Difficulty: Introductory to Intermediate | Prerequisites: Parts 1 and 2 of Chapter 4 study notes (classification, organism groups)
This final section of Chapter 4 pulls together the organisms you have already studied and asks: how do we describe and measure the variety of life in marine environments? It introduces the three levels of biodiversity (species, genetic, ecological), explains why marine biodiversity matters to both ecosystems and humans, and then covers the vocabulary of ecology (ecosystem, habitat, niche, population, community) and the biotic and abiotic factors that shape organism distribution. It closes with the practical fieldwork techniques marine ecologists use to estimate populations and investigate distribution in the littoral zone.
Biodiversity operates at three levels (species, genetic, ecological) and underpins ecosystem stability, food security, climate regulation and medical research. Populations and communities are shaped by biotic factors (competition, predation, disease, symbiosis) and abiotic factors (temperature, salinity, light, substrate). Scientists measure populations using mark-release-recapture and the Lincoln index, and investigate distribution using quadrats and transects.
Biodiversity
A measure of the species, genetic and ecosystem diversity of different species in a given area. In simple terms, it describes how much variety of life there is.
Species diversity
The abundance and richness of species in a given place at one time. Abundance is the number of individuals per species; richness is the number of different species present.
Genetic diversity
The variety of forms of genes (alleles) within a species. Populations with low genetic diversity are more vulnerable to environmental change because they have fewer variations to draw on.
Ecological diversity
The variation of ecosystems or habitats on a regional or global level. Think of it as the range of different types of environment available (coral reef, hydrothermal vent, rocky shore, etc.).
Ecosystem
The living organisms (biota) in an area plus the non-living environmental factors that act on them. Can be as large as the Great Barrier Reef or as small as a hydrothermal vent.
Habitat
The natural environment where an organism lives. Defined by where it lives and how it moves (planktonic, nektonic, benthic).
Niche
The role of a species within an ecosystem, including its feeding relationships, spatial position and temporal activity. Think of it as the organism's "job" in its community.
Species
A group of similar organisms that can interbreed naturally to produce fertile offspring.
Population
All the individuals of the same species that live at the same place and time.
Community
An association of all the different populations of species occupying a habitat at the same time.
Biotic factors
The living parts of an ecosystem: the organisms and their effects on each other. Includes feeding relationships (food chains, food webs), predator-prey dynamics, disease and competition.
Abiotic factors
The non-living components of an ecosystem. Geological (substrate, topography), physical (temperature, light, wave action, tides, currents, hydrostatic pressure, turbidity) and chemical (pH, salinity, oxygen, CO₂, nutrient availability).
Intra-specific competition
Competition between individuals of the same species, for example two male fish of the same species competing for a mate.
Inter-specific competition
Competition between members of different species, for example two different predator species competing for the same prey.
Competitive exclusion principle
In a stable ecosystem, no two species can occupy exactly the same niche indefinitely. One will outcompete the other, which will either adapt, move, or die out.
Mark-release-recapture
A method for estimating the population size of mobile species. Individuals are captured, marked and released; a second sample is then taken and the proportion of marked individuals is used to estimate the total population.
Lincoln index
The mathematical formula used with mark-release-recapture data: N = (n₁ x n₂) / m₂, where N is the estimated population, n₁ is the number marked in the first sample, n₂ is the total captured in the second sample, and m₂ is the number of marked individuals recaptured.
Frame quadrat
A plastic or metal square (typically 10 to 100 cm) that sets a standard unit of area for studying the distribution of sessile (immobile) marine organisms.
Transect
A rope or tape marked at regular intervals (typically 30 to 50 m) that sets standard distances for studying organism distribution along an environmental gradient.
Line transect
A sampling method where species touching the transect line are recorded. Can be continuous (whole length) or systematic (at fixed intervals, e.g. every 0.5 m).
Belt transect
A sampling method where a frame quadrat is placed at regular intervals on one side of the transect, and the frequency or percentage cover of organisms is recorded. Gives data on species abundance as well as distribution.
Random sampling
Samples taken at randomly chosen locations within the study site, often selected using a numbered grid and a random number generator. Used when the habitat is fairly uniform.
