Download PowerPoint slides

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 microevolutionary 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
Basic concept : Communities with greater diversity have no
‘vacant niches’ and are therefore less invasible.
Implies saturation of communities!
Vacant Niche Hypothesis
Resource axis #2
• New realized niche – Species A, Species B
• Realized niche: Invader – Species C
• Realized niche: Invader – Species D
Resource axis #1
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!
Biodiversity hypothesis
Theoretical evidence:
Tilman (1999) Ecology 80: 1455-1474
• ↑ number species ↑ filling of niche space
• ↑ number species ↓ average
resources availability
• Each species has a minimum
average 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.
Biodiversity hypothesis
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
Biodiversity hypothesis
Evidence:
Kennedy et al. (2002) Nature 417: 636-638
Question: At a small scale (field), how does diversity [species richness &
density] influence invasion?
As ↑ native diversity:
• ↓ invader cover
• ↓ invader number
• ↓ invader
maximum size
• no effect of species
richness on mean
invader size
Biodiversity hypothesis
Evidence:
Kennedy et al. (2002) Nature 417: 636-638
Question: At a small scale (field), how does diversity [species richness &
density] influence invasion?
Invasion decreased with increasing native species richness
But what about native species density?
Biodiversity hypothesis
Evidence:
Kennedy et al. (2002) Nature 417: 636-638
Question: At a small scale (field), how does diversity [species richness &
density] influence invasion?
As diversity increased,
crowding also increased
As crowding increased,
maximum invader size
decreased
Biodiversity hypothesis
Evidence:
Kennedy et al. (2002) Nature 417: 636-638
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?
Biodiversity hypothesis
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
Biodiversity hypothesis
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
Biodiversity hypothesis
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.
Biodiversity hypothesis
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?
• Different ecosystems (deserts  rainforests) vary in their
extrinsic factors that influence ranges of biodiversity.
• Account for that range and then look at the biodiversity
hypothesis!
Biodiversity hypothesis
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
Biodiversity hypothesis
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
• Diversity patterns at different scales may explain paradox
in part
• Assumes competition is dominant driver structuring
communities
• Type of diversity examined
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.
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
Evidence:
Davis et al. (2000) Journal of Ecology 88: 528-534
Plant communities become more susceptible whenever there is an
increase in the amount of limiting resources
Resource uptake
Invasion increases as:
↑ availability (A→B)
↓ uptake (A→C)
A
B
D
C
Gross resource supply
Both (A→D)
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
Evidence:
Davis et al. (2000) Journal of Ecology 88: 528-534
Resource uptake
Plant communities become more susceptible whenever there is an
increase in the amount of limiting resources
A
B
D
C
Gross resource supply
This is not a static
attribute of the
community, but rather a
condition that will
fluctuate over time!
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
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
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
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.
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
Evidence:
Lepš et al . 2002. Applied Vegetation Science
Piper aduncum
Native range: Central America
Invaded range: Papua New Guinea
Invasive Piper should only be found
where there are fluctuating resources.
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
Evidence:
Lepš et al 2002. Applied Vegetation Science
Piper aduncum
Native range: Central America
Invaded range: Papua New Guinea
Invasive Piper should only be found where
there are fluctuating resources.
Where should resources fluctuate?
River banks, abandoned gardens, landslide
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
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?
Methods
Field and greenhouse experiments to
determine if observed patterns were influenced
by belowground factors
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
Example:
Gundale et al. (2008) Ecography 31:201-210
Variable resource availability hypothesis
a.k.a. Fluctuating resource hypothesis
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
Disturbance and land use hypothesis
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
•
Change can be abrupt or gradual
•
Change can be permanent or
transitory
•
Transition to original or new state
•
Transition can be natural or deliberate,
with deliberately different end states
Disturbance and land use hypothesis
How can disturbance and land
use changes enhance invasions?
• Changing resource
availability
• Increases the probability of
success for ruderals
• Changes in vegetation states
provide opportunities for other
species to exist
Disturbance and land use hypothesis
Evidence:
D’Antonio & Vitousek (1992)
•
Without invasive species, typically when woodlands are disturbed, they
eventually return back to woodlands
•
With alien grasses there is a novel disturbance: fire
•
Fire initiates a series of feedbacks that
virtually precludes re-establishment of
woody plants
Recovery
Disturbance
Disturbance and land use hypothesis
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.
Both weedy non-native species
AND total number of non-native
species increased with
development.
disturbance
Disturbance and land use hypothesis
Evidence:
Gelbard and Belnap (2003) Conservation Biology 17: 420-432
Examined the effect of road improvement on cover of non-native plants
Improved-surface roads
Paved roads
Graded roads
4-wheel drive tracks
Disturbance and land use hypothesis
Evidence:
Gelbard and Belnap (2003) Conservation Biology 17: 420-432
Disturbance and land use hypothesis
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 traits
Resource availability
Changes in competitive balance
Temporary “vacant” niche
What about plants that can establish without disturbance?