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Understanding Invasive Species: The Enemy Release and Biodiversity Hypotheses

This article explores two key hypotheses explaining why certain species become invasive: the Enemy Release Hypothesis and the Biodiversity Hypothesis. The Enemy Release Hypothesis posits that exotic species thrive in new environments because they escape their natural enemies, such as pests and diseases. Meanwhile, the Biodiversity Hypothesis emphasizes the role of ecological interactions and the importance of community diversity in invasion dynamics. Together, these concepts provide crucial insights into the factors influencing invasive species success and their ecological impacts.

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Understanding Invasive Species: The Enemy Release and Biodiversity Hypotheses

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  1. Readings, enemy release and biodiversity hypotheses • Enemy release hypothesis: Keane, r. Crawley, M. 2002. Exotic plant invasions and the enemy release hypothesis. TREE 17:164-170 2. Biodiversity hypothesis: Shea K, Chesson P. 2002. Community ecology theory as a framework for biological invasions. TREE 17:170-176

  2. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies

  3. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies • Alien species are immigrants to a new area • Aliens often arrive as seeds

  4. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies • Alien species are immigrants to a new area • Aliens often arrive as seeds • In other words, they arrive without the grazers, insect pests, diseases, parasites, etc. of their native range – their “enemies”

  5. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies • Alien species immigrate without enemies • Hence, alien species “escapes” from their enemies and are no longer affected by biotic constraints • Thus, alien growth and success is much greater in new range

  6. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies • Alien species immigrate without enemies • Aliens lack biotic constraints • However, alien success will depend on potential enemies in new range: • Are potential enemies generalists or specialists?

  7. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies • Alien species immigrate without enemies • Aliens lack biotic constraints • However, alien success will depend on potential enemies in new range: • Are potential enemies generalists or specialists? • Are population sizes of potential enemies large or small?

  8. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies • Alien species immigrate without enemies • Aliens lack biotic constraints • However, alien success will depend on potential enemies in new range: • Are potential enemies generalists or specialists? • Are population sizes of potential enemies large or small? • Do potential enemies feed on foliage or seeds?

  9. What makes a species invasive? • g) Escape from biotic constraints hypothesis • aka “Escape from enemy” hypothesis • “Enemy release” hypothesis • Basic concepts: • Species in their native range are suppressed by natural enemies • Alien species immigrate without enemies • Aliens lack biotic constraints • However, alien success will depend on potential enemies in new range: • Are potential enemies generalists or specialists? • Are population sizes of potential enemies large or small? • Do potential enemies feed on foliage or seeds? • Are there similar hosts for potential enemies in new area?

  10. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Chrysanthemoides native to South Africa but invasive in Australia • Acacia native to Australia but invasive in South Africa • For both species, few pests in invaded area

  11. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Chrysanthemoides native to South Africa but invasive in Australia • Acacia native to Australia but invasive in South Africa • For both species, few pests in invaded area • Compare performance of each species in native area vs. invaded Invaded Native

  12. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Chrysanthemoides native to South Africa but invasive in Australia • Acacia native to Australia but invasive in South Africa • For both species, few pests in invaded area • When Chrysanthemoides is invader, does much better (sometimes much much much better!!) Invaded Native > > > > > > >

  13. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Chrysanthemoides native to South Africa but invasive in Australia • Acacia native to Australia but invasive in South Africa • For both species, few pests in invaded area • When Acacia is invader, does much much much better Native Invaded < < <

  14. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Chrysanthemoides native to South Africa but invasive in Australia • Acacia native to Australia but invasive in South Africa • For both species, few pests in invaded area • When species is invader, does much (much) better Invaded Native Native Invaded > > > > < > > < > <

  15. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Flip side can also occur: New pest in an area devastates natives • Example is American chestnut (Castanea dentata) & chestnut blight (invasive fungus Endothia parasitica)

  16. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Flip side can also occur: New pest in an area • Example is American chestnut (Castanea dentata) & chestnut blight (invasive fungus Endothia parasitica) • Dramatic ↓ in chestnut after arrival of blight in 1934

  17. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Flip side can also occur: New pest in an area • Example is American chestnut (Castanea dentata) & chestnut blight (invasive fungus Endothia parasitica) • Dramatic ↓ in chestnut after arrival of blight in 1934 • Other trees had ↑

  18. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mack et al. (2000) • Flip side can also occur: New pest in an area • Example is American chestnut (Castanea dentata) & chestnut blight (invasive fungus Endothia parasitica) • Dramatic ↓ in chestnut after arrival of blight in 1934 • Other trees had ↑, or small changes

  19. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Wolfe (2002) American Naturalist 160:705-711 • Silene latifolia native to Europe but invasive in North America • Surveyed populations in both Europe and North America for generalist and specialist enemies

  20. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Wolfe (2002) American Naturalist 160:705-711 • Silene latifolia native to Europe but invasive in North America • More populations experience damage in native range (Europe) then invaded range (North America)

  21. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Wolfe (2002) American Naturalist 160:705-711 • Silene latifolia native to Europe but invasive in North America • More populations experience damage in native range • True for both generalists

  22. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Wolfe (2002) American Naturalist 160:705-711 • Silene latifolia native to Europe but invasive in North America • More populations experience damage in native range • True for both generalists and specialist

  23. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Wolfe (2002) American Naturalist 160:705-711 • Silene latifolia native to Europe but invasive in North America • More populations experience damage in native range • More individuals within a population are damaged in native range (Europe) then invaded range (North America)

  24. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Wolfe (2002) American Naturalist 160:705-711 • Silene latifolia native to Europe but invasive in North America • More populations experience damage in native range • More individuals within a population are damaged in native range • True for both generalists

  25. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Wolfe (2002) American Naturalist 160:705-711 • Silene latifolia native to Europe but invasive in North America • More populations experience damage in native range • More individuals within a population are damaged in native range • True for both generalists and specialists

  26. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Klironomos (2002) Nature 417: 67-70 • Enemies not necessarily insects • Tested if soil organisms can affect growth • Logic: In native soils, pathogens accumulate rapidly, ultimately reducing growth of natives. For invasives in new soil, pathogens accumulate much slower, and hence do not adversely affect growth of invasives.

  27. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Klironomos (2002) Nature 417: 67-70 • Logic:Pathogens accumulate in soils for natives but not invasives • Series of experiments that used 5 invasive & 5 rare species from Canadian meadows • From each species, isolated 2 fractions of soil micro-organisms • Pathogen / saprobe filtrate = Detrimental • AMF (mycorrhizal) spores = Beneficial • Grew plants with microbes from their own soil vs. microbes from other species’ soil

  28. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Klironomos (2002) Nature 417: 67-70 • Logic: Pathogens accumulate in soils for natives but not invasives • Used 5 invasive & 5 rare species from Canadian meadows • From each species, isolated 2 fractions of soil micro-organisms • Pathogen / saprobe filtrate = Detrimental • AMF (mycorrhizal) spores = Beneficial • Grew plants with microbes from their own soil vs. microbes from other species’ soil • Predictions: • If use sterile soil, should see no affect on growth for both invasives & rare species • If use AMF, should see beneficial growth for both • If use pathogens, negative effects only for rare species

  29. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Klironomos (2002) Nature 417: 67-70 • Logic: Pathogens accumulate in soils for natives but not invasives • Predictions: • If use sterile soil, no affect for both invasives & rare species

  30. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Klironomos (2002) Nature 417: 67-70 • Logic: Pathogens accumulate in soils for natives but not invasives • Predictions: • If use sterile soil, no affect for both invasives & rare species • If use AMF, beneficial for both

  31. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Klironomos (2002) Nature 417: 67-70 • Logic: Pathogens accumulate in soils for natives but not invasives • Predictions: • If use sterile soil, no affect for both invasives & rare species • If use AMF, beneficial for both • If use pathogens, • negative only for rare

  32. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Klironomos (2002) Nature 417: 67-70 • Logic: Pathogens accumulate in soils for natives but not invasives • Predictions: • If use sterile soil, no affect for both invasives & rare species • If use AMF, beneficial for both • If use pathogens, • negative only forrare • Thus, invasives • accumulate pathogens • @ slower rate because • they escape harmful • pathogens when invading • foreign territory

  33. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mitchell & Power (2003) Nature 421: 625-627 • Additional support that pathogens are important • Examined 473 plant species naturalized to North America from Europe • Examined occurrence of viruses and various fungal pathogens (rust, smut, powdery mildew) in native and naturalized ranges

  34. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mitchell & Power (2003) Nature 421: 625-627 • Additional support that pathogens are important • Compare pathogens on 473 species in native vs. naturalized range • Predictions: • Fewer pathogens in naturalized range • Because viruses are more easily transmitted and have broader host ranges then fungi, expected that ↓ for viruses would be smaller than that for fungi • The bigger the escape from pathogens, the more noxious • And vice versa: accumulate more pathogens, less noxious