Species diversity = abundance (number of individuals per species) + richness (number of different species). Coral reefs are among the most species-rich areas on Earth because they have many different ecological niches. Sandy shores, by contrast, have far fewer species.
Genetic diversity = the variety of alleles within a species. Each population has its own gene allele composition and allele frequency. Populations that are inbred have a smaller range of alleles and are less able to adapt to environmental changes. Genetic diversity can be assessed by finding allele frequency per gene.
Ecological diversity = the variation of ecosystems or habitats at a regional or global level. Difficult to measure because each ecosystem merges into the ones around it.
Maintaining stable ecosystems
The more species there are, and the more evenly individuals are distributed among those species, the greater the species diversity.
High species diversity tends to make ecosystems more stable and resistant to ecological change.
Stable, non-extreme environments (e.g. coral reefs, rocky shores) support richer biodiversity than unstable or extreme environments (e.g. hydrothermal vents, reef slopes).
Protection of the physical environment
Root systems of marine plants (seagrasses, mangroves) stabilise muddy substrates and reduce erosion of estuaries and mud flats during storms.
Fringing reefs act as physical barriers, absorbing wave energy and protecting coastlines from tropical storms.
Climate control
High-biodiversity producers (phytoplankton, seagrasses) regulate atmospheric CO₂. Seagrasses store twice as much carbon per hectare as terrestrial rainforests.
Photosynthesis by marine carbon sinks reduces ocean acidification and produces oxygen for aerobic respiration.
Providing food sources
High-biodiversity ecosystems include the primary producers that underpin marine food webs, nurturing the growth of macroalgae, crustaceans and fish that humans depend on.
Source of medicines
41% of beneficial chemicals derived from marine organisms are anti-cancer compounds. Other categories include anti-microbial (24%), anti-inflammatory (22%), anti-foulant (10%), nervous system (2%) and anti-ulcer (1%).
Current medical trials from marine sources include pain-relief drugs from cone snails, schizophrenia medication from marine worms, wound-healing drugs from corals, and anti-cancer drugs from bacteria, bryozoans, fungi, tunicates and nudibranchs.
Keyhole limpet hemocyanin (KLH): a protein from the giant keyhole limpet (Megathura crenulata) that stimulates the immune system. Used to treat autoimmune, inflammatory and infectious diseases; being tested as a vaccine for breast and bladder cancer. Over-harvesting has led to significant population decline; aquaculture research is underway.
Deep-sea trenches are the largest unexplored habitat on Earth. Dermacoccus abyssi, a bacterium from the Mariana Trench, produces dermacozines that may protect against African sleeping sickness. Australian sea sponges produce chondropsins with potential against bone cancer, Alzheimer's, viral infections, diabetes and cardiovascular disorders.
An ecosystem is all living organisms in an area plus the non-living environmental factors acting on them. Five key marine ecosystems (explored further in Chapter 5): the open ocean, tropical coral reef, rocky shore, sandy shore and mangrove forest.
A habitat is the natural environment where an organism lives. Organisms can be classified by how they move in their habitat:
Planktonic: drift in ocean currents (phytoplankton, zooplankton)
Nektonic: actively swim (fish, marine reptiles, mammals)
Benthic: live on the seabed (starfish, crabs, sea cucumbers)
Some organisms cross habitats during their life cycle (e.g. crabs and clams start as planktonic larvae but become benthic adults).
A niche is the role of a species within an ecosystem, including its feeding relationships, spatial position and temporal activity. Niches can be shared via feeding relationships, spatial relationships (different zones of the water column) or temporal relationships (diurnal vs nocturnal feeding).
A population is all individuals of one species at the same place and time. Population increases are caused by reproduction and immigration; decreases by death and emigration. The maximum population an environment can sustain is the carrying capacity. When resources fall short, limiting factors (biotic or abiotic) restrict growth.
A community is all the different populations occupying a habitat at the same time.
Biotic factors include:
Feeding relationships (food chains, food webs, predator-prey dynamics)
Symbiosis (three types: parasitism, commensalism, mutualism). The abundance of hosts affects symbionts (e.g. hard coral growth rates depend on mutualistic zooxanthellae).
