Micro-evolutionary change hypothesis Multiple colonizing events Founder effects Genetic bottlenecks Genetic drift Natural Selection New abiotic environment

Micro-evolutionary change hypothesis

• Multiple colonizing events• Founder effects• Genetic bottlenecks• Genetic drift

• Natural Selection• New abiotic environment • New biotic environment

• Hybridization• interspecific• intraspecific

Micro-evolutionary change hypothesis

Summary

• Likely that most (if not all) invasive species go through micro-evolutionary changes

• Good evidence for hybridization being beneficial

• But

• Have evidence of micro-evolutionary changes for only a limited number of species

• Limited evidence that changes are beneficial

• A species that undergoes micro or macro evolutionary changes does not automatically become invasive

• Adaptation by natives in response to invasion

Vacant Niche Hypothesis

• Niche describes how an organism or population responds to the distribution of resources and competitors

• Fundamental niche (Hutchinson 1957) = theoretical limits of existence for a species along n resource axes

• Realized niche = actual limits of existence for a species

Implies saturation of communities!

Basic concept : Communities with greater diversity have no ‘vacant niches’ and are therefore less invasible.

Resource axis #1

Reso

urce

axi

s #2

Vacant Niche Hypothesis

• New realized niche – Species A, Species B• Realized niche: Invader – Species C• Realized niche: Invader – Species D

Vacant Niche Hypothesis• SUMMARY:• May have some utility for tropical oceanic islands• Natural enemies should shift on to more similar new

species more easily (enemy escape hypothesis)• New life forms can be very successful (annual grasses in

NV)

BUT•Many potential invaders lack pollinators, symbionts, etc.•Actual demonstration of “vacant” niche is nearly impossible

Biodiversity hypothesis Basic concepts:• High biodiversity confers high community stability

• Stable communities are not easily invaded

• Shares features with vacant niche hypothesis BUT does not require a vacant niche• Uses niche concepts that:

(1) Different species have different niches(2) As ↑ number species, ↑ filling of niche

space

Highly diverse communities more difficult to invade!

Theoretical evidence: Tilman (1999) Ecology 80: 1455-1474

• ↑ number species ↑ filling of niche space

Biodiversity hypothesis

• ↑ number species ↓ average resources availability

• Each species has a minimumaverage resource need = R*

• Corresponds with a minimum species diversity = N*

At or below N*, species can invade

Biodiversity hypothesis Theoretical evidence: Tilman (1999) Ecology 80: 1455-1474 If do for all species in community, as diversity decreases, invasibility increases.

Evidence: Kennedy et al. (2002) Nature 417: 636-638

Question: At a small scale (field), how does diversity [species richness & density] influence invasion?

Method: • 147 plots seeded with up to 24 natives• 13 aliens invaded naturally through time


Constructed communities

‘Neighborhood’ size = 40 x 125 cm




As ↑ native diversity:• ↓ invader cover• ↓ invader number• ↓ invader

maximum size

• no effect of species richness on mean invader size




Invasion decreased with increasing native species richness

But what about native species density?




As diversity increased, crowding also increased

As crowding increased, maximum invader size decreased


Question: At a small scale (field), how does native diversity [species richness & density] influence invasion?

Invasion decreased with increasing native species richness

Invader performance decreased with increasing crowding


Diversity decreased invasion

Is this an artifact of the manipulated experiment?

Does the same pattern hold for natural situations?

Contrary evidence: Stolghren et al. (1999) Ecological Monographs 69: 25-46

Questions: (1)What is the relationship between native species richness

and foliar cover and invasion of exotic plant species?

(2)Are invasions patterns a matter of scale, or environment?

Methods:• Collected field data from 2 biomes• Multi-scale vegetation sampling


Evidence: Stolghren et al. (1999) Ecological Monographs 69: 25-46


At small spatial scales:• Cover of non-native species declined with increasing

native diversity BUT only in the Central Grasslands

• Increasing native diversity increased non-native richness in the Colorado Rockies

Evidence: Stolghren et al. (1999) Ecological Monographs 69: 25-46


At large spatial scales, areas of high native species richness were consistently more invaded than areas of low species richness.

Areas with high diversity ALSO had the highest soil fertility and precipitation.

Resolving the conflict: Shea and Chesson (2002) Trends in Ecology & Evolution 17: 170-

176

• Why is the comparison of biodiversity between very different ecosystems valid?


• Account for that range and then look at the biodiversity hypothesis!

• Different ecosystems (deserts rainforests) vary in their extrinsic factors that influence ranges of biodiversity.

Resolving the conflict: Shea and Chesson (2002) Trends in Ecology & Evolution 17: 170-

176

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


•Within ecosystems, more species = less invasible

• Across ecosystems, more diverse systems (more resources) = more invasible

Increasing biodiversity increases ecosystem stability which increases resistance to invasion (due to filled niche space= decreased resource availability).


Summary:• Logical arguments & data to support the

hypothesis

But • Logical arguments & data contrary to hypothesis• Thus, biodiversity alone does not account for invasibility

• Assumes competition is dominant driver structuring communities

• Type of diversity examined

• Diversity patterns at different scales may explain paradox in part

Variable resource availability hypothesis

a.k.a. Fluctuating resource hypothesis Background: Davis et al. (2000) Journal of Ecology 88: 528-534

• There is a finite amount of plant resources (nutrients, light, water, ‘space’) at a given site in a given time.

