Difficulty: Introductory | Prerequisites: Basic food web concepts
Coral reefs support more species per square metre than almost any other ecosystem on Earth, but they are fragile. This topic covers the abiotic and biotic factors that control reef health, how removing apex predators triggers trophic cascades, the key ecological principle of competitive exclusion, and the three levels of biodiversity. It also covers why marine biodiversity matters for human health and the global economy.
Coral reefs depend on light-dependent zooxanthellae, so any abiotic factor that blocks light (like turbidity) limits reef growth. Removing sharks triggers a trophic cascade that ends in algae smothering coral. Biodiversity is measured at three levels (genetic, species, ecosystem), and the competitive exclusion principle says two species sharing one niche cannot coexist indefinitely.
Abiotic factor
A non-living physical or chemical component of an ecosystem, such as temperature, salinity, light availability, or water turbidity. In simple terms, anything in the environment that is not alive.
Biotic factor
A living component of an ecosystem that affects other organisms, such as predation, competition, disease, or symbiosis.
Turbidity
A measure of how cloudy or murky water is, caused by suspended particles (sediment, plankton, pollution). High turbidity reduces light penetration, limiting photosynthesis by zooxanthellae in coral and by macroalgae.
Zooxanthellae
Symbiotic photosynthetic dinoflagellates living inside coral tissue. They provide the coral with up to 90% of its energy through photosynthesis. When stressed (by heat, turbidity, or pollution), coral expels them, causing bleaching.
Trophic cascade
An ecological chain reaction where a change at one trophic level ripples through the food web. Removing an apex predator can cause populations at lower levels to increase or decrease in alternating fashion.
Species richness
The total number of different species present in a given area. It counts species, not individuals.
Species abundance
The number of individuals of each species in a given area. A habitat can have high abundance (thousands of organisms) but low richness (only one or two species).
Genetic diversity
The variety of different gene alleles within a single population or species. Higher genetic diversity means a population is better equipped to adapt to environmental changes.
Competitive exclusion principle
The ecological rule stating that two species competing for identical resources in the same niche cannot coexist indefinitely. One will outcompete the other, causing it to die out or shift to a different niche. Also called Gause's principle.
Ecological niche
The full range of environmental conditions and resources a species uses, including its habitat, diet, activity period, and interactions with other species.
Abiotic factors that limit coral growth: high water turbidity (reduces light for zooxanthellae), temperature extremes (cause bleaching), ocean acidification (dissolves calcium carbonate skeletons), low salinity.
Biotic factors affecting reefs: predation by crown-of-thorns starfish, interspecific competition for space, bacterial disease outbreaks.
Exam tip: the question asks specifically for an abiotic factor. Predation, competition, and disease are all biotic. Turbidity is the abiotic answer.
Sharks are apex predators that regulate mid-level predators like groupers.
When sharks are removed (by overfishing, finning), grouper populations increase unchecked.
More groupers means more consumption of herbivorous fish (e.g. yellow tang, parrotfish).
Fewer herbivores means less grazing on algae.
Algae grow unchecked, outcompete and smother coral polyps.
The reef shifts from a coral-dominated to an algae-dominated system, with a dramatic loss of biodiversity.
This chain reaction (apex predator removal, mid-predator release, herbivore decline, algae takeover) is the textbook coral reef trophic cascade.
Two species occupying identical ecological niches in the same habitat cannot coexist indefinitely.
Competition for the same limited resources (food, space, light) will favour whichever species is better adapted.
The losing species either goes locally extinct or shifts to a different niche (niche partitioning).
Marine example: if an invasive predator is introduced that feeds on the same prey, at the same time, in the same place as a native species, the more efficient hunter will thrive while the other declines.
Genetic diversity: Variation in gene alleles within a single population. Measured at the DNA level. Higher genetic diversity means better adaptive potential.
Species diversity: Combines species richness (how many species) and species abundance (how many individuals per species). A community with 10 species evenly distributed is more diverse than one with 10 species where 95% of individuals belong to just one.
Ecosystem diversity: The variety of different ecosystems (coral reef, mangrove, kelp forest, open ocean) within a region.
Richness = count of different species. A reef with 200 fish species has higher richness than one with 50.
Abundance = count of individuals per species. A reef could have 10,000 fish but only 2 species (high abundance, low richness).
Both are components of species diversity, but they measure different things. Richness alone does not tell you how evenly organisms are spread across species.
Medicine: Marine organisms are sources of new pharmaceutical compounds (e.g. cone snail venom used in pain management research, compounds investigated for schizophrenia treatment).
Climate regulation: Seagrass beds, mangroves, and phytoplankton act as carbon sinks, absorbing atmospheric CO2.
Economy: Biodiversity supports commercial and recreational fisheries, and ecotourism industries (dive tourism, wildlife photography). A single whale shark in Belize can generate an estimated $2 million in ecotourism revenue over its lifetime.
