3)What makes a species invasive? d) Escape from biotic constraints hypothesis aka“Escape from enemy” hypothesis “Enemy release” hypothesis Basic concepts:

3) What makes a species invasive?d) 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



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



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”



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 constraintsThus, alien growth and success is much greater in new range



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?




new range:Are potential enemies generalists or specialists?Are population sizes of potential enemies large or small?




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?




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?


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


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


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

>>>>

>>>


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

InvadedNative

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<<


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

>>>>

>>>

InvadedNative

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<<


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)


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



(invasive fungus Endothia parasitica)• Dramatic ↓ in chestnut after arrival of blight in 1934• Other trees had ↑



(invasive fungus Endothia parasitica)• Dramatic ↓ in chestnut after arrival of blight in 1934• Other trees had ↑, or small changes


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


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)


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


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


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)


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


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


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.


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 = DetrimentalAMF (mycorrhizal) spores = Beneficial

• Grew plants with microbes from their own soil vs. microbes from other species’ soil


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 = DetrimentalAMF (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 speciesIf use AMF, should see beneficial growth for bothIf use pathogens, negative effects only for rare species


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 sterile soil, no affect for both invasives & rare speciesIf use AMF, beneficial for both



If use sterile soil, no affect for both invasives & rare speciesIf use AMF, beneficial for bothIf use pathogens,

negative only for rare



If use sterile soil, no affect for both invasives & rare speciesIf use AMF, beneficial for bothIf use pathogens,

negative only for rare• Thus, invasives

accumulate pathogens@ slower rate becausethey escape harmfulpathogens when invadingforeign territory


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


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 rangeBecause 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 noxiousAnd vice versa: accumulate more pathogens, less noxious



Fewer pathogens in naturalized range



Fewer pathogens in naturalized rangeSmaller ↓ for viruses (24%) than for fungi (84%)



Fewer pathogens in naturalized rangeSmaller ↓ for virusesEscape related to noxiousness

(a) As ↑ escape, ↑ noxiousness



Fewer pathogens in naturalized rangeSmaller ↓ for virusesEscape related to noxiousness

(a) As ↑ escape, ↑ noxiousness(b) As ↑ pathogens , ↓ noxiousness


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 behindHost switching does not occurGeneralist in new range avoid invader



But:• Assumes:


• Need to demonstrate that native enemies limit plant population in native range



But:• Assumes:



• Is the release through ↓ invader mortalityOR through adverse affects on natives causing ↓

competition?



But:• Assumes:



• Is the release through ↓ invader mortalityOR through adverse affects on natives causing ↓ competition?

• Long-lived species and species with long-lived seedbanks probably little affected by enemies

Documents

3)What makes a species invasive? d) Escape from biotic constraints hypothesis aka“Escape from enemy” hypothesis “Enemy release” hypothesis Basic concepts: