effect of an Abiotic Component on a Population

• Pleurococcus sp. is a unicellular green alga that forms a slimy layer on moist tree trunks, rocks and soil sheltered from intense sunlight.
• It reproduces rapidly by binary fission, occurs singly or in groups, has a thick cell wall that prevents excessive water loss and contains many chloroplasts for photosynthesis.
• To investigate light:
  • Place five transparent $10\text{ cm}\times 10\text{ cm}$ quadrats on different aspects of a tree trunk, one metre above ground.
  • Mark areas containing Pleurococcus sp.; count squares that are at least half covered.
  • Calculate percentage coverage and compare it with light exposure at each aspect.