3) What makes a species invasive?
i) 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
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Basic concepts:
• Invasive species are ruderals
• These types of species are typically associated with disturbed
types of habitats.
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Basic concepts:
• Invasive species are ruderals
• Ruderals associated with disturbed habitats
• Changes in land use cause changes in the extent and
frequency of disturbance to an ecosystem
3) What makes a species invasive?
i) 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
3) What makes a species invasive?
i) 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
3) What makes a species invasive?
i) 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?
i) 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
3) What makes a species invasive?
i) 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 back to original state
3) What makes a species invasive?
i) 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
3) What makes a species invasive?
i) 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
3) What makes a species invasive?
i) 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?
i) 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?
i) 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
3) What makes a species invasive?
i) 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
3) What makes a species invasive?
i) 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, provides
opportunities for other species to exist
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Evidence: D’Antonio & Vitousek (1992)
• Without invasive species, typically if disturb woodlands, they
eventually return back to woodlands
Recovery
Disturbance
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Evidence: D’Antonio & Vitousek (1992)
• Without invasive species, eventually return back to woodlands
• But if have invasions of alien grasses during or after land is
cleared, then alien grasses induce a novel disturbance: fire
Recovery
Disturbance
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Evidence: D’Antonio & Vitousek (1992)
• Without invasive species, eventually return back to woodlands
• With 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
Recovery
Disturbance
3) What makes a species invasive?
i) 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
Also 260 alien non-weeds
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Evidence: From Kalin Arroyo et al. in Mooney & Hobbs (2000)
• Number of alien plants in political regions of Chile
• For each political region, also determined density of roads and
how much land area was in agricultural and urban use
Computed a “Land Use Index” from these that goes from “low” (1)
to “intensive” (11)
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Evidence: From Kalin Arroyo et al. in
Mooney & Hobbs (2000)
• Number of alien plants in political
regions of Chile
• Corresponding Land Use Index:
from “low” (1) to “intensive” (11)
• For alien weeds (A), number
increased significantly with
index
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Evidence: From Kalin Arroyo et al. in
Mooney & Hobbs (2000)
• Number of weedy plants in
political regions of Chile
• Corresponding Land Use Index:
from “low” (1) to “intensive” (11)
• Alien weeds increase with intesity
of land use
• Similarly for Total Number of
Alien Species (B)
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Summary: Disturbance and land use hypothesis
• Consistent with ecological theories
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Summary: Disturbance and land use hypothesis
• Consistent with ecological theories
• Evidence from a variety of ecosystems
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Summary: Disturbance and land use hypothesis
• Consistent with ecological theories
• Evidence from a variety of ecosystems
• Empirical correlations
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Summary: Disturbance and land use hypothesis
• 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
3) What makes a species invasive?
i) Disturbance and land use hypothesis
Summary: Disturbance and land use hypothesis
• Consistent with ecological theories
• Evidence from woodland / grasslands
• Empirical correlations
But
• Is disturbance / land use the factor?
• Plant establishment and invasion occur without disturbance
3) What makes a species invasive?
j) Anthropogenic hypothesis
Basic concepts:
• Human activities are the cause of invasive species
3) What makes a species invasive?
j) Anthropogenic hypothesis
Basic concepts:
• Human activities are the cause of invasive species
• Humans help invasives overcome barriers; e.g. Richardson et
al. (2000)
Humans are the vector for long distance transport
Geographic
Home range
Introduced
Environment area Reproduce
Disperse
Disturbed
area
Natural
area
3) What makes a species invasive?
j) 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 alter the environment that allows plants to grow and
reproduce
Geographic
Home range
Introduced
Environment area Reproduce
Disperse
Disturbed
area
Natural
area
3) What makes a species invasive?
j) 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?
j) Anthropogenic hypothesis
Basic concepts:
• Human activities are the cause of invasive species
• Humans help invasives overcome barriers
• Thus, humans and invasive species are interdependent
3) What makes a species invasive?
j) 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?
j) 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?
j) 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?
j) 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?
j) Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction?
3) What makes a species invasive?
j) 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?
j) 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?
j) 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?
j) 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?
j) 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”)
3) What makes a species invasive?
j) 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?
j) 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?
j) Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
3) What makes a species invasive?
j) Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
Example: Riparian areas in western US
Originally “cottonwood gallery forest”
Cottonwood and willow dominated ecosystem
Open forest with understory of shrubs and herbs
Used by diverse array of animals and birds
Re-establish after spring floods:
Produce abundant wind-dispersed seed in late-spring
Colonize moist, exposed sediments from floods
Grow rapidly (not shade tolerant)
3) What makes a species invasive?
j) Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
Example: Riparian areas in western US
Originally “cottonwood gallery forest”
First large impacts came during Spanish settlement
Primarily ranching and irrigated agriculture
3) What makes a species invasive?
j) Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
Example: Riparian areas in western US
Originally “cottonwood gallery forest”
Spanish settlement: ranching and irrigated agriculture
Expansion of US during mid-1800’s
Trappers removed upstream beavers; resulted in ↑ sediment
loads
Large-scale ranching, irrigated agriculture, logging, & mining;
resulted in land clearing and extensive disturbance
3) What makes a species invasive?
j) Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
Example: Riparian areas in western US
Originally “cottonwood gallery forest”
Spanish settlement: ranching and irrigated agriculture
US expansion (mid-1800’s): sedimentation, clearing, disturbance
Early 1900’s: Construction of dams, levees, drainage ditches
Prevented spring flood cycle
Dried fuel and litter accumulated → ↑ fires
3) What makes a species invasive?
j) Anthropogenic hypothesis
Evidence:
• Humans as vector for introduction
• Human alterations of ecosystem properties
Example: Riparian areas in western US
Originally “cottonwood gallery forest”
Spanish settlement: ranching and irrigated agriculture
US expansion (mid-1800’s): sedimentation, clearing, disturbance
Dam construction (early 1900’s): prevented spring flood; fires
Saltcedars & Russian olive introductions (mid-late 1800’s)
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?
j) Anthropogenic hypothesis
Summary: Abundant evidence that
• Humans are major vector for introduction
• Humans greatly alter ecosystem properties
3) What makes a species invasive?
j) Anthropogenic hypothesis
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