Year 11 Biology Module 4 · Ecosystem Dynamics ⏱ ~35 min 5 MC · 3 Short Answer Lesson 14 of 23

Keystone Species, Introduced Species and Ecological Disruption

In 1974, James Estes and John Palmisano published their study of sea otter populations in the Aleutian Islands, comparing kelp forest ecosystems with and without otters. In otter territories, sea urchin density averaged 0.5 per m², and kelp formed dense forests. In otter-free zones just kilometres away, urchin density reached 50 per m², and the seafloor was a barren. Each otter ate 25% of its own body weight in urchins per day, and its removal allowed urchins to graze kelp to the roots. Estes and Palmisano's paper is the most-cited keystone species study in ecology and defined the concept that one species can hold an entire ecosystem together.

Today's hook: Estes and Palmisano's 1974 Aleutian Islands study found otter territories had 0.5 sea urchins per m² (dense kelp), while otter-free zones had 50 urchins per m² (barren rock). A 100-fold difference in urchin density from the presence or absence of one predator. Using this logic, what would you predict happens to the dingo's ecosystem if dingoes are removed from grazing land, and is a student's claim that "removing dingoes only helps farmers" ecologically justified?

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Orient and predict

Warm up on earlier lessons, then commit to your own prediction about what one species can do to a whole ecosystem.

Beyond the syllabus. This whole lesson is classified extension: keystone species and introduced-species disruption are a useful application rather than a required standalone knowledge area in Module 4. The species interactions themselves (predation, competition, symbiosis) are core and examinable — treat this lesson as enrichment that deepens them.

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Worksheets

Practise this lesson

Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.

Build Foundations & guided practice Apply Application & data response Master Mastery tasks Exam Exam-style questions

Comparison of three symbiotic relationships, mutualism (plus/plus), commensalism (plus/zero) and parasitism (plus/minus), with a definition and examples for each.

Before You Read, Think First

warm-up

Q1. If dingoes were eradicated from the Simpson Desert, predict what would happen to the spinifex grassland and the animals that live in it within five years. Consider herbivores, vegetation, and soil in your prediction.

Q2. The cane toad was introduced to Australia to control beetle pests in sugar cane. It failed to control the beetles but thrived in the wild. Predict why an introduced species might thrive even when it fails at its intended purpose.

Next up

Frame the vocabulary

Load the terms that separate a keystone species from a merely abundant one, and an introduced species from an invasive one.

Key Terms, scan these before reading

vocab

Keystone speciesA species whose impact on ecosystem structure and function is disproportionately large relative to its biomass or abundance. Its removal causes disproportionate ecosystem collapse or restructuring.

Enemy release hypothesisIntroduced species often thrive because they leave behind their co-evolved natural enemies (predators, competitors, pathogens) that would normally limit population growth in the native range.

Mesopredator releaseAn increase in mid-level predator populations (foxes, cats) when apex predators (dingoes) are removed, because the suppression effect of the apex predator is lost.

Biological controlThe deliberate introduction of a natural enemy of an invasive species to suppress it. Can be highly effective but carries risks of non-target effects and irreversibility.

Novel weapons hypothesisIntroduced species may produce chemical or physical traits that native species have not evolved resistance to, giving the invader a competitive advantage.

Cross-lesson links: L13 showed trophic cascades in shark-dugong-seagrass systems. L14 examines the same cascade in a different biome, Estes and Palmisano's sea otter study is the most-cited keystone species paper in ecology and directly tests your food web knowledge from L03.

Next up

Keystone species

See why impact is measured against biomass, not abundance, and what happens when a keystone is removed.

Core Content

Keystone Species, Disproportionate Impact

+5 XP

A keystone species has an impact disproportionately large relative to its biomass. Removing it causes ecosystem-wide collapse, not just local decline.

In 1974, James Estes and John Palmisano surveyed two sites in the Aleutian Islands: one where sea otters were present, one where they were absent. Otter territories had 0.5 sea urchins per m² and dense kelp forests. Otter-free zones had 50 urchins per m² and barren rock. The otter comprised a tiny fraction of the ecosystem's biomass, yet its presence or absence restructured everything above and below it in the food web. Each otter consumed 25% of its body weight in urchins daily, just enough to prevent the urchin population from exploding. Remove that one species, and the entire kelp forest ecosystem disappears. That disproportionate structural importance is what makes a species a keystone.

