What you'll learn
Ecology is the study of how organisms interact with one another and with their surroundings. This topic covers the vocabulary ecologists use, the biotic and abiotic factors that shape where organisms live, feeding relationships in food chains and webs, the flow of energy through an ecosystem, the carbon and nitrogen cycles, the relationships between species, and the sampling methods used to study populations. It carries more marks in CSEC Biology than almost any other single area, partly because it is examined both as recall and as data interpretation. The definitions are worth learning precisely, because several of them look similar and examiners test exactly that distinction.
Key terms and definitions
Ecosystem — a community of organisms together with the non-living surroundings with which they interact.
Habitat — the place where an organism lives.
Population — all the organisms of one species living in a habitat at one time.
Community — all the populations of different species living together in a habitat.
Biotic factor — a living feature of the environment, such as predation or competition.
Abiotic factor — a non-living feature of the environment, such as temperature, light or pH.
Producer — an organism that makes its own food by photosynthesis.
Consumer — an organism that obtains energy by feeding on other organisms.
Decomposer — an organism that breaks down dead material, releasing nutrients back into the environment.
Trophic level — the position an organism occupies in a food chain.
Food chain — a sequence showing the transfer of energy from one organism to the next.
Food web — a set of interconnected food chains in an ecosystem.
Niche — the role an organism plays within its community.
Core concepts
Levels of organisation in ecology
The terms build outwards from the individual. A single organism lives in a habitat. All the individuals of its species in that habitat form a population. All the populations living there together form a community. The community together with the abiotic surroundings forms the ecosystem.
The distinction between population and community is worth fixing: a population is one species, a community is all the species. Questions frequently offer the wrong one as a distractor.
Biotic and abiotic factors
Abiotic factors are the physical and chemical conditions: light intensity, temperature, water availability, soil pH, mineral ion concentration, wind, and in aquatic habitats the dissolved oxygen concentration and salinity. These determine which organisms can survive in a place at all.
Biotic factors are the effects of other living things: competition for food, water, light or space; predation; disease; and the availability of food or of mates. A mangrove or coral reef habitat in the Caribbean illustrates both at once — salinity and tidal exposure set the physical limits, while competition for light and space among the organisms determines who occupies which zone.
Producers, consumers and feeding relationships
Almost every ecosystem begins with producers, usually green plants or algae, which convert light energy into chemical energy by photosynthesis. Primary consumers are herbivores that eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. A food chain shows this sequence, and the arrows always point in the direction of energy transfer — from the eaten to the eater. Drawing an arrow the wrong way is one of the easiest marks to lose in the whole topic.
Real ecosystems are not single chains. Most organisms eat more than one kind of food and are eaten by more than one predator, so the chains interconnect to form a food web. A food web is more realistic, and it also shows why removing one species can have effects that spread widely: predators of that species lose a food source, while organisms it fed on may increase in number.
Decomposers, mainly bacteria and fungi, feed on dead organisms and waste. They are essential because they release locked-up nutrients back into the soil or water for producers to reuse. Without them, nutrients would remain in dead material and the ecosystem would run down.
Energy flow and pyramids
Energy enters an ecosystem as light and leaves it as heat. At each transfer between trophic levels, most of the energy is lost rather than passed on. It is used in respiration to power movement and other processes, released as heat, and lost in undigested material in faeces, in urine, and in parts not eaten. Only a small proportion of the energy in one trophic level becomes new tissue in the next.
Two consequences follow, and both are examined. First, food chains are short — usually four or five levels at most — because after several transfers too little energy remains to support another level. Second, a pyramid of biomass normally narrows towards the top, because the total mass of living material supported falls at each level.
A pyramid of numbers counts individuals rather than mass, and it can be an awkward shape. A single large tree supporting many insects gives a narrow base and a wide second level. A pyramid of biomass avoids this problem, which is why it is generally the more useful representation.
The carbon cycle
Carbon moves continuously between the atmosphere and living things. Photosynthesis removes carbon dioxide from the air and fixes it into glucose and then into the tissues of plants. Respiration by plants, animals and decomposers returns carbon dioxide to the air. Feeding passes carbon compounds along food chains. When organisms die, decomposition releases their carbon as carbon dioxide, and where dead material is buried without decaying it may over very long periods form fossil fuels. Combustion of those fuels, and of wood, returns carbon dioxide to the atmosphere.
The cycle is balanced only if removal matches return. Burning fossil fuels and clearing forests both push it out of balance, adding carbon dioxide while reducing the photosynthesis that would remove it.
The nitrogen cycle
Plants need nitrogen to make proteins, but they cannot use nitrogen gas from the air directly. They absorb nitrates from the soil instead, and four groups of bacteria control the supply.
Nitrogen-fixing bacteria, some free-living in soil and some in the root nodules of legumes such as peas and beans, convert nitrogen gas into nitrogen compounds. Decomposers break down dead material and release ammonium compounds. Nitrifying bacteria convert those ammonium compounds into nitrites and then nitrates, the form plants take up. Denitrifying bacteria work in the opposite direction, converting nitrates back into nitrogen gas, and they thrive in waterlogged soils where oxygen is scarce.
This last point explains a common examination question about why farmers drain and plough soil: aeration favours nitrifying bacteria and discourages denitrifying ones, so more nitrate remains available to crops.
