An observation:

1
Invasive plants THE 10 hypotheses

• An observation
• Tens Rule From Williamson Fitter (1996)
  Ecology 771661-1666
• Only 10 of imported species escape to wild
• Only 10 of species that make it to the wild
  become naturalized
• Only 10 of naturalized species become pests

2
Why are only some species invasive?
3
Why are only some species invasive?
If so few species become invasive, does a plant
species become invasive because it has certain
characteristics?
4
Why are only some species invasive?

• Species characteristics Plant Life History
  Traits (Chapter 3 of NRC 2002)
• Reproductive system
• Dioecious (male female flowers on separate
  plants) vs. Monoecious (on same plant)

5
Why are only some species invasive?

• Species characteristics Plant Life History
  Traits (Chapter 3 of NRC 2002)
• Reproductive system
• Dioecious vs. Monoecious
• Self-incompatible pollen vs. Self-compatible
  pollen

6
Why are only some species invasive?

• Species characteristics Plant Life History
  Traits (Chapter 3 of NRC 2002)
• Reproductive system
• Dioecious vs. Monoecious
• Self-incompatible pollen vs. Self-compatible
  pollen
• Some type of asexual reproduction
• Apomixis produce viable seed without
  fertilization
• Vegetative reproduction regenerate from stem or
  root fragments
• Clonal propogation new individuals produced
  through rhizomes

7
Why are only some species invasive?

• Species characteristics Plant Life History
  Traits (Chapter 3 of NRC 2002)
• Reproductive system (tend to be self-compatible
  monoecious w/ asexual reproduction)

8
Why are only some species invasive?

• Species characteristics Plant Life History
  Traits (Chapter 3 of NRC 2002)
• Reproductive system (tend to be self-compatible
  monoecious w/ asexual reproduction)
• Flowering fruiting periods (tend to be long)
• Juvenile period (tend to be short)
• Seed production (tend to be high)
• Germination cues (tend to be present)
• Light requirements (tend to be highly efficient,
  plastic, competitive)

9
Why are only some species invasive?

• Species characteristics Plant Life History
  Traits (Chapter 3 of NRC 2002)
• Reproductive system (tend to be self-compatible
  monoecious w/ asexual reproduction)
• Flowering fruiting periods (tend to be long)
• Juvenile period (tend to be short)
• Seed production (tend to be high)
• Germination cues (tend to be present)
• Light requirements (tend to be highly efficient,
  plastic, competitive)
• BUT
• Very few invasive plants have all these traits
• Some invasive plants have few or none of these
  traits

10
Why are only some species invasive?

• Species characteristics Plant Life History
  Traits (Chapter 3 of NRC 2002)
• Reproductive system (tend to be self-compatible
  monoecious w/ asexual reproduction)
• Flowering fruiting periods (tend to be long)
• Juvenile period (tend to be short)
• Seed production (tend to be high)
• Germination cues (tend to be present)
• Light requirements (tend to be highly efficient,
  plastic, competitive)
• BUT
• Very few invasive plants have all these traits
• Some invasive plants have few or none of these
  traits
• THUS must be more than just the species
  characteristics

11
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis
• Belowground chemical warfare hypothesis
• Environmental change hypothesis
• Variable resource availability hypothesis
• Competition hypothesis
• Microevolutionary change hypothesis
• Escape from biotic constraints hypothesis
• Biodiversity hypothesis
• Disturbance and land use hypothesis
• Anthropogenic hypothesis

12
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, fundamental niche Species A
Success
Resource axis 1
13
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, fundamental niche Species A
Success
Success
Resource axis 1
Resource axis 2
14
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, fundamental niche Species A
Resource axis 2
Resource axis 1
15
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, fundamental niche Species A
Resource axis 2
Resource axis 1
16
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, fundamental niche Species A Native,
fundamental niche Species B
Resource axis 2
Resource axis 1
17
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, fundamentalrealized niche Species
A Native, fundamentalrealized niche Species B
Resource axis 2
Resource axis 1
18
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, fundamentalrealized niche Species
A Native, fundamentalrealized niche Species
B Invader, fundamental niche Species C Invader,
fundamental niche Species D
Resource axis 2
Resource axis 1
19
Why are only some species invasive? 1) Vacant
Niche Hypothesis
Native, new realized niche Species A, Native,
new realized niche Species B Invader, realized
niche Species C Invader, realized niche
Species D
Resource axis 2
Resource axis 1
20
Why are only some species invasive? 1) Vacant
Niche Hypothesis

• Appealing ecological theory, BUT
• Actual demonstration of vacant niche is nearly
  impossible
• Many potential invaders lack pollinators,
  symbionts, etc.
• May have some utility for tropical oceanic islands

21
Why are only some species invasive? 2)
Belowground Chemical Warfare Hypothesis (aka
allelopathy)

• Allelopathy one plant releases chemicals that
  are toxic to another
• In natural environment, invader releases
  allelochemicals
• But the other members of the plant community have
  evolved with the invader
• Thus other plants are relatively immune to the
  allelochemicals
• In new invaded environment, invader releases
  allelochemicals
• Now the allelochemicals are novel to the other
  members of the plant community
• Thus other plants are susceptible to damage by
  the allelochemicals

22
Why are only some species invasive? 2)
Belowground Chemical Warfare Hypothesis

• Callaway Aschehoug (2000)
• C. diffusa releases chemicals that are NOT toxic
  to species in native Caucasus habitat, but
  chemicals are toxic to species in new Montana
  habitat
• Bais et al. (2003) Science 3011377-1380
• Identified the specific allelochemical
  ()-catechin that has higher concentration in C.
  diffusa soils and inhibits germination and growth
  of Montana grasses

23
Why are only some species invasive? 2)
Belowground Chemical Warfare Hypothesis

• Excellent support for some species (C. diffusa)
• BUT
• How many other species?

24
Why are only some species invasive? 3)
Environmental ltGlobalgt Change Hypothesis

• Global Changes
• Increasing atmospheric CO2 has 2 direct effects
  on plants
• Increases photosynthesis ? more C for growth
• Closes leaf stomates ? uses less water

Keeling Whorf (2004) CDIAC
25
Why are only some species invasive? 3)
Environmental ltGlobalgt Change Hypothesis
Global Changes Increasing atmospheric CO2 From
Smith et al. (2000) Red brome (Bromus
madritensis ssp. rubens)
26
Why are only some species invasive? 3)
Environmental ltGlobalgt Change Hypothesis
Global Changes Increasing atmospheric CO2 From
Dukes in Mooney Hobbs (2000) Fig. 5.1
Stimulation of growth by elevated CO2 for
invasives vs. noninvasives
27
Why are only some species invasive? 3)
Environmental ltGlobalgt Change Hypothesis

• Global Changes
• Increasing atmospheric CO2
• Evidence in some specific cases
• But not all invasives benefit
• Similar results for other global change factors
  (N deposition, warming)

28
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
• Competition hypothesis
• Microevolutionary change hypothesis
• Escape from biotic constraints
• Biodiversity hypothesis
• Disturbance and land use hypothesis
• Anthropogenic hypothesis

29
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• In most plant communities at most times, most of
  the resources that are available are taken up by
  the plants
• Plant community becomes susceptible whenever
  there is an increase in the amount of unused
  resources (? availability, ? uptake, or both)
• Changes in availability uptake naturally occur
  through time
• Assumes invaders have access to variable
  resources

30
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• Competition hypothesis General support, but
  exceptions

31
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• Competition hypothesis General support, but
  exceptions
• Microevolutionary change hypothesis Specific
  cases how many?

32
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• Competition hypothesis General support, but
  exceptions
• Microevolutionary change hypothesis Specific
  cases how many?
• Escape from biotic constraints General support
  but exceptions

33
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• Competition hypothesis General support, but
  exceptions
• Microevolutionary change hypothesis Specific
  cases how many?
• Escape from biotic constraints General support
  but exceptions
• Biodiversity hypothesis Conflicting support

34
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• Competition hypothesis General support, but
  exceptions
• Microevolutionary change hypothesis Specific
  cases how many?
• Escape from biotic constraints General support
  but exceptions
• Biodiversity hypothesis Conflicting support
• Disturbance and land use hypothesis Special
  case of 4 10

35
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• Competition hypothesis General support, but
  exceptions
• Microevolutionary change hypothesis Specific
  cases how many?
• Escape from biotic constraints General support
  but exceptions
• Biodiversity hypothesis Conflicting support
• Disturbance and land use hypothesis Special
  case of 4 10
• Anthropogenic hypothesis Broad support, few
  exceptions

36
Why are only some species invasive?

• 10 hypotheses compiled from literature
• Vacant niche hypothesis No support
• BCW hypothesis Specific cases, but how many?
• Environmental change hypothesis Limited support
• Variable resource availability hypothesis
  Limited support
• Competition hypothesis General support, but
  exceptions
• Microevolutionary change hypothesis Specific
  cases how many?
• Escape from biotic constraints General support
  but exceptions
• Biodiversity hypothesis Conflicting support
• Disturbance and land use hypothesis Special
  case of 4 10
• Anthropogenic hypothesis Broad support, few
  exceptions
• Any missing hypotheses?
• Is summary accurate for each hypothesis?