What you'll learn
Ecology, conservation and pollution studies organisms in their environment, how populations change, how communities develop over time, and how human activity affects them. At CAPE level the topic is quantitative as well as descriptive: you must be able to sample populations using appropriate techniques, apply the mark-release-recapture method, interpret population growth curves, and evaluate conservation strategies. The Caribbean context matters throughout, since coral reefs, mangroves and hurricane disturbance are examinable examples. By the end of this topic you should be able to define the ecological terms precisely, describe sampling methods and their limitations, explain population growth and limiting factors, describe succession, explain the major forms of pollution, and evaluate conservation measures.
Key terms and definitions
Ecosystem — a community of organisms interacting with one another and with their abiotic environment
Habitat — the place where an organism lives
Niche — the role of an organism in its ecosystem, including how it obtains energy and interacts with other species
Population — all the organisms of one species in a habitat at one time
Community — all the populations of all species in a habitat
Carrying capacity — the maximum population size an environment can sustain indefinitely
Abiotic factor — a non-living environmental factor
Biotic factor — a living environmental factor
Density-dependent factor — a factor whose effect increases as population density increases
Succession — the directional change in a community over time
Climax community — the stable community at the end of a succession
Biodiversity — the variety of species and genetic variation within an ecosystem
Indicator species — a species whose presence or absence indicates environmental conditions
Eutrophication — nutrient enrichment of water causing algal growth and oxygen depletion
Core concepts
The niche concept
The niche is more than where an organism lives: it includes what it eats, when it is active, what eats it, its tolerance of abiotic conditions, and every interaction it has with other species.
The competitive exclusion principle follows from this. Two species occupying exactly the same niche in the same habitat cannot coexist indefinitely, because one will compete more successfully and the other will be eliminated or forced to occupy a different niche.
This explains niche differentiation, in which similar species partition a resource — feeding at different heights in a tree, at different times of day, or on different sizes of the same prey — so that competition is reduced and coexistence becomes possible.
Sampling techniques
Counting every organism is rarely possible, so populations are sampled and the results scaled up.
Quadrats are used for plants and slow-moving animals. A frame of known area is placed and the organisms within are recorded. Placement must be random to avoid bias, and random coordinates from a grid are used to achieve this. The population is estimated by multiplying the mean number per quadrat by the total area divided by the quadrat area.
Records may be taken as density, the number per quadrat; frequency, the proportion of quadrats in which the species occurs; or percentage cover, the proportion of the quadrat area occupied, which is useful for species difficult to count as individuals.
Reliability increases with the number of quadrats, and a suitable number can be determined by plotting cumulative mean against number of quadrats and identifying where the curve levels off.
Transects are used where distribution changes across a habitat, for example from the sea inland or up a shore. A line is laid and quadrats are placed at intervals along it, giving a systematic rather than random sample because the point is to detect the pattern of change.
Mark-release-recapture is used for mobile animals. A sample is captured, marked in a way that does not affect survival or behaviour, and released. After sufficient time for mixing, a second sample is captured and the number of marked individuals recorded.
The population estimate is the number in the first sample multiplied by the number in the second sample, divided by the number of marked individuals in the second sample.
The method rests on assumptions that are examinable in their own right: the marked individuals mix randomly with the population; the mark does not affect survival, behaviour or the chance of being recaptured; the mark is not lost; there is no significant migration, birth or death between the samples; and sufficient time is allowed for mixing but not so much that the population changes.
Abiotic factors are measured alongside, using a light meter, thermometer, pH meter, oxygen probe or salinity meter, so that distribution can be correlated with conditions.
Population growth
A population introduced into a favourable environment shows a characteristic sigmoid growth curve with four phases.
The lag phase shows little growth as the organisms acclimatise, and the population is small so few reproduce.
The exponential or log phase shows rapid growth. Resources are abundant, there is little competition, and the birth rate greatly exceeds the death rate.
The transitional phase shows slowing growth as resources become limiting and competition increases.
The stationary or plateau phase shows a stable population fluctuating around the carrying capacity, where the birth rate equals the death rate.
The carrying capacity is set by limiting factors, and these are classified in a way that questions often require.
Density-dependent factors have a greater effect as the population becomes denser: competition for food, water, light and space; predation, since predators concentrate where prey is abundant; disease, which spreads more readily in a dense population; and accumulation of waste. These are biotic and they regulate populations around the carrying capacity.
Density-independent factors affect the population regardless of its density: temperature, drought, flood, fire and hurricanes. These are abiotic, and in the Caribbean hurricanes are a significant example, capable of reducing a population sharply whatever its size.
Predator and prey populations often show linked cycles, with the predator population peaking shortly after the prey population, since a rise in prey allows more predators to survive, and the resulting predation then reduces the prey, after which the predator population falls.
Succession
Succession is the directional change in a community over time, and it follows a consistent pattern.