Disease: illnesses caused by pathogens such as viruses and bacteria that decrease population size and abundance. Example: stony coral tissue loss disease (SCTLD), first sighted in Florida in 2014, had spread to nearly half of stony coral species on the Florida Reef Tract by 2019.
Intra-specific competition: competition within a species (e.g. two males competing for a mate).
Inter-specific competition: competition between different species (e.g. two predators competing for the same prey). Can lead to niche overlap, greater niche specialisation, or, ultimately, extinction of one species.
Competitive exclusion principle: in a stable ecosystem, no two species can occupy exactly the same niche. If the niches are identical, one species will outcompete the other. However, inter-specific competition more often leads to niche differentiation and specialisation rather than extinction.
Abiotic factors include:
Geological: substrate type and shape of the seafloor (topography)
Physical: temperature, exposure to air, wind and sunlight, wave action, tides, currents, hydrostatic pressure, water turbidity, light availability (intensity and wavelength)
Chemical: pH, salinity, oxygen concentration, CO₂ concentration, nutrient availability (nitrate, phosphate)
Each of these factors can be a limiting factor if it falls outside the tolerance range of a species. For example, hard coral species are limited to waters between 30°N and 30°S, requiring warm, clear, low-turbidity water so that light can reach zooxanthellae for photosynthesis. Too many nutrients (e.g. nitrate and phosphate from agricultural run-off) cause eutrophication, excessive algal growth that smothers coral polyps.
Mark-release-recapture
Used for mobile species where counting every individual is impractical.
Procedure: capture a sample, mark each individual (e.g. non-toxic waterproof paint on molluscs, numbered tags on fish, turtles or sharks), record the number marked (n₁), release them. One to two days later, capture a second sample at the same location. Count the total captured (n₂) and the number of marked individuals recaptured (m₂).
Lincoln index formula:
N = (n₁ x n₂) / m₂
Where N = estimated population size.
Assumptions (if violated, the estimate becomes less accurate):
Marked individuals are unaffected by the tagging process.
Marked individuals disperse throughout the unmarked population.
All animals have the same probability of being marked initially.
Markings are not lost between the two samples.
The second sample is random.
Emigration, immigration, mortality and recruitment are negligible.
All marked animals seen in the second sample are reported.
Worked example: 120 dog whelks marked on first visit; 200 captured on second visit, 40 of which were marked. N = (120 x 200) / 40 = 600.
Investigating distribution and abundance in the littoral zone
Frame quadrat: a plastic or metal square (10 to 100 cm) used to sample sessile species. Best for estimating populations of immobile organisms.
Transect: a rope or tape (30 to 50 m) placed between two points in the direction of an environmental gradient (e.g. high tide mark to low tide mark).
Line transect: species touching the transect line are recorded.
Continuous sampling: species recorded along the entire length.
Systematic sampling: species recorded at fixed intervals (e.g. every 0.5 m).
Belt transect: a frame quadrat is placed at regular intervals on one side of the transect. Frequency or percentage cover of organisms is recorded. This is an example of systematic sampling because samples are taken at fixed intervals. Used to gather data on the zonation of marine species along a gradient (e.g. exposure from high to low tide).
Random sampling: quadrats placed at randomly selected locations within the study site. Often uses a numbered grid and a computer-generated random number to choose sample squares, eliminating observer bias. Best when the habitat is fairly uniform and large (e.g. a flat seagrass meadow).
Ethical and safe methods
An ethical method evaluates and chooses ways to protect the natural habitat under investigation (e.g. accessing a coral reef via a single file from the beach rather than having students walk over the reef in multiple paths).
A safe method identifies potential hazards and reduces their risk (e.g. checking tides and weather forecasts before a rocky shore survey).
The mark-release-recapture method is used worldwide to monitor fish, whale, turtle and shark populations, informing fishery quotas and conservation policy. Quadrat and transect surveys are standard practice in environmental impact assessments before coastal development. The discovery of bioactive compounds in deep-sea organisms (dermacozines, chondropsins, KLH) is driving multi-billion-dollar pharmaceutical research pipelines and making the case for protecting unexplored marine habitats.
Students often confuse species richness with species diversity. Richness counts how many species are present; diversity also takes into account how evenly individuals are distributed among those species.
