EARTH INVADERS: Crazy Ants on Christmas Island

For my first invasive species study, I’m going to talk about one of the most infamous invaders: the Yellow Crazy Ant. Yes, seriously, that’s what they’re called. I love science.

It’s crazy (get it?) that something so tiny can cause so much destruction. Image from Stephen Belcher

These little blighters have caused mayhem across a huge range of tropical and sub-tropical ecosystems. Christmas Island, an Australian territory in the Indian Ocean, knows the effects of the crazy ant all too well. Since their unwanted arrival, the ants have restructured ecosystems across the entire island. Ants as a whole make fantastic (well, awful) invasive species, but why is that?

Why are Ants such Successful Invaders?

  • Huge population sizes can enable ants to achieve ‘competitive dominance’
    • This essentially means that they hog all of the resources in an ecosystem like the greedy little invertebrates they are
  • A characteristic called ‘unicoloniality’ also helps certain ants to become invasive
    • Unicolonial species form massive colonies with numerous, spatially-separate but interconnected nests
    • This is termed a ‘supercolony’, and it allows ant populations to grow rapidly, resulting in invasive effects
  • Polygyny—the presence of more than one queen—also allows ants to quickly become invasive
  • Reduced intraspecific aggression can make ants more succesful as invasive species
    • This means that ants are nice to members of their own species. If only they could learn to be nice to everyone else, too.

Ecosystem Reshuffling on Christmas Island

Crazy ants have formed a series of super colonies on Christmas Island. These high-density colonies are sustained 24 hours a day, 365 days a year, both on the ground and in the tree canopy. You can’t fault their commitment to being a pain in the a**.  Crazy ants were accidentally introduced to the island somewhere between 1915 and 1934, and the first super colony was discovered in 1989. By 2005, crazy ants were found at densities of 20 million ants per hectare. I bet whoever brought their pet ant on vacation feels bad now.

Like myself, the crazy ant has a broad and opportunistic diet. This means that it eats pretty much everything in its path. Unfortunately, this includes a very important species: the red land crab.

On Christmas Island, crazy ants kill red land crabs using sheer force and numerical dominance. They spray formic acid over the eyes and mouthparts of the crabs, which unsurprisingly leads to death within 48 hours. Adding insult to (literal) injury, crazy ants storm the red crab’s burrows. They kill and consume any resident crabs before moving into the burrow themselves. This is hugely problematic; on Christmas Island, red land crabs are a keystone species. They play a vital role in litter decomposition and seed input, effectively structuring the rainforest.  The image on the right shows what happens to the forest after crazy ants take over. It’s easy to think that the bottom picture is an improvement. In reality, the forest becomes overgrown with a carpet of seedlings and leaf litter. The structure and composition of the forests which cover Christmas Island changes completely, resulting in huge levels of ecosystem disruption.

The plight of the Christmas Island pipistrelle demonstrates how damaging the crazy ants are. These tiny bats have become extinct as an indirect result of the crazy ants; giant millipedes have increased in abundance due to reduced red crab numbers, and these millipedes prey on pipistrelles. Invasive species often trigger a cascade of effects which indirectly affect a huge number of species, which is one of the reasons why they’re so harmful.

This image shows the forests on Christmas Island before (A) and after (B) invasion by crazy ants. Images courtesy of O'Dowd et al., 2003

Saving Christmas Island

different group of introduced insects are likely to have played a role in facilitating the invasion of crazy ants. Tiny bugs known as scale insects extract honeydew from tree branches, and this sugary solution forms a large part of the ant’s diet. Researchers have hypothesised that the scale insects are partially responsible for the crazy ant crisis: the food they provide enables the ant population to increase rapidly.

So, how do you solve an insect-induced disaster?


By adding more insects!


I’m not joking. A non-native wasp, Tachardiaephagus somervillei, has been introduced to Christmas Island in an attempt to take down the crazy ants. Queue a collective sigh from everyone who has read about the cane toad disaster. The wasp is a natural predator of scale insects, so researchers hope that their introduction could reduce the amount of honeydew available to crazy ants, resulting in their eventual starvation. In trial sites, researchers have observed reductions in crazy ant populations following the introduction of these wasps—but it’s probably too soon to start celebrating just yet.

Spare a thought for these little guys next time you complain about suitcase checks at the airport. Image from DIAC images

In summary: the accidental introduction of crazy ants to Christmas Island has resulted in the restructuring of the entire island ecosystem  and the death of key native organisms. Although methods to control the crazy ants are now in place, it’s unlikely that the ecosystem will ever revert to its natural state.  It’s quite easy to shrug this off when it’s occurring so far away from home—but invaders could soon change our landscapes, too.

In my next blog, I’m going to talk about an invasive species which has recently been spotted in the UK: the Asian Hornet. If this one didn’t make you say “damn nature you scary”, the next one definitely will. Stay tuned!

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