Invasive Species – Part II:The ENEMY RELEASE HYPOTHESIS

Effects of flea beetles (Apthona spp.) on leafy spurge in MT

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LOCAL FLORA – Lecture 13 Dr. Donald P. Althoff

LEC 13

ERH: The enemy release hypothesis

• Proposed as an explanation for the mechanism of invasions for exotic plants (or at least in some instances)

• Originally described in:Williamson, M. 1996. Biological Invasions.

Chapman and Hall, London, England.Crawley, M.J., editor. 1997. Plant

Ecology. Blackwell Scientific, Oxford,England

ERH: The enemy release hypothesis

• Also known as the….

_________________________ hypothesis

_________________________ hypothesis

_________________________ hypothesis

ERH: basic premise

• “…that plant species, on introduction to an exotic region, should experience a ______________ ________________ by herbivores and other natural enemies, resulting in an increase in distribution and abundance”

Keane, R.M. and M.J. Crawley. 2002. Exotic plantinvasions and the enemy release hypothesis.

Trends in Ecology and Evolution 17(4):164-170.

ERH: 3-point logical argument• Natural enemies are important _____________

of plant populations

• Enemies have a _______________________ than on exotic species

• Plants are able to _________________________in enemy regulation, resulting in increased population growth.

1

2

3

ERH: some assumptions and assertions• The potential and/or level for the ERH to apply to

individual species (i.e., exotics) varies by species

• Enemies of exotics (as portrayed in the ERH) might play (explain) the most important role of success for some invasions….but other factors, such as ___________, might be equally important or more important in some situations.

• In the absence of disturbance, ERH might well be the best explanation for the success

Consider “native” community

• Study species could have “_________________” and “generalist” enemies as well as native competitors

Consider “exotic” community

• Study species could be ______ from “specialist enemies” but “generalist” enemies as well as native competitors are still present

Specialistenemies

Generalistenemies

Specialistenemies

Study Species(exotic inits native

community)

NativeCompetitorsNative

community

Specialistenemies

Generalistenemies

Specialistenemies

Study Species(exotic inits native

community)

NativeCompetitorsExotic

community

ERH: looking at the model (exotic community—last slide)….

• ERH predicts there will be few ___________________ attacking the study species (=exotic)

• _________________ by native specialists will be _______ (as indicated by “faint dashed line” in model)

• Generalist enemies will have greater impact on the native competitors….resulting in _________________ against the ______________ (=exotic)

Consider…..• Specialist enemies are just that….they have __________,

in the native community, with the exotic. By definition, a specialist enemy attacks a __________ species.

• Therefore, when the _________ is introduced into a new community, its specialist enemy(ies) won’t be there

• However, two main mechanisms can result in which specialist enemies can be found in the exotic community: __________________ and ________________________

Most likely….• Every plant species has something that eats/attacks

it….even those that are most unpalatable.

• At the individual scale, herbivory and pathogen attack generally have a negative impact on the _________ of the individual plant. To counteract, many plants have evolved ____________________ to ______ enemy attack

• At the population scale, enemies can have significant impact on plant communities. Consider selective grazing by vertebrate herbivores (rabbits, sheep, deer, elk, bison, etc.)—on ___________________________________ ______________________________

Capitalizing on reduction in enemy regulation…or not?

• If an exotic plant species is impacted by enemies in its native region, removal (lack) of special enemies would logically allow the exotic to be more ______________

• This __________________ if the exotic is released into an unsuitable climate…or marginally suitable

• This ________________ if the exotic is not as competitive as the native species. How can this happen?…because the native species have

evolved there, therefore more adaptedto all the biotic and abiotic conditions present

One other consideration….

• If the exotic lacks an enemy specialists (in it’s new exotic community), the plant which always is exposed to limited resources may ____ have to use those limited resources to cover both

___________________________

…it can focus it’s resources on being __________________, thereby giving it an advantage over native plant species

In conclusion…• ERH is an _______________ clear and clean explanation of

exotic plant invasions

• However, a full understanding of the assumptions underlying ERH (refer back to slide #5 with 1, 2, 3) is required to properly __________.

• That said, some ___________________, where “specialist enemies” have been introduced (ex. release of flea beetles where leafy spurge has invaded resulted in a reduction of the leafy spurge….see opening slide) would support ERH