An observation:• “Tens Rule” From Williamson & Fitter (1996) Ecology 77:1661-1666

Only ~10% of imported species escape to wildOnly ~10% of species that make it to the wild become naturalizedOnly ~10% of naturalized species become pests

Invasive plants: THE 10 hypotheses

Why are only some species invasive?

If so few species become invasive, does a plant speciesbecome invasive because it has certain characteristics?

Why are only some species invasive?

Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002)

1. Reproductive system• Dioecious (male & female flowers on separate plants) vs.

Monoecious (on same plant)

Why are only some species invasive?

Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002)

1. Reproductive system• Dioecious vs. Monoecious• Self-incompatible pollen vs. Self-compatible pollen

Why are only some species invasive?

Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002)

1. Reproductive system• Dioecious vs. Monoecious• Self-incompatible pollen vs. Self-compatible pollen• Some type of asexual reproduction

Apomixis – produce viable seed without fertilizationVegetative reproduction – regenerate from stem or root fragmentsClonal propogation – new individuals produced through rhizomes

Why are only some species invasive?

Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002)

1. Reproductive system (tend to be: self-compatible monoecious w/ asexual reproduction)

Why are only some species invasive?

Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002)

1. Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction)

2. Flowering & fruiting periods (tend to be: long)3. Juvenile period (tend to be: short)4. Seed production (tend to be: high)5. Germination cues (tend to be: present)6. Light requirements (tend to be: highly efficient, plastic, &

competitive)

Why are only some species invasive?

Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002)

1. Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction)

2. Flowering & fruiting periods (tend to be: long)3. Juvenile period (tend to be: short)4. Seed production (tend to be: high)5. Germination cues (tend to be: present)6. 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

Why are only some species invasive?

Species characteristics: Plant Life History Traits (Chapter 3 of NRC 2002)

1. Reproductive system (tend to be: self-compatible & monoecious w/ asexual reproduction)

2. Flowering & fruiting periods (tend to be: long)3. Juvenile period (tend to be: short)4. Seed production (tend to be: high)5. Germination cues (tend to be: present)6. 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

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis2. Belowground chemical warfare hypothesis3. Environmental change hypothesis4. Variable resource availability hypothesis5. Competition hypothesis6. Microevolutionary change hypothesis7. Escape from biotic constraints hypothesis8. Biodiversity hypothesis9. Disturbance and land use hypothesis10.Anthropogenic hypothesis

Why are only some species invasive?

Native, fundamental niche – Species A

Resource axis #1

Suc

cess

Why are only some species invasive?1) Vacant Niche Hypothesis

Native, fundamental niche – Species A

Resource axis #1

Suc

cess

Resource axis #2

Suc

cess

Why are only some species invasive?1) Vacant Niche Hypothesis

Native, fundamental niche – Species A

Resource axis #1

Res

ourc

e ax

is #

2

Why are only some species invasive?1) Vacant Niche Hypothesis

Native, fundamental niche – Species A

Resource axis #1

Res

ourc

e ax

is #

2

Why are only some species invasive?1) Vacant Niche Hypothesis

Native, fundamental niche – Species ANative, fundamental niche – Species B

Resource axis #1

Res

ourc

e ax

is #

2

Why are only some species invasive?1) Vacant Niche Hypothesis

Native, fundamental=realized niche – Species ANative, fundamental=realized niche – Species B

Resource axis #1

Res

ourc

e ax

is #

2

Why are only some species invasive?1) Vacant Niche Hypothesis

Native, fundamental=realized niche – Species ANative, fundamental=realized niche – Species BInvader, fundamental niche – Species CInvader, fundamental niche – Species D

Resource axis #1

Res

ourc

e ax

is #

2

Why are only some species invasive?1) Vacant Niche Hypothesis

Native, new realized niche – Species A, Native, new realized niche – Species BInvader, realized niche – Species CInvader, realized niche – Species D

Resource axis #1

Res

ourc

e ax

is #

2

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

Why are only some species invasive?1) Vacant Niche Hypothesis

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 communityThus other plants are susceptible to damage by the

allelochemicals

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

• 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 301:1377-1380Identified the specific allelochemical: (–)-catechin that has higher

concentration in C. diffusa soils and inhibits germination and growth of Montana grasses

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

• Excellent support for some species (C. diffusa)

BUT• How many other species?

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

Mauna Loa

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Atm

osph

eric

[C

O2]

(pp

mv)

300

320

340

360

380

400

Global Changes• Increasing atmospheric CO2 has 2 direct effects on plants:

Increases photosynthesis → more C for growthCloses leaf stomates → uses less water

Keeling & Whorf (2004) CDIAC

Why are only some species invasive?3) Environmental <Global> Change Hypothesis

Global Changes – Increasing atmospheric CO2

From Smith et al. (2000): Red brome (Bromus madritensis ssp. rubens)

Why are only some species invasive?3) Environmental <Global> 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

Why are only some species invasive?3) Environmental <Global> Change Hypothesis

Global Changes• Increasing atmospheric CO2

Evidence in some specific casesBut not all invasives benefit

Similar results for other global change factors (N deposition, warming)

Why are only some species invasive?3) Environmental <Global> Change Hypothesis

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis5. Competition hypothesis6. Microevolutionary change hypothesis7. Escape from biotic constraints8. Biodiversity hypothesis9. Disturbance and land use hypothesis10.Anthropogenic hypothesis

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. 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

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis Limited support5. Competition hypothesis General support, but exceptions

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis Limited support5. Competition hypothesis General support, but exceptions6. Microevolutionary change hypothesis Specific cases ; how many?

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis Limited support5. Competition hypothesis General support, but exceptions6. Microevolutionary change hypothesis Specific cases ; how many?7. Escape from biotic constraints General support; but exceptions

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis Limited support5. Competition hypothesis General support, but exceptions6. Microevolutionary change hypothesis Specific cases ; how many?7. Escape from biotic constraints General support; but exceptions8. Biodiversity hypothesis Conflicting support

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis Limited support5. Competition hypothesis General support, but exceptions6. Microevolutionary change hypothesis Specific cases ; how many?7. Escape from biotic constraints General support; but exceptions8. Biodiversity hypothesis Conflicting support9. Disturbance and land use hypothesis Special case of #4 & #10

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis Limited support5. Competition hypothesis General support, but exceptions6. Microevolutionary change hypothesis Specific cases ; how many?7. Escape from biotic constraints General support; but exceptions8. Biodiversity hypothesis Conflicting support9. Disturbance and land use hypothesis Special case of #4 & #1010.Anthropogenic hypothesis Broad support, few exceptions

Why are only some species invasive?

10 hypotheses compiled from literature:1. Vacant niche hypothesis No support2. BCW hypothesis Specific cases, but how many?3. Environmental change hypothesis Limited support4. Variable resource availability hypothesis Limited support5. Competition hypothesis General support, but exceptions6. Microevolutionary change hypothesis Specific cases ; how many?7. Escape from biotic constraints General support; but exceptions8. Biodiversity hypothesis Conflicting support9. Disturbance and land use hypothesis Special case of #4 & #1010.Anthropogenic hypothesis Broad support, few exceptions

Any missing hypotheses?Is summary accurate for each hypothesis?

Why are only some species invasive?