Relationships between species
Competition occurs when two organisms need the same limited resource. Intraspecific competition is between members of the same species, and it is usually the more intense because their needs are identical. Interspecific competition is between different species.
Predation is one organism killing and eating another. Predator and prey numbers often rise and fall in linked cycles: as prey become plentiful, predators increase; the increased predation then reduces prey numbers, and predator numbers fall in turn.
Three further relationships are distinguished by who benefits. In mutualism both organisms gain, as with nitrogen-fixing bacteria in root nodules, which receive sugars and supply nitrogen compounds. In parasitism one gains at the other's expense, as with a tapeworm in an intestine. In commensalism one benefits while the other is largely unaffected.
Sampling populations
Counting every organism is rarely possible, so ecologists sample. A quadrat is a frame of known area placed on the ground to record the plants or slow-moving animals within it. Quadrats must be placed randomly — for instance by using random numbers as coordinates on a grid — so the sample is not biased towards spots that look interesting. Estimating the population means multiplying the mean number per quadrat by the total area.
A transect is a line along which quadrats are placed at intervals, used when studying how a community changes across a habitat, such as from the water's edge up a shore. Using a transect where random quadrats were needed, or the reverse, is a standard examination trap: random quadrats estimate abundance, transects show change across a gradient.
Taking more samples improves reliability, because it reduces the effect of chance variation between quadrats.
Worked examples
Example 1. A food chain is written: grass → grasshopper → lizard → hawk. Identify the producer and the secondary consumer, and state what the arrows represent.
The producer is the grass, because it makes its own food by photosynthesis. The secondary consumer is the lizard, because it eats the grasshopper, which is the primary consumer. The arrows represent the direction in which energy is transferred, from the organism eaten to the organism that eats it.
Example 2. Ten quadrats each of area 1 m² are placed randomly in a field of area 400 m². The mean number of a plant species per quadrat is 6. Estimate the population.
Each quadrat covers 1 m² and the mean count is 6 per m², so the estimated population is 6 × 400 = 2 400 plants. The estimate assumes the quadrats were placed randomly and that the species is distributed fairly evenly.
Example 3. Explain why a food chain rarely has more than five trophic levels.
Most of the energy entering a trophic level is lost rather than passed on — used in respiration and released as heat, and lost in faeces, urine and uneaten parts. Because only a small fraction becomes new tissue at each transfer, after four or five levels too little energy remains to support a further population.
Example 4. Waterlogged soil produces poor crop growth even when fertiliser has been added. Suggest why, referring to the nitrogen cycle.
Waterlogged soil contains little oxygen, which favours denitrifying bacteria. These convert nitrates in the soil into nitrogen gas, which plants cannot absorb, so the nitrate added as fertiliser is lost and less is available for the crop to make proteins.
Common mistakes and how to avoid them
Drawing food chain arrows the wrong way. The arrow points from the organism eaten towards the organism eating it, showing energy transfer. Check every arrow before moving on.
Confusing population and community. One species is a population; all the species together are a community.
Saying energy is "lost" without saying how. The mark requires a route: respiration and heat, faeces, urine, or parts not eaten.
Mixing up biotic and abiotic. Predation and competition are biotic; temperature, light and pH are abiotic. A useful test is whether the factor is alive or produced by something alive.
Treating a pyramid of numbers as always pyramid-shaped. One large producer supporting many small consumers gives an inverted lower section. Only biomass pyramids reliably narrow upwards.
Using a transect to estimate abundance. Random quadrats estimate how many; transects show how a community changes across a gradient.
Claiming plants absorb nitrogen gas. They absorb nitrates. Only nitrogen-fixing bacteria use the gas directly.
Exam technique for Ecology and Ecosystems
Learn the definitions word for word. Several marks in every paper come from stating precisely what an ecosystem, a population or a niche is, and approximate wording often fails.
For data questions, read the axes and units before drawing any conclusion, and quote figures from the data when explaining a trend. An answer that says "the number increased" earns less than one that says where the increase began and ended.
When asked about the effect of removing a species from a food web, trace the consequences in both directions: what loses a food source, and what is no longer eaten. Two-directional answers score better than one.
For sampling questions, expect to justify a method as well as name it. Explain why placement was random, why several quadrats were used, and what the estimate assumes.
Quick revision summary
- Habitat → population (one species) → community (all species) → ecosystem (community plus abiotic surroundings).
- Abiotic: light, temperature, water, pH, salinity, dissolved oxygen. Biotic: competition, predation, disease, food supply.
- Food chain arrows show energy transfer, pointing from eaten to eater.
- Energy is lost at each trophic level through respiration and heat, faeces, urine and uneaten parts, so chains are short.
- Biomass pyramids narrow upwards; number pyramids may not.
- Carbon cycle: photosynthesis removes CO₂; respiration, decomposition and combustion return it.
- Nitrogen cycle: nitrogen-fixing, decomposing, nitrifying and denitrifying bacteria; plants absorb nitrates, not nitrogen gas.
- Waterlogged soil favours denitrifying bacteria and loses nitrate.
- Mutualism both gain; parasitism one gains at the other's cost; commensalism one gains, the other unaffected.
- Random quadrats estimate abundance; transects show change across a gradient; more samples improve reliability.