Impacts of Invasions
• Measuring impact is complex
– What should be measured and how?
− For individual plant, individual species,
or multiple species?
− Over what time frame?
− Lack of comprehensive data
Ecological Impacts
• Invasive species affect different
community & ecosystem processes
1. Disturbance regimes
• Fire
2. Resource dynamics
• Nutrients: N and C cycling
• Water: amount, timing, location
Ecological Impacts
Ecosystem engineers: species able to physically
alter habitats (Crooks 2002)
•Alter ecosystem physical
processes (sedimentation,
water availability, N
cycling)
•Change habitat structure
(more or less complexity)
•Effects cascade through
community
Jager et al (2009) J of Ecology 97:1252-1263
Ecological Impacts
• Impacts on community structure
1. Productivity
Ecological Impacts
• Impacts on community structure
2. Community dynamics
Ecological Impacts
• Changes in productivity and community
dynamics often observed as:
1. Direct competition
2. Large scale species displacement
Ecological Impacts: Competition
Question: How is invasion by exotic grasses impacting native
recruitment in dry forests of Hawaii?
Methods: Experimental field studies removing invasive grasses
Dodonea viscosa
A’ali’i
Pipturus
Acaciaalbidus
koa
Mamaki
Koa
Denslow et al. (2006) Oecologia 148: 118-128
Sophora chrysophylla
Mamane
Denslow et al. (2006) Oecologia 148: 118-128
Ecological Impacts: Replacement
Question: How has invasion by Cape ivy
affected 3 coastal habitats in
SF Bay Area?
Methods: Comparative and experimental
field studies
Delairea odorata - Cape Ivy
Alvarez & Cushman (2002) Ecological Applications 12:1434-1444
Alvarez & Cushman (2002) Ecological Applications 12:1434-1444
Alvarez & Cushman (2002) Ecological Applications 12:1434-1444
Alvarez & Cushman (2002) Ecological Applications 12:1434-1444
Alvarez & Cushman (2002) Ecological Applications 12:1434-1444
Ecological Impacts: Mutualisms
• Question: How does invasion by Alliaria petiolata alter
plant composition of NA forests?
• Methods: Examined mycorrhizal colonization of tree
roots from soil cultured with Alliaria
Stinson et al. (2006) PLoS Biology 4: 140
Stinson et al. (2006) PLoS Biology 4: 140
Ecological Impacts
• Species impacts can be by:
• Indirect effects on community structure or function
• Direct species replacement
Ecological Impacts
Threatened & endangered species
~ 614 animals and 749 plants are federally listed in US
~ half of federally listed species are believed to be
associated with invasives (includes plants plus
other organisms)
~ a quarter of species are believed to be
threatened by direct effects of invasive species
http://www.fws.gov/Endangered/wildlife.html#Species
Ecological Impacts: T&E Species
Question: Is there a
relationship between
species invasion and
imperiled species in CA?
Methods: examined
distribution of 834 exotic
plants in CA & correlated it
to imperiled species using
multivariate analyses
(CCA, SEM)
Seabloom et al. (2006) