Enemy release hypothesis

The 'enemy release hypothesis' is the idea that invasive species are less impacted by enemies (e.g., herbivores) than native species, because in the new geographical location, the invasives species are freed from the parasites that kept their growth in check in their native environ ment. Therefore, invasive species are thought to compete more successfully with native species in their invasive environment, because the native species have not been released from their traditional enemies. The enemy release hypothesis is the basis for biocontrol programs.

The introduction of the pests of invasive species can have a dramatic impact on the growth and spread of invasive species in biocontrol programs, which is evi dence that the enemy release hypothesis is operating. Also, plant species are sometimes (but not always) taller in their introduced ranges, and this is often attributed to the absence of their traditional parasites. Most invasive plant species have an average of 16 parasites in their native range, but only 3 in their introduced range.

Related to the idea that invasive species may be larger in their introduced ranges, is the idea that invasive species may have introgressed genes from hybridization with native species, so that invasive species not be the same type as their progenitors in their region of origin (intro gression hypothesis; see the section on 'Types of invasive species'). With novel genes introduced, the invasive spe cies with introgressed genes may be larger and with advantages over the original invasive type.

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