Sea Otter (North Pacific)

Sea otters control sea urchin populations. Without otters, urchins overgraze kelp forests, converting them to "urchin barrens", bare rock with almost no biodiversity. With otters, kelp forests support hundreds of fish, invertebrate, and bird species. The otter's biomass is tiny compared to the kelp forest it protects.

African Elephant (African Savanna)

Elephants create waterholes by digging during drought, knock down trees to create grassland patches for grazers, and disperse seeds over vast distances. Their browsing prevents any single tree species from dominating, maintaining habitat diversity for dozens of herbivore species.

Bees and Pollinators (Global)

Approximately 75% of crop species and 90% of flowering plant species depend on animal pollination. Bees are the dominant pollinators in most terrestrial ecosystems. Their loss would collapse plant reproductive systems, eliminating food and habitat for herbivores, insectivores, and carnivores.

How to identify a keystone species in an exam

Look for a species with low biomass but high impact
Check whether its removal causes ecosystem-wide restructuring (not just local decline)
Identify whether it controls a dominant competitor or resource monopolist
Confirm that its impact is greater than would be predicted from its abundance alone

A keystone species has a disproportionately large impact on ecosystem structure relative to its biomass or abundance, its removal causes ecosystem-wide collapse, not just local decline.

Pause, copy the highlighted definition into your book before continuing.

Which characteristic best defines a keystone species?

Next up

The dingo, an Australian keystone predator

Read the dingo fence as a continent-scale natural experiment in top-down control.

The Dingo, Australia's Keystone Predator

+5 XP

We just identified what makes a species a keystone. That raises a question: does Australia have its own keystone predators, and what is the evidence? This card answers it the dingo case study provides direct experimental evidence via exclusion fences.

The dingo arrived with humans ~4,000 years ago but has become an ecological keystone, shaping communities from desert spinifex to temperate woodland

1. Dingo exclusion fences

Inside the dingo fence in South Australia (no dingoes): kangaroo density is 10× higher, ground cover is 50% lower, and soil erosion is severe. Outside the fence (dingoes present): kangaroo populations are controlled, vegetation is healthier, and erosion is reduced.

2. Mesopredator suppression

Dingoes kill foxes and feral cats, smaller predators devastating to native mammals, birds, and reptiles. Where dingoes are present, fox and cat activity is lower, and native species have higher survival rates.

3. Vegetation structure

By controlling kangaroo, rabbit, and wallaby grazing, dingoes indirectly protect ground cover, shrub regeneration, and tree seedlings. This maintains habitat complexity for ground-dwelling fauna.

4. Desert ecosystems

In the Simpson Desert, dingoes suppress introduced house mice and rabbits during boom years, preventing them from stripping spinifex grassland to bare sand. Without dingoes, rodent plagues can convert grassland to desert.

Australian context: The debate over dingo management is contested. Livestock producers view dingoes as pests; ecologists view them as essential ecosystem engineers. The science is clear: where dingoes persist, biodiversity is higher. Some researchers advocate for management that preserves dingo populations in conservation areas while controlling hybrid dogs near livestock.

Dingo removal causes mesopredator release (more foxes and cats) and increases herbivore density, the exclusion fence provides direct evidence that dingoes are a keystone predator shaping vegetation structure and native mammal survival.

Pause, copy the highlighted dingo evidence summary into your book.

The dingo exclusion fence in South Australia provides evidence that:

Next up

Introduced species as disruptors

Work out why a species that is unremarkable at home can dominate somewhere new.

Introduced Species, Ecological Disruptors

+5 XP

We've seen how a native predator can be a keystone. Now the question flips: what happens when a species arrives from outside with no natural controls? This card explains the enemy release hypothesis and Australia's most damaging introduced species.

When a species is moved to a new ecosystem, it leaves behind its natural predators, competitors, and diseases. Released from these constraints, it can grow exponentially and displace native species. This is the enemy release hypothesis.

Enemy release

No co-evolved predators, parasites, or pathogens to limit population growth.

Novel weapons

Chemical or physical defences that native species have not evolved resistance to.

Vacant niches

The new ecosystem may lack competitors for the same resource, allowing rapid colonisation.

