A framework to unite thinking on biological invasions. Comment on individual components.


H1: Introduced populations experience a release from enemies (ERH).
a. Prediction: Introduced populations have fewer enemies compared to populations in the native range.
b. Prediction: Reduction in enemy diversity leads to less damage compared to populations in the native range, which facilitates demographic expansion.

H2: Introduced species experience biotic resistance in new region (BRH)
a. Prediction: Introduced species compete with native species
b. Prediction: Introduced species accumulate novel enemies over time

H3: Introduced species diverge evolutionarily from native populations
a. Prediction: Introduced populations experience a reduction in genetic diversity compared to native populations (bottlenecks)
b. Prediction: Multiple independent introductions to one region increases genetic diversity (hybridizing and outcrossing with native species or other invaders)
c. Prediction: (adaptive evolution) Introduced species locally adapt to new biotic environment, and members of the invaded community adapt to the presence of the invader.

Examples of integration of the above hypotheses

H1a + H2a + H3c = Reduction of parasitism gives introduced species an advantage when competing with heavily parasitized native species
a. Prediction: Resources diverted from supporting parasites to support host growth and reproduction
b. Prediction: EICA, costly defenses selected against, freeing resources for growth and reproduction

H2b+ H3c = Introduced species will experience an increase in enemies over time as enemy relationships in the new range evolve.
a. Prediction: Historical invasions will have a subset of enemies from the range of origin and a subset from the current range.
b. Prediction: Historical invasions will be parasitized by a greater proportion of local parasite species compared to contemporary invasions
c. Prediction: Historical invasions provide a glimpse into the evolutionary future of contemporary invasions

H1a + H2a + H2b +H1b = Introduced species invading communities with phylogenetically similar species will experience a smaller demographic advantage from lack of parasites
a. Prediction: Parasites will shift from native to use phylogenetically similar introduced species
b. Prediction: Introduced species invading areas with close relatives should be less invasive

H1a + H2b + H3b + H3c = Hybridization and outcrossing can alter relationships with natural enemies, competitors or other novel aspects of the environment, directly facilitating invasion, and providing variation necessary for adaptive evolution.
a. Prediction: The most invasive species are likely to be those that are outcrossed and experience enemy release.