Course: Intro to Marine Biology, Florida International University | Chapter 2 Source: CH 2 Key Terms Difficulty: Beginner Prerequisites: None, though a basic grasp of general biology terms (organism, cell, energy) will help. Tags: marine ecosystems, biodiversity, ecological niche, trophic levels, food chain, habitat, population ecology, marine biology key terms
This chapter lays the vocabulary foundation for the entire marine biology course. Before you can discuss coral reefs, deep-sea vents, or fisheries management, you need a shared language for how living things are organised and how they relate to one another. The terms here, from "ecosystem" down to "trophic level," reappear in every later chapter, so treating them as throwaway definitions is a mistake. If you are coming in cold, this is the right place to start: everything that follows in the course builds on the hierarchy of species, populations, communities, and ecosystems introduced here.
Marine ecosystems are built from a hierarchy: species form populations, populations form communities, and communities plus their physical environment make up ecosystems. Every organism occupies a niche (broad or narrow) and sits at a trophic level, either making its own energy (producer) or obtaining it by eating others (consumer). Biodiversity measures how much variety exists across species, habitats, and genes in a given area.
Ecosystem
All the living organisms in an area plus the non-living (abiotic) environmental factors that act on them. Think of it as the full package: the fish, the water, the temperature, the salinity, and every interaction between them.
Habitat
The natural environment where an organism lives, providing food, protection, shelter, and mates. In simple terms, it is an organism's address.
Population
All the individuals of the same species living in the same place at the same time and able to reproduce with one another. Think of it as every member of one species in one location.
Community
A group of interdependent organisms of different species inhabiting the same region and interacting with each other at the same time. In simple terms, this is all the different species sharing a neighbourhood.
Species
A group of similar organisms that can interbreed naturally to produce fertile offspring. This is the biological species concept, and "fertile offspring" is the key phrase: a horse and a donkey can mate, but the mule they produce is sterile, so horses and donkeys remain separate species.
Biodiversity
The amount of biological diversity per unit area. It encompasses three levels: species diversity (number and variety of species), habitat diversity (range of different environments), and genetic diversity (variation in genes within a species).
Ecological niche
The specific role a species plays within its ecosystem, including its use of resources and its relationships with other species. Think of it as a species' job description rather than its address (which is the habitat).
Generalised niche
An organism that can eat a wide range of food sources and live in a broad range of habitats. Raccoons and rats are classic generalists: they thrive almost anywhere.
Specialised niche
An organism with a narrow range of physical, biological, or chemical parameters, living in a specific habitat and relying on a very specific diet. Koalas eating almost exclusively eucalyptus leaves are a good example.
Producer
An organism that makes its own food using energy from its surroundings, typically via photosynthesis or chemosynthesis. Also called an autotroph. Phytoplankton are the ocean's most important producers.
Consumer
An organism that obtains energy by feeding on other organisms. Also called a heterotroph. Consumers can be herbivores, carnivores, or omnivores depending on what they eat.
Predator
An animal that hunts, kills, and eats other animals for food.
Prey
An organism that is killed and eaten by another organism.
Trophic level
A position in a food chain occupied by a group of organisms with a similar feeding mode. Producers sit at the first trophic level, primary consumers at the second, secondary consumers at the third, and so on. Energy decreases at each step (roughly 10% is transferred up).
Species is the smallest unit: organisms that can interbreed and produce fertile offspring.
Population groups individuals of the same species in one place.
Community brings together populations of different species in the same region.
Ecosystem adds the abiotic (non-living) factors: temperature, salinity, light, substrate.
The hierarchy runs: species → population → community → ecosystem. Exam questions often ask you to arrange these in order.
A habitat is where an organism lives (its address).
A niche is what an organism does (its job): how it feeds, what it competes with, what role it fills.
Generalised niche (generalist): broad diet, flexible habitat. These species are more resilient to environmental change because they have options.
Specialised niche (specialist): narrow diet, specific habitat. These species are efficient in stable conditions but vulnerable when conditions shift.
Key distinction: habitat = location, niche = function. This is a favourite exam question.
Producers (autotrophs) form the base. In the ocean, phytoplankton do the heavy lifting, converting sunlight into chemical energy via photosynthesis.
Consumers (heterotrophs) occupy higher trophic levels:
Primary consumers eat producers (herbivores, e.g. zooplankton).