High reproduction

Many invasive species are selected for rapid reproduction and dispersal in disturbed habitats.

European Rabbit

Introduced in 1859 for hunting. By the 1920s, rabbits covered 70% of Australia. They dig warrens that destabilise soil, overgraze pastures competing with native bilbies and bandicoots, and prevent tree regeneration by eating seedlings. Their impact on soil structure and vegetation is still visible 160 years later.

Cane Toad

Introduced in 1935 to control beetle pests in Queensland cane fields. The toads failed to control beetles but thrived in the wild, spreading across northern Australia. They are toxic to most native predators, quolls, goannas, snakes, and freshwater crocodiles die after eating them. This is toxic disruption of predator populations, causing trophic cascades in reverse.

Prickly Pear

Introduced as a garden plant in the 1800s. By the 1920s, Opuntia stricta covered 24 million hectares of Queensland and NSW, an area larger than Britain. It smothered pasture, prevented stock movement, and eliminated native ground cover.

The enemy release hypothesis states that introduced species thrive because they leave behind their co-evolved predators, competitors, and pathogens, allowing exponential population growth in the new ecosystem.

Pause, copy the highlighted enemy release definition into your book.

Next up

Biological control

Weigh the Cactoblastis success against the myxoma and cane toad outcomes, and note that release is irreversible.

Biological Control, Fighting Species with Species

+5 XP

We've seen introduced species thrive through enemy release. That raises a management question: can we fight invasions by deliberately reintroducing enemies? This card examines biological control, from spectacular success to cautionary failure.

When chemical pesticides fail or cause collateral damage, ecologists sometimes introduce a natural enemy of the invasive species to suppress it. The results range from spectacular success to catastrophic failure.

Success: Cactoblastis Moth vs Prickly Pear

In 1925, the Cactoblastis cactorum moth was released from South America. Its larvae eat prickly pear from the inside. Within 10 years, the moth reduced prickly pear cover by over 90%, reclaiming millions of hectares. It remains one of the most celebrated biological control successes in history.

Partial Success: Myxoma Virus vs Rabbits

Released in 1950, the myxoma virus initially killed 99.8% of infected rabbits. But both host and pathogen evolved. Rabbits developed genetic resistance, and the virus evolved lower virulence. Today, myxoma still suppresses rabbit numbers but no longer controls them alone. The calicivirus (RHDV) was introduced as a supplementary control agent.

Risks of biological control: A control agent may attack non-target native species. The cane toad itself was a failed biological control attempt. Any proposed biocontrol agent must undergo years of host-specificity testing in quarantine before release. Even then, evolution can shift host preferences over time. Biological control is irreversible, once released, the agent cannot be recalled.

Biological control uses a natural enemy to suppress an invasive species. Success: Cactoblastis moth reduced prickly pear by >90%. Partial success: myxoma virus reduced rabbits by 99.8% initially but coevolution reduced effectiveness. Risks: non-target effects, irreversibility.

Pause, copy the highlighted biological control summary into your book.

Last learn step

Analyse a keystone, evaluate a control

Analyse a keystone species yourself, then judge biological control against its risks and alternatives.

Activity 1, Keystone Species Analysis

ClassifyBand 3–4

In a coastal woodland, researchers noticed that where goannas were abundant, ground-nesting bird species were also abundant. Where goannas had been removed by landowners, snake populations increased, ground-nesting bird eggs were heavily predated, and shrub regeneration declined because seeds were not dispersed.

Explain whether the goanna is likely to be a keystone species in this ecosystem. Use the definition of keystone species in your answer. (2 marks)
Describe one direct effect and one indirect effect of goanna removal on the woodland ecosystem. (2 marks)
Connect this scenario to Lesson 13. How does goanna removal resemble dingo removal in terms of trophic cascade? (1 mark)

Activity 2, Biological Control: Benefits, Risks and Alternatives

EvaluateBand 5

Prickly pear cactus (Opuntia stricta) was introduced to Queensland and spread across 24 million hectares by 1925. The Cactoblastis moth was introduced as biological control and reduced prickly pear to less than 1% of its former range within 10 years.

Explain why prickly pear is classified as an introduced species rather than a keystone species. (2 marks)
Evaluate the success of the Cactoblastis moth introduction using two criteria: effectiveness and safety. (3 marks)
A student suggests biological control is always preferable to chemical control. Evaluate this claim using the prickly pear case and one counter-example. (3 marks)

According to the enemy release hypothesis, why do introduced species often thrive in new ecosystems?

