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Invasive Species – Part II: The ENEMY RELEASE HYPOTHESIS

LOCAL FLORA – Lecture 13 Dr. Donald P. Althoff. LEC 13. ?. Invasive Species – Part II: The ENEMY RELEASE HYPOTHESIS. Effects of flea beetles ( Apthona spp. ) on leafy spurge in MT. ERH: The enemy release hypothesis.

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Invasive Species – Part II: The ENEMY RELEASE HYPOTHESIS

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  1. LOCAL FLORA – Lecture 13 Dr. Donald P. Althoff LEC 13 ? Invasive Species – Part II: The ENEMY RELEASE HYPOTHESIS Effects of flea beetles (Apthonaspp.) onleafy spurge in MT

  2. 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

  3. ERH: The enemy release hypothesis • Also known as the…. _______________ hypothesis _______________ hypothesis _______________ hypothesis

  4. 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 plant invasions and the enemy release hypothesis. Trends in Ecology and Evolution 17(4):164-170.

  5. ERH: 3-point logical argument 1 • 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. 2 3

  6. 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

  7. 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

  8. Specialist enemies Generalist enemies Specialist enemies Native Competitors Study Species (exotic in its native community) Native community

  9. Specialist enemies Generalist enemies Specialist enemies Native Competitors Study Species (exotic in its native community) Exotic community

  10. 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 study species (=exotic)

  11. 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 ________________________

  12. 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 _____________________ __________________________________________

  13. 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?

  14. 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

  15. In conclusion… • ERH is an intuitively clear and clean explanation of exotic plant invasions • However, a full understanding of the assumptions underlying ERH (refer back to slide with 1, 2, 3) is required to properly test it. • 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

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