A single sea otter eats about 9 kg of sea urchins per day. Without otters, urchin populations explode, overgrazing kelp forests into barrens that support almost nothing. The otter is a keystone species — its impact is wildly out of proportion to its numbers. Australia has its own keystone predators, and it also carries some of the world’s most devastating introduced species. This lesson connects both stories.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Before you read, commit to a prediction. You will revisit these at the end.
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.
Most species in an ecosystem are redundant — if one small herbivore disappears, another fills its place. But a few species hold the entire structure together. Remove them, and the ecosystem collapses. These are keystone species.
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.
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.
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:
The dingo is not native to Australia in the deep evolutionary sense — it arrived with humans approximately 4,000 years ago. But it has become an ecological keystone, shaping communities from desert spinifex to temperate woodland.
1. Dingo exclusion fences
The dingo fence in South Australia provides a natural experiment. Inside the fence (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 that are 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 populations of 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.
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.
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.
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 not predation; it is toxic disruption of predator populations, causing trophic cascades in reverse.
Introduced as a garden plant and living fence in the 1800s. By the 1920s, prickly pear (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.
When chemical pesticides fail or cause collateral damage, ecologists sometimes turn to biological control: introducing a natural enemy of the invasive species to suppress it. The results range from spectacular success to catastrophic failure.
In 1925, the Cactoblastis cactorum moth was released from South America. Its larvae eat prickly pear from the inside. Within 10 years, the moth had reduced prickly pear cover by over 90%, reclaiming millions of hectares for pasture and native vegetation. It remains one of the most celebrated biological control successes in history.
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 (a dead host is a dead home for the virus). Today, myxoma still suppresses rabbit numbers but no longer controls them alone. The calicivirus (RHDV) was introduced as a supplementary control agent.
Apply keystone species and invasive species concepts to Australian scenarios.
Read the following scenario:
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.
A new invasive ant species arrives in northern Queensland. It has no natural predators in Australia, forms supercolonies that monopolise food resources, and attacks native insects and small vertebrates.
Definition
Keystone species: A 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.
Australian example
Dingo: Keystone predator in Australian ecosystems. Controls kangaroo, rabbit, fox, and cat populations. Dingo exclusion fences show 10× higher kangaroo density and 50% lower ground cover inside (no dingoes) vs outside.
Enemy release hypothesis
Introduced species thrive because they leave behind co-evolved predators, competitors, and pathogens. Without these natural enemies, their populations grow exponentially.
Invasive species examples
European rabbit (overgrazing, soil destabilisation); cane toad (toxic to native predators); prickly pear (covered 24 million ha by 1920s).
Biological control
Cactoblastis cactorum moth successfully controlled prickly pear. Myxoma virus initially controlled rabbits (99.8% mortality) but both host and pathogen evolved. Risks: non-target effects, irreversibility.
Syllabus link
ACSBL050, ACSBL053, ACSBL060: Predict how ecosystem changes affect species distribution and abundance; analyse effects of species on ecosystems; apply keystone species and invasive species concepts.
Now that you have completed the lesson, review your initial answers. What did you get right? What surprised you?
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.
In this lesson you learned: