Sept.
16- Hypotheses continued
(Variable resource, competition, novel weapons)
18- Hypotheses finish!
(disturbance, biodiversity, anthropogenic)
23- Nevada Cooperative Extension outreach programs (Earl Creech)
25- State weed & invasive plants management (Scott Marsh)
30- Federal invasive plants research (Mark Weltz)
Oct.
2- Review
7- Exam 1 (Dr. Nowak is back)
Notes: Hopefully we can get the last 6 hypotheses done before the guest
lectures. If we can, on Oct 2 I’ll bring in old exams and we can review.
*** Information presented by the guest lecturers will be on the exam! ***
3) What makes a species invasive?
Biodiversity hypothesis
Basic concepts:
• High biodiversity → high community stability
• Stable communities not invaded
• 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
3) What makes a species invasive?
Biodiversity hypothesis
Theoretical basis:
Tilman 1999. Ecology.
• ↑ number species ↑ filling of niche
space
• ↑ number species ↓ average
resources availability
3) What makes a species invasive?
Biodiversity hypothesis
Theoretical basis:
Tilman 1999. Ecology.
• ↑ number species ↑ filling of niche
space
• ↑ number species ↓ average
resources availability
• Each species has some minimum
average resource need = R*
• Corresponds with some minimum
species diversity = N*
3) What makes a species invasive?
Biodiversity hypothesis
Theoretical basis:
Tilman 1999. Ecology.
• ↑ number species ↑ filling of niche
space
• ↑ number species ↓ average
resources availability
• Each species has some minimum
average resource need = R*
• Corresponds with some minimum
species diversity = N*
• Above N*, that species cannot
invade because average
community resource level
is less then minimum for
that species (R*)
• At or below N*, can invade
3) What makes a species invasive?
Biodiversity hypothesis
Experimental evidence: From
Kennedy et al. (2002) Nature
Had 147 plots originally seeded
with up to 24 natives
(1,2,4,6,8,12,24)
Observed 13 aliens invaded
naturally through time
As ↑ native diversity:
↓ invader cover
↓ invader number
↓ invader max size
NS median size
3) What makes a species invasive?
Biodiversity hypothesis
Experimental evidence: From
Kennedy et al. (2002) Nature
Had 147 plots originally seeded
with up to 24 natives
(1,2,4,6,8,12,24)
Observed 13 aliens invaded
naturally through time
As ↑ native diversity:
↓ invader cover
↓ invader number
↓ invader max size
NS median size
Conclusion: Fewer invaders and
reduced invader performance
with increasing native
diversity.
3) What makes a species invasive?
Biodiversity hypothesis
Experimental evidence: From
Kennedy et al. (2002) Nature
Invaders do more poorly with ↑
native diversity
Why?
With high native diversity
provides
• Increasing density
• Increasing species richness
• and crowded neighborhoods
3) What makes a species invasive?
Biodiversity hypothesis
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
Research question: The biodiversity
hypothesis seems to work well in
controlled experiments, but what
about out in the world?
3) What makes a species invasive?
Biodiversity hypothesis
Contrary evidence: From Levine
(2000)
Surveyed similarly sized tussocks
• 3 invaders (Agrostis, Plantago,
Cirsium)
• All invaders had ↑ occurrence
with ↑ diversity, contrary to
biodiversity hypothesis
3) What makes a species invasive?
Biodiversity hypothesis
Contrary evidence: From Levine
(2000)
Surveyed similarly sized tussocks
• 3 invaders (Agrostis, Plantago,
Cirsium)
• All invaders had ↑ occurrence
with ↑ diversity, contrary to
biodiversity hypothesis
3) What makes a species invasive?
Biodiversity hypothesis
Contrary evidence: From Levine
(2000)
Surveyed similarly sized tussocks
• 3 invaders (Agrostis, Plantago,
Cirsium)
• All invaders had ↑ occurrence
with ↑ diversity, contrary to
biodiversity hypothesis
Why?
• To support high diversity,
must have lots of resources
• Thus, diverse sites are “best”
sites
• Best sites most likely to be
invaded
3) What makes a species invasive?
Biodiversity hypothesis
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”
(dashed lines) and “non-reserves”
(solid lines)
3) What makes a species invasive?
Biodiversity hypothesis
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”
(dashed lines) and “non-reserves”
(solid lines)
• As expected, number invaders:
Islands > Mainlands
Non-reserves > Reserves
• But, for all sites, more
invaders with greater
diversity
3) What makes a species invasive?
Biodiversity hypothesis
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’.
3) What makes a species invasive?
Biodiversity hypothesis
Resolving the conflict: Shea and
Chesson 2002
Why is the comparison of
biodiversity between two very
different ecosystems valid?
Different ecosystems (deserts 
rainforests) will have different
natural ranges of biodiversity.
Account for that range and then
look at the biodiversity
hypothesis!
3) What makes a species invasive?
Biodiversity hypothesis
Increasing biodiversity increases ecosystem stability which increases
resistance to invasion (due to filled niche space= decreased resource
availability).
Summary:
Logical arguments to support the hypothesis
But logical arguments contrary to hypothesis
Data that support the hypothesis
But other data contrary to hypothesis
Thus, biodiversity alone does not account for invasibility
Diversity patterns at different scales may explain paradox in part
3) What makes a species invasive?
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 usually associated with disturbed
habitats
3) What makes a species invasive?
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 usually associated with disturbed
habitats
• Every system has a natural disturbance regime (fire return,
flooding interval, etc)
• So changes in land use can alter the natural disturbance
regime (more or less frequent, bigger or smaller events…)
3) What makes a species invasive?
Disturbance and land use hypothesis
Basic concepts:
• Invasive species are ruderals
• Ruderals associated with disturbed
habitats
• Land use changes affect disturbance
Examples from Hobbs in Mooney &
Hobbs (2000)
Change can be abrupt or gradual
Change can be permanent
3) What makes a species invasive?
Disturbance and land use hypothesis
Basic concepts:
• Invasive species are ruderals
• Ruderals associated with disturbed
habitats
• Land use changes affect disturbance
Examples from Hobbs in Mooney &
Hobbs (2000)
Change can be abrupt or gradual
Change can be permanent or transitory
--Sites can recover to the original state
or an alternate state
3) What makes a species invasive?
Disturbance and land use hypothesis
Basic concepts:
• Invasive species are ruderals
• Ruderals associated with disturbed
habitats
• Land use changes affect disturbance
Examples from Hobbs in Mooney &
Hobbs (2000)
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
3) What makes a species invasive?
Disturbance and land use hypothesis
Basic concepts:
• Invasive species are ruderals
• Ruderals associated with
disturbed habitats
• Land use changes affect
disturbance
• How can disturbance and land
use changes enhance
invasions?
3) What makes a species invasive?
Disturbance and land use hypothesis
Basic concepts:
• Invasive species are ruderals
• Ruderals associated with
disturbed habitats
• Land use changes affect
disturbance
• Disturbance and land use
changes enhance invasions by:
--Changing the amount or flow of
resources hence inducing a
change in resource availability
--Increases the probability of
success for ruderals
--Because of the changes in
vegetation states there is
opportunities for other species
to exist
3) What makes a species invasive?
Disturbance and land use hypothesis
Evidence: D’Antonio & Vitousek
(1992)
Without invasive species, typically
when woodland are disturbed,
they eventually return back to
woodlands
Recovery
Disturbance
3) What makes a species invasive?
Disturbance and land use hypothesis
Evidence: D’Antonio & Vitousek
(1992)
Without invasive species, typically
when woodland are disturbed,
they eventually return back to
woodlands
With ruderal alien grass invasion:
a novel disturbance = fire
Once have a fire, initiate a series of
feedback effects that virtually
preclude the re-establishment
of woody plants
3) What makes a species invasive?
Disturbance and land use hypothesis
Evidence:From Kalin Arroyo et al.
in Mooney & Hobbs (2000)
Determined number of alien plants
in 12 political regions of Chile
• Total of 430 alien weeds
and 260 alien non-weeds
Developed a land use index using
data about agriculture use,
urban areas and road density.
disturbance
3) What makes a species invasive?
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 woodland / grasslands
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 establish without disturbance?
3) What makes a species invasive?
Anthropogenic hypothesis
“Human activity moves species from plant to place both accidentally
and deliberately and it does so at rates that are without precedent
in the last tens of millions of years. As a result, taxa that evolved in
isolation from each other are being forced into contact in an instant
of evolutionary time”
-D’Antonio and Vitousek 1992
3) What makes a species invasive?
Anthropogenic hypothesis
Basic concepts:
• Human activities are the cause of invasive species
• Humans help invasives overcome barriers; e.g. Richardson et al.
(2000)
• Humans overcome geographic barriers
• Humans overcome environmental & reproductive barriers
• Humans help disperse invasives in new range
Geographic
Home range
Introduced
Environment area Reproduce
Disperse
Disturbed
area
Natural
area
3) What makes a species invasive?
Anthropogenic hypothesis
Basic concepts:
• Human activities are the cause of invasive species
• Humans help invasives overcome barriers
• Thus, humans and invasive species are interdependent
Geographic
Home range
Introduced
Environment area Reproduce
Disperse
Disturbed
area
Natural
area
3) What makes a species invasive?
Anthropogenic hypothesis
Basic concepts: From Hobbs in Mooney & Hobbs (2000)
• Humans and invasive species are interdependent
Direct effects by introducing invading species
3) What makes a species invasive?
Anthropogenic hypothesis
Basic concepts: From Hobbs in Mooney & Hobbs (2000)
• Humans and invasive species are interdependent
Direct effects by introducing invading species
Direct effects by altering ecosystem properties
3) What makes a species invasive?
Anthropogenic hypothesis
Basic concepts: From Hobbs in Mooney & Hobbs (2000)
• Humans and invasive species are interdependent
Direct effects by introducing invading species
Direct effects by altering ecosystem properties
Cascading, indirect effects
3) What makes a species invasive?
Anthropogenic hypothesis
Basic concepts: From Hobbs in Mooney & Hobbs (2000)
• Humans and invasive species are interdependent
Direct effects by introducing invading species
Direct effects by altering ecosystem properties
Cascading, indirect effects, which sets up an invasive cycle
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction?
• Any examples of introduced plants that are now a problem?
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence: From Mack et al. (2000)
• Humans as vector for introduction
Lantana camara (lantana; shrub verbena) shrub native to tropical
New World
Prized for showy flowers and fragrant leaves
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence: From Mack et al. (2000)
• Humans as vector for introduction
Lantana camara native to tropical New World
Introduced as a horticultural species repeatedly throughout the
world, especially tropics and sub-tropics
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence: From Lonsdale (1999) Ecology 80:1522-1536
• Humans as vector for introduction
Asked: Do the number of invasive species increase with number of
visitors to natural areas?
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence: From Lonsdale (1999)
• Humans as vector for introduction
Do invasives ↑ with visitation?
First, control for larger parks have
more natives which draws
more visitors (a)
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence: From Lonsdale (1999)
• Humans as vector for introduction
Do invasives ↑ with visitation?
First, more natives → more visitors
Then look at visitors corrected for
number of natives (= “visitor
residuals”) vs. number of
exotics (b)
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence: From Lonsdale (1999)
• Humans as vector for introduction
Do invasives ↑ with visitation?
First, more natives → more visitors
Number of exotics significantly
(P<0.001; adj. r2=0.68) with
visitors
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
Example: Riparian areas in western US
• Originally “cottonwood gallery forest”
• Settlement brought ranching and irrigated
agriculture (sedimentation, clearing,
disturbance)
• Dam construction (early 1900’s):
prevented spring flood; fires
3) What makes a species invasive?
Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
Example: Riparian areas in western US
Saltcedars & Russian olive introductions (mid-late
1800’s) for windbreaks, erosion control &
stream bank stabilization, ornamentals
• Originally encouraged and subsidized by
governments
• More drought tolerant; less exacting on
seed dispersal & establishment
requirements
• Now dominant species on much of the
riparian areas of western US
3) What makes a species invasive?
Anthropogenic hypothesis
Humans have broken down barriers that once kept plants in their
native area.
Summary:
Abundant evidence that
• Humans are major vector for introduction
• Humans greatly alter ecosystem properties
But:
• Invasive species problems are no longer just a natural, ecological
problem
3) What makes a species invasive?
10 hypotheses:
1. Vacant niche hypothesis
2. Allelopathy hypothesis
3. Environmental change hypothesis
4. Variable resource availability hypothesis
5. Competition hypothesis
6. Microevolutionary change hypothesis
7. Escape from biotic constraints hypothesis
8. Biodiversity hypothesis
9. Disturbance and land use hypothesis
10. Anthropogenic hypothesis
3) What makes a species invasive?
10 hypotheses:
1. Vacant niche hypothesis No support
2. Allelopathy hypothesis
3. Environmental change hypothesis
4. Variable resource availability hypothesis
5. Competition hypothesis
6. Microevolutionary change hypothesis
7. Escape from biotic constraints hypothesis
8. Biodiversity hypothesis Conflicting support
9. Disturbance and land use hypothesis
10. Anthropogenic hypothesis
3) What makes a species invasive?
10 hypotheses:
1. Vacant niche hypothesis No support
2. Allelopathy hypothesis
3. Environmental change hypothesis
4. Variable resource availability hypothesis Very limited support
5. Competition hypothesis
6. Microevolutionary change hypothesis
7. Escape from biotic constraints hypothesis
8. Biodiversity hypothesis Conflicting support
9. Disturbance and land use hypothesis Special case of #10
10. Anthropogenic hypothesis
3) What makes a species invasive?
10 hypotheses:
1. Vacant niche hypothesis No support
2. Allelopathy hypothesis Specific cases, but how many?
3. Environmental change hypothesis
4. Variable resource availability hypothesis Very limited support
5. Competition hypothesis
6. Microevolutionary change hypothesis Specific cases ; how many?
7. Escape from biotic constraints hypothesis
8. Biodiversity hypothesis Conflicting support
9. Disturbance and land use hypothesis Special case of #10
10. Anthropogenic hypothesis
3) What makes a species invasive?
10 hypotheses:
1. Vacant niche hypothesis No support
2. Allelopathy hypothesis Specific cases, but how many?
3. Environmental change hypothesis Broad support, but exceptions
4. Variable resource availability hypothesis Very limited support
5. Competition hypothesis Broad support, but exceptions
6. Microevolutionary change hypothesis Specific cases ; how many?
7. Escape from biotic constraints Broad support;exceptions
8. Biodiversity hypothesis Conflicting support
9. Disturbance and land use hypothesis Special case of #10
10. Anthropogenic hypothesis
3) What makes a species invasive?
10 hypotheses:
1. Vacant niche hypothesis No support
2. Allelopathy hypothesis Specific cases, but how many?
3. Environmental change hypothesis Broad support, but exceptions
4. Variable resource availability hypothesis Very limited support
5. Competition hypothesis Broad support, but exceptions
6. Microevolutionary change hypothesis Specific cases ; how many?
7. Escape from biotic constraints Broad support;exceptions
8. Biodiversity hypothesis Conflicting support
9. Disturbance and land use hypothesis Special case of #10
10. Anthropogenic hypothesis Broad support, few exceptions
The literature on invasion ecology contains
a growing number of theories and
generalization, with much duplication,
redundancy and reinventing of the wheel.
There is a need to distill the fundamental
issues from the different theories, while
realizing that invasions are context
specific.
• Richardson and Pysek from Fifty years of
invasion ecology –the legacy of Charles
Elton, 2008