One of the oldest ideas in invasion ecology attributes the success of invasive weeds to being freed from their natural pests. The natural enemies of plants include vertebrate and invertebrate herbivores, fungi, bacteria, and viral diseases. Some natural enemies are specifically adapted to attacking one or a few plant species. These specialist enemies help keep plant populations in check in the weed’s native range. They often do not exist in the introduced range.
In the absence of natural enemies, invasive weeds have a competitive advantage over native plants. Without natural enemies, invaders allocate more of their resources to growth, acquiring water and nutrients, and to reproduction. This advantage might not show up right away. Invaders can evolve over time, slowly adjusting to their new environment. In contrast, native plants must continue battling their natural enemies, plus compete with new weed neighbors.
Bological control efforts try to capitalize on the natural enemies of invasive weeds. These enemies are brought from their native range and released into introduced weed populations. This doesn’t eradicate the invasive weeds, but may reduce their vigor and give native plants a chance to compete. This can be a tricky strategy because the introduced plant pests don’t always perform as scientists and managers think they might.
The lack of natural enemies probably doesn’t account for all of an invasive weed’s success. Multiple weed tactics are usually at play.
-Beetle on leafy spurge: Public domain.
-Beetle release: Patrick Kane.
-Blossey, B. and R. Notzold. 1995. Evolution of increased competetive ability in invasive nonindigenous plants: A hypothesis. The Journal of Ecology. Vol. 83 No. 5 pp. 887-889.
-Keane, R.M. and M.J. Crawley. 2002. Exotic plant invasions and the enemy release hypothesis. Trends in Ecology & Evolution. Vol. 17 No. 4 pp. 164-170.