Classification and Ecology Basics

This page covers the starting point of Section A: how organisms are grouped, how ecologists describe the environment, and the vocabulary needed for food webs, population questions, and conservation topics.

Ecology questions often begin with definitions, then move into application. Strong answers name the term, explain it clearly, and link it to the example in the question.

Classification

Classification is the grouping of organisms based on shared features. It helps biologists identify organisms, compare them, and understand relationships among living things.

In field work, organisms may first be grouped using visible features. Useful features include hairiness, colour, shape, leaf venation, number of legs, number of wings, antennae, body covering, and body segmentation.

In a school garden, insects could be separated from spiders by counting legs: insects have six legs, while spiders have eight.

A useful starting point is the five-kingdom system:

Animal Kingdom Subdivisions

The Animal Kingdom can also be subdivided into phyla, then classes, then smaller groups down to species.

The order is:

For CSEC, the main skill is using visible features to place organisms into sensible groups. Features such as body segmentation, number of legs, wings, shell, backbone, and body covering are useful.

Arthropods are especially common in classification questions because their visible features are easy to compare.

Chordates include the vertebrate classes students meet most often:

In classification questions, the best feature is usually the one that separates organisms cleanly. Number of legs, wings, antennae, segmentation, and backbone are often more useful than colour.

Ecology

Ecology is the scientific study of interactions between organisms and their environment.

An environment includes:

• living factors, called biotic factors
• non-living factors, called abiotic factors

Biotic and Abiotic Factors

Biotic factors are the living parts of the environment, such as plants, animals, fungi, bacteria, and humans. Abiotic factors are the non-living parts, such as light intensity, temperature, water availability, soil type, pH, salinity, and air currents.

If mangrove seedlings grow poorly in very salty soil, salinity is an abiotic factor affecting their distribution.

Soil, Water, and Climate

Soil affects organisms because it provides water, mineral nutrients, oxygen for roots and soil organisms, anchorage, and different pH or salinity conditions. Air provides oxygen for respiration, carbon dioxide for photosynthesis, and nitrogen that can enter nutrient cycles through bacteria.

Light affects photosynthesis and activity patterns. Temperature affects enzyme-controlled reactions, growth, reproduction, and survival.

When an answer uses an abiotic factor, the effect matters: low light reduces photosynthesis, while very high salinity can make water uptake harder for plants.

Levels of Ecological Organisation

Ecology moves from one organism to the whole biosphere.

Habitat and Niche

A habitat is the place where an organism lives, such as a pond, forest, rotting log, coral reef, or mangrove swamp. A niche is the role a species plays in a community. It includes where the organism lives, what it eats, what eats it, when it is active, and how it affects other organisms.

A frog's habitat may be a pond. Its niche includes feeding on insects, being prey for birds and snakes, breeding in water, and helping to control insect populations.

Limiting Factors

A limiting factor is any biotic or abiotic factor that restricts the size, distribution, or survival of a population.

Common limiting factors include food, water, space, mates, temperature, predators, disease, and competition. When a population stops increasing, limiting factors are usually involved: food may run out, disease may spread more easily, or competition may become stronger.

Ecological Investigations

Ecologists use different tools and methods depending on the habitat and the organisms being studied. Choosing the right method matters: a quadrat works well for plants or slow-moving animals, but a sweep net or pitfall trap is better for mobile insects.

Sampling Methods

For plants, a quadrat can give percentage cover (how much of the frame is occupied) or density — the number of organisms per unit area sampled.

Mark-Release-Recapture

When animals move freely through a habitat, direct counting is difficult. The mark-release-recapture method estimates population size using the Lincoln index:

N = (M × C) ÷ R

• N — estimated population size
• M — number captured and marked in the first sample
• C — total number captured in the second sample
• R — number in the second sample that carry the original mark

Quick Comparisons

FLASHCARD SET • 12 CARDSClassification and Ecology Basics