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Can Native Species Become Invasive?

The forest may be home to several species of house ants, but one of those species is finding the slick life of the city to be much more appealing. According to a recent study at Purdue University, the odorous house ant (Tapinoma sessile) is not only thriving in urban areas; it’s actually taking over.

Moving to a non-native environment and forcing out native species is typically characteristic of an invasive species. Species native to a certain environment have to compete for food and shelter, and predators keep their populations in check.

But when those elements are absent, the study concludes, a native species can adapt qualities of invasive species, including excessive reproductive capacity and aggressive tendencies. According to a peer-reviewed article that appeared in Biological Invasions published February 2010, Purdue scientist Grzegorz Buczkowski discovered that the normally small, docile, single-queen colonies of odorous house ant quickly became massive and aggressive, housing a multitude of queens. In most instances, the sheer size of the colonies allows them to outcompete other ant species.

Odorous house ant

Odorous house ant

The odorous house ant is the most common species of house ant, named for its coconut-like odor when squashed. In the wild, they have one queen per colony, their colonies are small, and they co-exist with several other ant species. Their nests are not close to human dwellings.

However, in urban settings, their colonies expand from about 80 individuals per nest to nearly 900 individuals. Each nest has multiple queens, and nests tend to be located near buildings, landscape mulch, or piles of debris. Buczkowski says that besides the odorous house ant, only two other ant species tended to live in urban settings: cornfield ants (Lasius neoniger) and pavement ants (Tetramorium caespitum). The ants, normally not an aggressive species, also tend toward in-fighting in urban areas when their numbers are larger.

Native species do not typically move to a completely different habitat, and their tendency to stay in their original habitat makes them manageable. However, the habitat characteristics that facilitate invasive species seem to have attracted the odorous house ant out of its comfort zone: free food, warm shelter and a lack of enemies. Common landscape plants and compost provide abundant food sources. Heated dwellings and landscape mulch provide a warm living space. And with only two other ant competitors—whose numbers are not nearly as massive—the odorous house ant easily takes over.

However, most native species don’t travel far enough away from home to even reach a new habitat. With housing complexes being built near natural areas, the odorous house ant doesn’t have to travel far. Buczkowski hypothesizes that the ants make the transition from their wooded home to a new city life by traveling from the forest to a nearby residence.

Buczkowski’s research raises some interesting questions: if one native species can change so dramatically simply by moving to a new habitat, how many other native species might eventually do the same? How many already have? The issue raises some ethical questions as well. Integrated pest management focuses on preventing and mitigating exotic invasive species and replacing those species with native species. If a native species invades a new habitat, does it become invasive? And should it be treated accordingly?

Although Buczkowski does not answer those questions, he does suggest that researchers should not look only at foreign pest invaders, but also at species within our own borders that have decided to improve their standard of living.

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