• In most plant communities, at most times, resources are taken up by resident plants.

Plant communities become susceptible to invasion whenever there is an increase in the amount of limiting resources.

Evidence:Davis et al. (2000) Journal of Ecology 88: 528-534

Gross resource supply

Reso

urce

upt

ake

Resource

supply-

uptake iso

cline


a.k.a. Fluctuating resource hypothesis

Resistan

t to In

vasio

n

A

Easily

Invasib

le

B

C

Plant communities become more susceptible whenever there is an increase in the amount of limiting resources

Invasion increases as:

↑ availability (A→B)

↓ uptake (A→C)

Both (A→D)D

Evidence:Davis et al. (2000) Journal of Ecology 88: 528-534

Gross resource supply

Reso

urce

upt

ake

Resource

supply-

uptake iso

cline



Resistan

t to In

vasio

n

A

Easily

Invasib

le

B

C

Plant communities become more susceptible whenever there is an increase in the amount of limiting resources

D

This is not a static attribute of the community, but rather a condition that will fluctuate over time!

Evidence:Davis & Pelsor (2001) Ecology Letters 4: 421-428

Question: How do fluctuations in resource availability influence competition and invasion?

Methods:• Desmodium canadense, Dalea purpurea, and

Rudbeckia hirta were seeded into bare plots or plots established with non-native grasses

• Some plots weeded to reduce competiton

• Resource manipulated: water



Evidence:Davis & Pelsor (2001) Ecology Letters 4: 421-428

• Increasing the limiting resource (water) increased invasion for some species, even with high amounts of competition.



Evidence:Lepš et al . 2002. Applied Vegetation Science

Piper aduncumNative range: Central AmericaInvaded range: Papua New Guinea



Invasive Piper should only be found where there are fluctuating resources.

Evidence:Lepš et al 2002. Applied Vegetation

Science

Piper aduncumNative range: Central AmericaInvaded range: Papua New Guinea

Invasive Piper should only be found where there are fluctuating resources.



River banks, abandoned gardens, landslide

Where should resources fluctuate?

Example:Gundale et al. (2008) Ecography 31:201-210

Questions:Under what combination of soil resource conditions is invasion by cheatgrass favored or constrained?

How is this influenced by fire?

MethodsField and greenhouse experiments to determine if observed patterns were influenced by belowground factors



Example:Gundale et al. (2008) Ecography 31:201-210



Resources naturally fluctuate over time. When availability of the most limiting resource is greater than resource uptake, the system is vulnerable to invasion.



SUMMARY:• Conceptual appealing• Flexibility to accommodate space, time, & many different

resources• Experimental evidence

But• Low predictive power

• Different invaders respond differently to different resources• Have to know where/when availability increases

Basic concepts:• Many invasive species have a “ruderal” life history

strategy.• “Ruderal” = small, very-short lived plants that

grow and mature rapidly and that have a large reproductive effort, especially in response to stress

• These species are often associated with disturbed habitats

• Every system has a natural disturbance regime (fire return, flooding interval, etc)

• Changes in land use can alter the natural disturbance regime (more or less frequent, bigger or smaller events…)

Disturbance and land use hypothesis

Empirical Evidence:Hobbs in Mooney & Hobbs (2000)

Land use changes affect disturbance


• Transition can be natural or deliberate, with deliberately different end states

• Transition to original or new state

• Change can be permanent or transitory

• Change can be abrupt or gradual

How can disturbance and land use changes enhance invasions?


• Changing resource availability

• Changes in vegetation states provide opportunities for other species to exist

• Increases the probability of success for ruderals

Evidence: D’Antonio & Vitousek (1992)

• Without invasive species, typically when woodlands are disturbed, they eventually return back to woodlands

DisturbanceRecovery


• With alien grasses there is a novel disturbance: fire

• Fire initiates a series of feedbacks that virtually precludes re-establishment of woody plants

Evidence:Kalin Arroyo et al. in Mooney &

Hobbs (2000)

• Determined number of alien plants in 12 political regions of Chile

• Developed a land use index using data about agriculture use, urban areas and road density.

disturbance


Both weedy non-native species AND total number of non-native species increased with development.

Evidence:Gelbard and Belnap (2003) Conservation Biology 17: 420-432

Examined the effect of road improvement on cover of non-native plants


4-wheel drive tracksGraded roads

Improved-surface roads

Paved roads

Evidence:Gelbard and Belnap (2003) Conservation Biology 17: 420-

432


Changes in land use cause changes in the extent and frequency of disturbance to an ecosystem which are then opened up for ruderal plant establishment.

Summary:•Consistent with ecological theories•Evidence from a variety of ecosystems•Empirical correlations

But•Is disturbance / land use the factor, or is it something associated with these?

Species traitsResource availabilityChanges in competitive balanceTemporary “vacant” niche

What about plants that can establish without disturbance?


Documents

Micro-evolutionary change hypothesis Multiple colonizing events Founder effects Genetic bottlenecks Genetic drift Natural Selection New abiotic environment