Overview

Nature has borders (oceans, mountains). When humans move animals across these borders, chaos ensues. An invasive species is a weed that takes over the garden. It has no natural predators in the new land, so it eats everything and multiplies until the ecosystem collapses.

Core Idea

The core idea is Ecological Release. In its home, the species is kept in check by predators and diseases. In the new home, it is “released” from these checks. It becomes a super-predator.

Formal Definition

An organism that causes ecological or economic harm in a new environment where it is not native.

Intuition

Imagine dropping a lion into a flock of sheep that have never seen a lion. The sheep don’t run. The lion eats until it explodes. That is an invasive species.

Examples

  • Cane Toads (Australia): Introduced to eat beetles. Instead, they ate everything else. They are poisonous, so anything that tries to eat them (crocodiles, snakes) dies. They are an unstoppable biological tank.
  • Kudzu (USA): “The vine that ate the South.” Introduced from Japan to stop erosion. It grows a foot a day and smothers entire forests.
  • Rabbits (Australia): A settler released 24 rabbits for hunting. Within a few years, there were millions. They ate the continent bare.

Common Misconceptions

  • All non-natives are invasive: No. Tomatoes are not native to Italy (from Americas). Potatoes are not native to Ireland. Most crops are non-native but harmless. “Invasive” means harmful.
  • Nature will balance itself: Eventually, yes (in million years). But in the short term, it means mass extinction.
  • Ballast Water: Ships fill their tanks with water in one port and dump it in another. This is how zebra mussels invaded the Great Lakes.
  • Biocontrol: Introducing another species to eat the invasive one. (Risky: see Cane Toads).

Applications

  • Border Control: Customs agents aren’t just looking for drugs; they are looking for fruit and bugs that could destroy agriculture.

Criticism / Limitations

  • Xenophobia: Some critics argue the language of “invasion” is too militaristic and mirrors anti-immigrant rhetoric. We should focus on ecosystem function, not just “native vs. alien.”

Further Reading

  • Kolbert, Elizabeth. The Sixth Extinction. (Chapter on the “New Pangaea”).
  • Simberloff, Daniel. Invasive Species: What Everyone Needs to Know. 2013.