Students sometimes assume that all abiotic factors are physical. Chemical factors (pH, salinity, nutrient concentration) are also abiotic.
Students may think the competitive exclusion principle means competing species always go extinct. More often, inter-specific competition drives niche specialisation rather than extinction.
Students confuse line transects and belt transects. A line transect records only what touches the line. A belt transect adds quadrats at intervals alongside the line, giving abundance data as well as distribution data.
⚠️ Be able to define and distinguish species diversity, genetic diversity and ecological diversity.
⚠️ Know the Lincoln index formula and be able to apply it in a worked calculation (this appears frequently in exams).
⚠️ Understand the assumptions underlying mark-release-recapture and be able to explain how violating each one affects the accuracy of the estimate.
⚠️ Be able to explain the difference between biotic and abiotic factors, giving named examples of each.
⚠️ Know the difference between intra-specific and inter-specific competition, and be able to state the competitive exclusion principle.
⚠️ Be ready to describe and compare frame quadrats, line transects, belt transects, and random sampling, including when each is most appropriate.
True or false: Species richness and species diversity are the same thing.
Fill in the blank: The Lincoln index formula is N = (n₁ x ______) / m₂.
True or false: A belt transect records only the species that touch the transect line.
Fill in the blank: Competition between individuals of the same species is called ______ competition.
True or false: pH and salinity are examples of abiotic factors.
Q: Define species diversity, genetic diversity and ecological diversity.
A: Species diversity is the abundance and richness of species in a given place at one time. Genetic diversity is the variety of forms of genes (alleles) within a species. Ecological diversity is the variation of ecosystems or habitats on a regional or global level.
Q: A marine biologist captures and marks 100 Patagonian toothfish. On a second visit she captures 70 toothfish. If the estimated population is 700, how many of the recaptured fish were marked?
A: Using N = (n₁ x n₂) / m₂, rearrange to m₂ = (n₁ x n₂) / N = (100 x 70) / 700 = 10. Ten of the recaptured fish were marked.
Q: Explain the competitive exclusion principle and give a marine example.
A: The competitive exclusion principle states that no two species can occupy exactly the same niche in a stable ecosystem. If an introduced species occupies the same niche as a native species (e.g. two top predators feeding on the same prey in the same habitat at the same time), one will outcompete the other, which will decline and potentially become locally extinct.
Q: Describe how a belt transect is used to investigate the zonation of organisms on a rocky shore.
A: A transect line (rope or tape) is placed from the high tide mark down to the low tide mark, in the direction of the environmental gradient. A frame quadrat is placed at regular intervals (e.g. every 2 to 5 m) on one side of the transect. At each point, the species present are identified and their frequency or percentage cover is recorded. This gives data on how species distribution and abundance change with exposure to tidal conditions.
Q: State three assumptions of the mark-release-recapture method.
A: (Any three of) Marked individuals are unaffected by the tagging process; marked individuals disperse throughout the unmarked population; all animals have the same probability of being marked initially; markings are not lost between samples; the second sample is random; emigration, immigration, mortality and recruitment are negligible; all marked animals in the second sample are reported.
Biodiversity measurement links directly to Chapter 5 (marine ecosystems) where you will apply these sampling techniques in specific habitats. Abiotic factors such as temperature, salinity and nutrient run-off connect to Chapter 9 (human impacts on the marine environment), particularly eutrophication and climate change. The competitive exclusion principle and niche theory are core concepts in ecology that run through the entire course.
Biodiversity, species diversity, species richness, species abundance, genetic diversity, allele frequency, ecological diversity, ecosystem, habitat, niche, species, population, community, biome, carrying capacity, limiting factors, biotic factors, abiotic factors, food chain, food web, predator-prey, symbiosis, parasitism, commensalism, mutualism, disease, SCTLD, stony coral tissue loss disease, intra-specific competition, inter-specific competition, competitive exclusion principle, mark-release-recapture, Lincoln index, population estimation, frame quadrat, transect, line transect, belt transect, continuous sampling, systematic sampling, random sampling, ethical method, safe method, littoral zone, zonation, coral reefs, rocky shores, hydrothermal vents, marine medicines, KLH, keyhole limpet hemocyanin