  35. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mitchell & Power (2003) Nature 421: 625-627 • Additional support that pathogens are important • Compare pathogens on 473 species in native vs. naturalized range • Predictions: • Fewer pathogens in naturalized range

  36. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mitchell & Power (2003) Nature 421: 625-627 • Additional support that pathogens are important • Compare pathogens on 473 species in native vs. naturalized range • Predictions: • Fewer pathogens in naturalized range • Smaller ↓ for viruses (24%) than for fungi (84%)

  37. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mitchell & Power (2003) Nature 421: 625-627 • Additional support that pathogens are important • Compare pathogens on 473 species in native vs. naturalized range • Predictions: • Fewer pathogens in naturalized range • Smaller ↓ for viruses • Escape related to noxiousness • As ↑ escape, ↑ noxiousness

  38. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Evidence: from Mitchell & Power (2003) Nature 421: 625-627 • Additional support that pathogens are important • Compare pathogens on 473 species in native vs. naturalized range • Predictions: • Fewer pathogens in naturalized range • Smaller ↓ for viruses • Escape related to noxiousness • As ↑ escape, ↑ noxiousness • As ↑ pathogens , ↓ noxiousness

  39. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Summary: Escape from biotic constraints hypothesis • Intuitively clear • Strong evidence in a number of cases • Underlying concept for biological control

  40. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Summary: Escape from biotic constraints hypothesis • Intuitively clear • Strong evidence in a number of cases • Underlying concept for biological control • But: • Assumes: • Native specialist enemies are left behind • Host switching does not occur • Generalist in new range avoid invader

  41. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Summary: Escape from biotic constraints hypothesis • Intuitively clear • Strong evidence in a number of cases • Underlying concept for biological control • But: • Assumes: • Native specialist enemies are left behind • Host switching does not occur • Generalist in new range avoid invader • Need to demonstrate that native enemies limit plant population in native range

  42. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Summary: Escape from biotic constraints hypothesis • Intuitively clear • Strong evidence in a number of cases • Underlying concept for biological control • But: • Assumes: • Native specialist enemies are left behind • Host switching does not occur • Generalist in new range avoid invader • Need to demonstrate that native enemies limit plant population in native range • Is the release through ↓ invader mortality • OR through adverse affects on natives causing ↓ competition?

  43. What makes a species invasive? • g) Escape from biotic constraints hypothesis • Summary: Escape from biotic constraints hypothesis • Intuitively clear • Strong evidence in a number of cases • Underlying concept for biological control • But: • Assumes: • Native specialist enemies are left behind • Host switching does not occur • Generalist in new range avoid invader • Need to demonstrate that native enemies limit plant population in native range • Is the release through ↓ invader mortality • OR through adverse affects on natives causing ↓ competition? • Long-lived species and species with long-lived seedbanks probably little affected by enemies

  44. What makes a species invasive? • h) Biodiversity hypothesis • Basic concepts: • High biodiversity confers high community stability

  45. What makes a species invasive? • h) Biodiversity hypothesis • Basic concepts: • High biodiversity → high community stability • Stable communities are not easily invaded

  46. What makes a species invasive? • h) Biodiversity hypothesis • Basic concepts: • High biodiversity → high community stability • Stable communities not invaded • Shares features with vacant niche hypothesis • NOTE: Biodiversity hypothesis does not require vacant niche

  47. What makes a species invasive? • h) Biodiversity hypothesis • Basic concepts: • High biodiversity → high community stability • Stable communities not invaded • Shares features with vacant niche hypothesis • Biodiversity hypothesis does not require vacant niche • But uses niche concepts that: • Different species have different niches

  48. What makes a species invasive? • h) Biodiversity hypothesis • Basic concepts: • High biodiversity → high community stability • Stable communities not invaded • Shares features with vacant niche hypothesis • Biodiversity hypothesis does not require vacant niche • But uses niche concepts that: • Different species have different niches • As ↑ number species, ↑ amount of potential niche space that is filled

  49. What makes a species invasive? • h) Biodiversity hypothesis • Basic concepts: • High biodiversity → high community stability • Stable communities not invaded • Shares features with vacant niche hypothesis • Biodiversity hypothesis does not require vacant niche • But uses niche concepts that: • Different species have different niches • As ↑ number species, ↑ filling of niche space • Thus highly diverse communities more difficult to invade

  50. What makes a species invasive? • h) Biodiversity hypothesis • Theoretical evidence: From Tilman (1999) • ↑ number species ↑ filling of niche space

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