What makes the cane toad particularly destructive to Australian ecosystems?

Independent practice

Show what you can do without prompts

Answer a fresh multiple-choice set and three exam-style short answers, then compare your reasoning with the model answers.

Multiple Choice

+5 XP

A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct

Short Answer

+5 XP

ApplyBand 4(4 marks) 1. Define keystone species and explain why the sea otter is considered a keystone species in North Pacific kelp forest ecosystems.

AnalyseBand 4(4 marks) 2. Explain the enemy release hypothesis and describe how it applies to the European rabbit in Australia. In your answer, explain why rabbits were able to reach such high population densities after introduction.

EvaluateBand 5–6(6 marks) 3. If dingoes were eradicated from the Simpson Desert, predict the effects on spinifex grassland, native mammals, and introduced species over five years. Integrate at least three concepts from this lesson or Lesson 13 in your answer.

Show all answers

Short Answer Model Answers

Q1 (4 marks): A keystone species is a species whose impact on ecosystem structure and function is disproportionately large relative to its biomass or abundance. Removing it causes disproportionate ecosystem collapse or restructuring (1 mark). The sea otter is a keystone species because its biomass is tiny compared to the kelp forest it protects (1 mark). Sea otters prey on sea urchins, preventing urchin populations from exploding. Without otters, urchins overgraze kelp, converting kelp forests to "urchin barrens" with drastically reduced biodiversity (1 mark). With otters present, kelp forests persist and support hundreds of fish, invertebrate, and bird species. The otter's impact is far greater than its numbers would predict (1 mark).

Q2 (4 marks): The enemy release hypothesis states that introduced species often thrive because they leave behind their co-evolved natural enemies, predators, competitors, parasites, and pathogens, that would normally limit their population growth in their native range (1 mark). European rabbits in Australia were released from control by native predators, diseases, and competitors that existed in Europe (1 mark). In Australia, rabbits faced few effective predators (dingoes were persecuted, and eagles could not control populations at ground level). No native diseases specifically targeted rabbits (1 mark). Released from these constraints, rabbit populations grew exponentially, reaching densities of over 600 million by the 1920s and covering 70% of the continent (1 mark).

Q3 (6 marks): Spinifex grassland: without dingoes, kangaroo, rabbit, and house mouse populations would increase due to reduced predation pressure (trophic cascade from L13). Increased herbivory would reduce spinifex cover, exposing soil to wind erosion and reducing habitat for ground-dwelling reptiles and invertebrates (1 mark). Soil erosion would increase, potentially converting productive grassland to degraded scrub (1 mark). Native mammals: dingo removal would cause mesopredator release, feral cats and foxes would increase because dingoes currently suppress them (1 mark). Higher cat and fox densities would increase predation on small native mammals (bilbies, mulgaras, dunnarts), causing population declines or local extinctions (1 mark). Introduced species: rabbits and house mice would thrive due to enemy release combined with reduced dingo predation. During rainfall boom years, rodent plagues could strip grassland to bare sand (1 mark). The loss of dingo suppression would amplify all existing impacts of introduced herbivores, potentially triggering further invasive species expansion (1 mark).

Final step

Retrieve, reflect and finish

Check what actually stuck, revisit your opening predictions, and save the completed lesson.

Check what actually stuck

Take the full module quiz

quiz

A full module quiz covering every lesson in this module, not just this one. Set aside a decent block of time and treat it like a real assessment.

Start the module quiz →

Revisit Your Thinking

Estes and Palmisano's 1974 Aleutian Islands study found a 100-fold difference in sea urchin density (0.5 vs 50 per m²) between otter-present and otter-absent zones. Kelp forests existed in one and not the other. The sea otter was a keystone species: its removal restructured the entire ecosystem even though it represented a tiny fraction of total biomass. The same principle applies to the dingo in Australian rangelands, remove the apex predator, mesopredators (foxes, cats) release, herbivores overgraze, vegetation cover falls, soil erodes, native mammals decline. The cascade logic is identical.

Return to your Think First response. Write one definition of keystone species and one definition of enemy release hypothesis from memory, then apply each to one Australian example.

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