Primary succession begins on bare ground with no soil, such as new volcanic rock or a sand dune.
Pioneer species colonise first. They are adapted to harsh conditions, typically tolerating extremes, reproducing rapidly and dispersing widely, and often able to fix nitrogen. Lichens are the classic example on rock.
The pioneers change the environment. They break down the substrate, and when they die their decomposition forms humus, so a thin soil develops that retains water and nutrients.
These changes make conditions less hostile and suitable for other species, which colonise and outcompete the pioneers. Mosses and small herbaceous plants follow, then grasses and shrubs, then trees.
Each community changes the environment further, making it suitable for the next. The general principle, which is what examiners want stated, is that each stage makes the environment less suitable for the existing species and more suitable for the next.
Through the succession, biomass, biodiversity, soil depth and the complexity of food webs all increase, and the community becomes more stable.
The climax community is the final stable stage, determined by climate.
Secondary succession occurs where soil already exists, for example after a fire, a hurricane or the abandonment of farmland. It proceeds much faster than primary succession because soil, nutrients and a seed bank are already present.
Deflected succession occurs where human activity prevents the climax being reached, as when grazing or mowing maintains grassland that would otherwise become woodland. The resulting community is a plagioclimax.
Pollution
Water pollution includes sewage, agricultural run-off, industrial effluent and oil.
Eutrophication is the most examined mechanism and must be given as a full chain. Nitrate and phosphate from fertiliser or sewage enter the water, causing algae to grow rapidly and form a surface bloom. This blocks light from the plants below, which die. Saprobiotic bacteria decompose the dead material and multiply, and because they respire aerobically they consume the dissolved oxygen. The biochemical oxygen demand rises, dissolved oxygen falls, and fish and invertebrates die.
Indicator species allow water quality to be assessed. Stonefly and mayfly nymphs require high oxygen concentrations and indicate clean water; bloodworms and sludgeworms tolerate low oxygen and indicate polluted water. Lichens serve the same purpose for air quality, being sensitive to sulfur dioxide.
Air pollution includes sulfur dioxide and oxides of nitrogen producing acid rain, which damages vegetation, leaches nutrients from soil and acidifies lakes; particulates causing respiratory disease; and carbon dioxide and methane contributing to the enhanced greenhouse effect.
Bioaccumulation and biomagnification are important and are often confused. Bioaccumulation is the build-up of a persistent substance within a single organism over its lifetime, because it is not excreted or broken down. Biomagnification is the increase in concentration along a food chain, because each consumer eats many organisms from the level below and retains the substance from all of them. Top predators are therefore most affected, which is why persistent pesticides such as DDT caused breeding failure in birds of prey.
Land pollution includes non-biodegradable plastic waste, landfill leachate and pesticide residues.
Conservation
Conservation is the active management of ecosystems to maintain biodiversity, and it differs from preservation, which leaves an area untouched. Conservation allows sustainable use.
The reasons for conserving biodiversity are examinable and fall into groups. Ecological: species are interdependent, and removing one affects others, while diverse ecosystems are more stable and resilient. Economic: species provide food, timber, medicines and tourism revenue, and a substantial proportion of pharmaceuticals derive from natural compounds. Genetic: wild relatives of crops hold alleles that may be needed for future breeding. Ethical and aesthetic: many argue that species have a right to exist and that we have an obligation to future generations.
In situ methods conserve species in their natural habitat, through national parks and marine protected areas, legal protection, habitat restoration and control of invasive species. The advantages are that the species remains in its natural environment where it continues to evolve and where the whole community is protected, and it is usually cheaper. The disadvantage is that the threat may persist within the area and enforcement can be difficult.
Ex situ methods conserve species outside their habitat, through zoos, botanic gardens, captive breeding and seed banks. The advantages are close protection and control of breeding to maintain genetic diversity. The disadvantages are high cost, limited numbers, loss of natural behaviour, and difficulty in reintroducing animals successfully.
Caribbean examples are worth citing. Coral reefs are threatened by rising sea temperature causing bleaching, by sedimentation from coastal development, by overfishing and by damage from anchors and tourism, and are conserved through marine protected areas, mooring buoys and fishing regulation. Mangroves protect coasts from storm surge and erosion, act as nurseries for fish and sequester carbon, and are threatened by clearance for development. Sea turtles are protected through legislation, nesting beach protection and control of lighting near beaches.
Sustainable management balances use with conservation. Sustainable forestry uses selective logging, coppicing, rotational felling and replanting; sustainable fishing uses quotas, minimum mesh and catch sizes, closed seasons during breeding, and no-take zones from which stocks spread into surrounding waters.
Worked examples
Example 1: A mark-release-recapture calculation (4 marks)
In a study of a beetle population, 60 individuals were captured, marked and released. A second sample of 80 individuals contained 15 marked beetles. Estimate the population size and state two assumptions.