Coastal protection: Coral reefs and mangroves buffer coastlines against storm surge and erosion.
Food security: Bony fish (especially Peruvian anchoveta) are a primary global protein source.
Students often confuse species richness with species abundance. Richness is the number of species; abundance is the number of individuals. A pond with 5,000 organisms of one species has high abundance but a richness of just 1.
Students often classify predation or disease as abiotic factors. Anything involving a living organism (predator, pathogen, competitor) is biotic. Turbidity, temperature, salinity, and pH are abiotic.
Students often think the competitive exclusion principle means the weaker species always goes extinct globally. In practice, the losing species may persist by shifting to a slightly different niche (niche partitioning), though the exam typically frames it as local extinction or exclusion.
Students often describe "genetic diversity" as the number of species. Genetic diversity operates within a single species or population, referring to allele variation. Species diversity is the one that counts different species.
⚠️ "Name an abiotic factor limiting coral reef growth" is a common multiple-choice question. The answer is high water turbidity. Do not pick predation, competition, or disease.
⚠️ The shark-removal trophic cascade (sharks, groupers, herbivores, algae, coral) is a high-value short answer or essay question.
⚠️ Know the distinction between species richness and species abundance. This appears as a 5-mark short answer.
⚠️ Genetic diversity refers to allele variation within a population. "Which level of biodiversity describes variation of gene alleles?" is a standard MCQ.
⚠️ The competitive exclusion principle + a marine example is a 5-mark question.
The trophic cascade concept explains why marine protected areas that ban shark fishing often see healthier coral reefs downstream. The economic arguments for biodiversity (medicine, fisheries, tourism) are increasingly used in policy debates about marine conservation funding.
Is predation by crown-of-thorns starfish an abiotic or biotic factor?
Fill in the blank: Genetic diversity describes variation in ______ within a single population.
True or false: Species richness accounts for the number of individuals per species.
What happens to herbivore populations on a coral reef when sharks are removed?
Name three reasons marine biodiversity matters for human welfare.
Answers: 1. Biotic. 2. Gene alleles. 3. False (that is species abundance). 4. They decline, because mid-level predators (groupers) increase and overconsume them. 5. Any three of: pharmaceutical discovery, climate regulation via carbon sinks, commercial fisheries, ecotourism revenue, coastal storm protection.
Q: Which of the following is an abiotic factor that could limit coral reef growth: predation by crown-of-thorns starfish, high water turbidity, interspecific competition, or bacterial disease?
A: High water turbidity. It is a physical (non-living) factor that reduces light penetration, limiting photosynthesis by the coral's mutualistic zooxanthellae.
Q: Explain how the loss of sharks can indirectly lead to declining biodiversity on a coral reef.
A: Sharks regulate mid-level predators like groupers. Without sharks, grouper populations rise and overconsume herbivorous fish (e.g. yellow tang). Fewer herbivores means algae grow unchecked, outcompete coral for space, and the reef shifts from coral-dominated to algae-dominated, with a loss of overall biodiversity.
Q: Distinguish between species richness and species abundance.
A: Species richness is the total number of different species in an area. Species abundance is the number of individuals of each of those species. Both contribute to species diversity, but richness does not account for how individuals are distributed, and high abundance does not guarantee high diversity.
Q: Which level of biodiversity describes the variation of gene alleles within a single population?
A: Genetic diversity.
Q: Define the competitive exclusion principle and give one marine example.
A: The principle states that two species cannot occupy the same ecological niche in the same habitat indefinitely. One will outcompete the other for limited resources. Example: if an invasive marine predator feeds on the same prey as a native species in the same area and at the same time, the more efficient hunter thrives while the other declines or is excluded.
Q: Identify three reasons why maintaining marine biodiversity is important for human health or the global economy.
A: (1) Marine organisms are sources of new medicines (e.g. compounds from cone snails). (2) Carbon sinks like seagrass and phytoplankton regulate atmospheric CO2. (3) Biodiversity supports commercial fisheries and ecotourism (e.g. a single whale shark in Belize can generate $2 million in lifetime dive tourism revenue).
The trophic cascade here mirrors the sea otter / sea urchin / kelp cascade in kelp forest ecology, both demonstrating top-down control. The biodiversity levels (genetic, species, ecosystem) connect to field methods, since tools like mark-release-recapture estimate population sizes that feed into abundance data. Competitive exclusion links to niche theory and resource partitioning, which appear in broader ecology modules.
Coral reef, abiotic factor, biotic factor, turbidity, zooxanthellae, coral bleaching, trophic cascade, food web, apex predator, mesopredator release, shark removal, grouper, herbivore, algae overgrowth, species richness, species abundance, species diversity, genetic diversity, allele, ecosystem diversity, competitive exclusion, Gause's principle, ecological niche, niche partitioning, marine biodiversity, pharmaceuticals, carbon sink, ecotourism, whale shark, fisheries, coastal protection, OCB 3043, FIU marine biology