Experimental evidence of the effect of allelochemicals on plant production.

One important topic in the area of population ecology concerns exotic species, species moved from a native location to another location, usually by humans. Such species sometimes spread or invade so aggressively that they crowd out native species. It is estimated that more than 800 of the 5,000 plant species found in Canada are actually exotic introductions that have successfully established populations in the wild. Over half were brought in for gardening, horticulture, or landscaping purposes. Invasive exotic plants have traditionally been thought to succeed because they have escaped their natural enemies, primarily insects that remained in the country of origin and were not transported to the new locale. One way of controlling exotic species, therefore, has been to import the plant's natural enemies. This is known as biological control. However, new research on the population ecology of diffuse knapweed (Centaurea diffusa), an invasive Eurasian plant that has established itself in many areas of North America and causes serious problems in the arid southwestern interior of Canada, especially British Columbia, suggests a different reason for their success.