Secondary consumers eat primary consumers (carnivores, e.g. small fish).
Tertiary consumers eat secondary consumers (top predators, e.g. sharks).
Energy transfer between trophic levels is inefficient: roughly 10% passes up, 90% is lost as heat. This is why top predators are always rarer than the organisms they feed on.
Predator and prey describe a specific feeding relationship within this structure.
Three components to remember:
Species diversity: the number and variety of species in an area.
Habitat diversity: the range of different environments available.
Genetic diversity: the variation in genes within a single species.
Higher biodiversity generally means a more stable, resilient ecosystem. If one species is lost, others can fill its role.
The 10% energy-transfer rule explains why overfishing top predators (tuna, sharks) collapses marine food webs disproportionately: there is very little energy at the top, and populations recover slowly. Understanding niche specialisation matters for conservation: specialist species like certain coral-dwelling fish are the first to disappear when reefs degrade, because they have no alternative habitat. Biodiversity assessments (species, habitat, and genetic diversity) are the standard tool marine biologists use to evaluate the health of an area before, during, and after events like oil spills or coastal development.
Students often confuse habitat and niche. Habitat is where an organism lives; niche is what it does there (its role, diet, interactions). They are tested separately.
Students sometimes think community and ecosystem are interchangeable. A community is only the living organisms. An ecosystem includes the non-living factors (water, temperature, light) as well.
The idea that "higher trophic level = more energy" is backwards. Energy decreases as you move up. Top predators have the least available energy, which is why they exist in smaller numbers.
Biodiversity is not just "how many species." Students forget the habitat diversity and genetic diversity components. All three are part of the definition.
Expect a question asking you to arrange the levels of organisation in order: species → population → community → ecosystem.
The habitat vs. niche distinction is a perennial exam favourite. Be able to define both and explain the difference in one sentence.
Know the three components of biodiversity (species, habitat, genetic) and be prepared to give an example of each.
The 10% energy-transfer rule across trophic levels comes up frequently. Be ready to explain why top-predator populations are small.
Generalised vs. specialised niche: know an example of each and be able to say which is more vulnerable to environmental change and why.
True or false: A community includes both living organisms and non-living factors.
False. That describes an ecosystem. A community is only the living organisms.
Fill in the blank: An organism that makes its own food is called a ______.
Producer (or autotroph).
True or false: A specialist species is more vulnerable to habitat loss than a generalist.
True. Specialists depend on narrow conditions and specific resources.
Fill in the blank: Roughly ______% of energy is transferred from one trophic level to the next.
10%.
True or false: Biodiversity refers only to the number of species in an area.
False. It also includes habitat diversity and genetic diversity.
Q: Define "ecosystem" and explain how it differs from a "community."
A: An ecosystem includes all living organisms in an area plus the non-living environmental factors (temperature, salinity, light, substrate) that act on them. A community includes only the living organisms interacting in that area. The difference is the abiotic component.
Q: Explain the difference between a generalised niche and a specialised niche. Give one example of each.
A: A generalist can exploit a wide range of food sources and habitats (e.g. raccoons, rats). A specialist depends on a narrow set of conditions or a specific diet (e.g. koalas eating eucalyptus, or coral-dwelling gobies). Specialists are more efficient under stable conditions but more vulnerable to environmental change.
Q: Why are top predators typically less abundant than organisms at lower trophic levels?
A: Only about 10% of energy is transferred from one trophic level to the next. By the time energy reaches top predators, there is far less of it available, so fewer individuals can be supported.
Q: Name and briefly define the three components of biodiversity.
A: Species diversity (the number and variety of species), habitat diversity (the range of different environments), and genetic diversity (the variation in genes within a species).
Q: A coral reef contains clownfish, parrotfish, sea urchins, algae, and various coral species, all interacting with each other. Is this a population, a community, or an ecosystem? Explain.
A: As described (multiple species interacting), it is a community. If you also include the abiotic factors, such as water temperature, salinity, light penetration, and the calcium carbonate substrate, then it becomes an ecosystem.
This chapter connects directly to food webs and energy flow (typically Ch. 3 or later), where the trophic-level concept is expanded into full food-web diagrams. The biodiversity definitions here underpin conservation biology chapters that discuss threats to marine ecosystems (pollution, overfishing, climate change). The generalist-vs.-specialist framework returns when you study adaptation and natural selection: specialists evolve under stable conditions, generalists under variable ones.
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