The population estimate is the number in the first sample multiplied by the number in the second sample, divided by the number of marked individuals recaptured. That is 60 multiplied by 80, divided by 15, which gives 4,800 divided by 15, equalling 320 beetles.
Two assumptions are that the marked individuals mixed randomly with the rest of the population before the second sample was taken, and that the marking did not affect the beetles' survival, behaviour or likelihood of being recaptured.
Other valid assumptions include that the mark was not lost, and that there was no significant migration, birth or death between the two samples.
Example 2: Explaining a sigmoid growth curve (5 marks)
Explain the shape of the growth curve of a population introduced into a new habitat.
Initially there is a lag phase with little increase, because the population is small so few individuals are reproducing, and the organisms are acclimatising to the new conditions.
An exponential phase follows, in which the population increases rapidly. Resources such as food, water and space are abundant, there is little intraspecific competition, and the birth rate greatly exceeds the death rate.
As the population becomes large, growth slows. Resources become limiting and competition increases, while disease and predation have greater effects at higher density, so the death rate rises and the birth rate falls.
Finally the population stabilises around the carrying capacity, fluctuating about it as the birth rate and death rate become approximately equal. The carrying capacity is the maximum population the environment can sustain given the available resources.
Example 3: Evaluating conservation methods (5 marks)
Compare in situ and ex situ conservation of an endangered Caribbean species.
In situ conservation protects the species in its natural habitat, for example through a marine protected area or national park with legal protection and habitat restoration.
Its advantages are that the species remains in the environment to which it is adapted, so it continues to behave naturally and to evolve in response to selection pressures; the whole community and its interactions are protected rather than one species alone; and the population can be maintained at a larger size than captivity permits, preserving genetic diversity. It is also generally cheaper.
Its disadvantages are that the original threat, such as pollution or poaching, may continue within the protected area, and enforcement over a large area may be difficult and costly.
Ex situ conservation, through captive breeding in zoos or seed banks, allows close protection from predators and disease and controlled breeding to maintain genetic diversity, and provides a reserve population should the wild population be lost.
Its disadvantages are high cost, the small numbers that can be maintained, the loss of natural behaviours that may make reintroduction unsuccessful, and the risk of inbreeding in a small captive population.
The two are best used together, with captive breeding supporting reintroduction into a protected habitat.
Common mistakes and how to avoid them
The most frequent error is defining a niche as a habitat. The niche is the organism's role, including its feeding, timing and interactions, not merely its location.
Students often confuse bioaccumulation with biomagnification. Bioaccumulation is within one organism over time; biomagnification is along a food chain.
Another common slip in succession answers is describing the sequence of species without explaining the mechanism. Each community must be said to change the environment so that it becomes suitable for the next.
In mark-release-recapture questions, candidates frequently divide by the wrong figure. The divisor is the number of marked individuals in the second sample.
In eutrophication answers, stopping at the algal bloom loses most of the marks. The chain must run through to oxygen depletion and the death of fish.
Finally, candidates often use random placement for transects. Transects are deliberately systematic, because the aim is to detect change along a gradient.
Exam technique for "Ecology, conservation and pollution"
State the sampling method and the reason for choosing it. Quadrats for sessile organisms, transects for gradients, mark-release-recapture for mobile animals.
For any estimate, show the calculation and then state at least two assumptions, since the assumptions are frequently a separate question part.
When classifying limiting factors, use the density-dependent and density-independent distinction explicitly and give an example of each.
In succession answers, note what increases through the sequence — biomass, biodiversity, soil depth, stability — as this is often a separate mark.
For evaluation questions, give advantages and disadvantages of each approach and finish with a judgement, ideally referring to the two approaches being complementary rather than alternatives.
Quick revision summary
A niche is an organism's role, and two species with identical niches cannot coexist, leading to niche differentiation. Quadrats placed randomly sample sessile organisms by density, frequency or percentage cover; transects sample systematically along a gradient; and mark-release-recapture estimates mobile populations as first sample times second sample divided by marked recaptures, subject to assumptions about mixing, marking and no migration. Population growth shows lag, exponential, transitional and stationary phases, stabilising at the carrying capacity set by density-dependent factors such as competition, predation and disease, and disturbed by density-independent factors such as hurricanes. Succession proceeds from pioneer species, which change the environment and make it suitable for successors, through increasing biomass, biodiversity and stability to a climax community, with secondary succession faster because soil already exists and plagioclimax resulting from human intervention. Eutrophication runs from nutrient enrichment through algal bloom, light blocking, plant death, bacterial respiration and oxygen depletion to the death of fish, and indicator species reveal water and air quality. Bioaccumulation occurs within an organism and biomagnification along a food chain. In situ conservation protects species in their habitat and ex situ in captivity, and the two are complementary.