What if an invasive species entered an ecosystem?

Every place has its own plants and animals. They live together.

What if a new animal arrived from far away? Would it fit in, or cause trouble?

Every ecosystem, from a forest to a pond, is a web of plants and animals that live together. Some eat, some are eaten, and many compete for the same food and space.

People sometimes carry plants and animals to places where they don't naturally live, on purpose or by accident. Many cause no trouble. But some spread fast and harm the living things already there. Those are called invasive species. What happens when one arrives? This lab explores two real stories.

Not every non-native species is invasive. The U.S. National Invasive Species Information Center stresses that the terms are not interchangeable: domestic cows, for example, are non-native to North America but beneficial in agriculture. A species is invasive when its introduction causes, or is likely to cause, economic, environmental or health harm.

This thought experiment follows two documented introductions to Australia, cane toads and European rabbits, and asks what changed, what is known, and what remains reasoning rather than measurement.

In a healthy place, many kinds of plants and animals share the land.

Some animals eat plants. Some animals eat other animals. Each one has its place.

In an ecosystem, living things are connected. Plants grow, plant-eaters feed on them, and predators eat the plant-eaters. Over a very long time, the plants and animals of a place have come to live alongside each other.

The animals already living in a place are called native species. The plants they eat, the places they shelter and the animals that eat them are part of their everyday life.

Native species in an ecosystem interact through predation, competition, herbivory and other relationships. Those interactions, along with food supply and habitat, influence how large each population becomes.

When a species is introduced, it enters that web of relationships from outside. NOAA notes that invasive species can compete with native organisms for limited resources, alter habitats, reduce biodiversity and even cause extinctions.

In 1935, people brought cane toads to Australia. They hoped the toads would eat beetles.

In 1859, a man got 24 rabbits from England. Both animals spread far across the land.

In 1935, an insect scientist brought 102 cane toads from Hawaii to Queensland, Australia. He bred them, and that August 2,400 toads were let go, in the hope that they would eat beetle grubs that were killing sugar cane. There is no record that anyone tested first whether the toads would eat those beetles. Cane toads are poisonous, and native animals that try to eat them often die. Today they live across Queensland, the Northern Territory and into New South Wales and Western Australia.

In 1859, a settler named Thomas Austin received 24 rabbits from England. Rabbits spread across Australia at more than 60 miles a year, and about 200 million live there now. Genetic work suggests most of them may come from Austin's 24.

In June 1935 the entomologist Reginald Mungomery captured 102 toads in Hawaii; all but one survived the trip to Gordonvale, Queensland, where they were left to breed, and on 19 August 2,400 toads were released around Gordonvale as a biological control for cane beetles. There is no evidence of pre-release testing of whether they ate the beetles, and it went ahead despite warnings from other scientists. Their toxins stop the heart of most animals that try to eat them.

Rabbits, probably mostly from a shipment of 24 that reached Thomas Austin at Christmas 1859 spread at more than 60 miles a year, covering the continent within half a century. Australia now has roughly 200 million, threatening around 300 kinds of plants and animals.

Rabbits have lots of babies. They eat grass and crops.

Cane toads are poisonous. Animals that try to eat them get very sick.

Each invasive species has its own reasons for spreading. European rabbits eat through pastures and crops and compete with native animals for food. Scientists think the rabbits Austin received had wild ancestors, which may have helped them survive in the wild.

Cane toads harm native animals in three ways, according to Australian invasive-species experts: they eat them, they compete with them, and they poison animals that try to eat them. A newcomer might also spread faster when few predators hunt it. For rabbits, Smithsonian Magazine reported that people at the time were hunting the rabbits' predators and turning land into pasture. How much each reason mattered is an idea, not a measurement.

PestSmart, from Australia's Centre for Invasive Species Solutions, lists the cane toad's impacts as predation, competition and lethal poisoning of animals that ingest its bufadienolide toxins. Geneticists who traced Australia's rabbits suggest the wild ancestry of Austin's animals may partly explain why they thrived, since wild rabbits are probably better than domestic ones at avoiding predators.

Why a particular introduction becomes invasive differs from case to case. For rabbits, Smithsonian (citing Science) adds that 19th-century settlers were turning land into pasture and hunting rabbits' predators, which made survival easier, while ecologist David Peacock argues that several introductions, not Austin's alone, drove the invasion. How much each factor mattered is not measured here, so in this lab those explanations are labeled ideas.

People work hard to stop new pests arriving.

You can help too: never let a pet go free outside.

Experts say invasive species are spread mostly by people, often by accident: in the water ships carry for balance, in releases of unwanted pets or aquarium animals, and in other ways. That means people can also help stop them.

The U.S. Fish and Wildlife Service works to prevent, remove and control invasive species. Australia's invasive-species experts say the lessons of the cane toad likely helped shape the country's strict quarantine and risk checks today.

The National Invasive Species Information Center states that invasive species are spread primarily by human activities, often unintentionally, with released pets one pathway; NOAA adds ships' ballast water and accidental releases of aquaculture species, aquarium specimens or bait. The U.S. Fish and Wildlife Service works to prevent, eradicate and control them.

PestSmart concludes that the cane toad story, in which pre-release testing was skipped and initial controls were overturned under industry pressure, has likely influenced Australia's strict quarantine laws and risk-assessment procedures.