Readings, enemy release and biodiversity hypotheses 1.Enemy release hypothesis: Keane, r. Crawley,...

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Readings, enemy release and biodiversity hypotheses

1. 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

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

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

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

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

3) What makes a species invasive?h) Biodiversity hypothesis

Basic concepts:• High biodiversity confers high community stability

Basic concepts:• High biodiversity → high community stability• Stable communities are not easily invaded

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

Biodiversity hypothesis does not require vacant nicheBut uses niche concepts that:

Different species have different niches

Different species have different nichesAs ↑ number species, ↑ amount of potential niche space

that is filled

Different species have different nichesAs ↑ number species, ↑ filling of niche spaceThus highly diverse communities more difficult to invade

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• Thus, as ↑ number species

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• As ↑ number species, availability of resources, on average,

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• As ↑ number species, availability of resources, on average, ↓

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• ↑ number species ↓ average resources availability

Defined mathematically in A

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• ↑ number species ↓ average resources availability• Each species has some minimum

average resource need = R*

average resource need = R*• Corresponds with some minimum

species diversity = N*

species diversity = N*• Above N*, that species cannot

invade

invade because averagecommunity resource levelis less then minimum forthat species (R*)

species diversity = N*• Above N*, species cannot invade• At or below N*, can invade

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• ↑ number species ↓ average resources availability• Each species has R*• At or below N*, species can invade

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• ↑ number species ↓ average resources availability• Each species has R*• At or below N*, species can invade• If do for all species in community,

can determine relativeinvasibility as diversity changes

Theoretical evidence: From Tilman (1999)• ↑ number species ↑ filling of niche space• ↑ number species ↓ average resources availability• Each species has R*• At or below N*, species can invade• If do for all species in community,

invasibility ↓ as diversity ↑

Experimental evidence: From Kennedy et al. (2002) Nature 417: 636-638

• Had 147 plots originally seeded with up to 24 natives• Observed 13 aliens invaded naturally through time

• Had 147 plots originally seeded with up to 24 natives• Observed 13 aliens invaded naturally through time• As ↑ native diversity:

(a) ↓ invader cover

(a) ↓ invader cover(b) ↓ invader number

(a) ↓ invader cover(b) ↓ invader number(c) ↓ invader max size

(a) ↓ invader cover(b) ↓ invader number(c) ↓ invader max size(d) NS median size

• Conclude: Invaders do more poorly with ↑ native diversity

• Invaders do more poorly with ↑ native diversity• Why?

As ↑ native diversity:↑ number of neighbors

As ↑ native diversity:↑ number of neighbors↑ number of neighboring species

As ↑ native diversity:↑ number of neighbors↑ number of neighboring species↑ crowding

With high native diversity, have dense, species rich, crowded neighborhoods

3) What makes a species invasive?h) Biodiversity

hypothesis

Contrary evidence: Allcock and Hik 2003

• Grazed woodland in Australia

• Habitats included alluvia (rich) grasslands (intermediate) and woodlands (resource poor)

• Negative relationship between exotic and native diversity

• Natives –ve relationship to high biomass; exotics +ve relationship.

Contrary evidence: From Levine (2000)• Examined riparian communities along South Fork Eel River, CA• Dominated by native tussock sedge Carex nudata• Each tussock is discrete island (neighborhood) colonized by up to

20 perennial plants & mosses

Contrary evidence: From Levine (2000)• Surveyed similarly sized tussocks over 7 km stretch of river• Recorded incidence of 3 invaders (Agrostis, Plantago, Cirsium)

Contrary evidence: From Levine (2000)• Surveyed similarly sized tussocks• 3 invaders (Agrostis, Plantago, Cirsium)• All invaders had ↑ occurrence with

↑ diversity, contrary to biodiversityhypothesis

• Why?To support high diversity, must have

lots of resources

lots of resourcesThus, diverse sites are “best” sites

lots of resourcesThus, diverse sites are “best” sitesBest sites most likely to be invaded

Contrary evidence: From Lonsdale (1999) Ecology 80: 1522-1536• Global survey – compiled data from 184 sites around the world• Separated into “Island” vs. “Mainland”

Within each group, broke down further into nature “reserves” and “non-reserves”

• As expected, for givendiversity, number invaders:Islands > Mainlands

• As expected, for givendiversity, number invaders:Islands > MainlandsNon-reserves > Reserves

• But, for all sites, moreinvaders with greaterdiversity

Resolving the conflict: Shea and Chesson 2002

• Within ecosystems, more species = less invasable• Among ecosystems, more diverse systems (more

resources) = more vulnerable

Resolving the conflict: Shea and Chesson 2002

• Within ecosystems, more species = less invasable• Among ecosystems, more diverse systems (more

resources) = more vulnerable

• Within ‘clusters’ extrinsic factors (e.g. climate) are similar

• Factors differ across ‘clusters’.

Summary:• Logical arguments to support the hypothesis

But logical arguments contrary to hypothesis

But logical arguments contrary to hypothesis• Data that support the hypothesis

But other data contrary to hypothesis

But other data contrary to hypothesis• Thus, biodiversity alone does not account for invasibility• Diversity patterns at different scales may explain paradox in

Readings, enemy release and biodiversity hypotheses 1.Enemy release hypothesis: Keane, r. Crawley,...

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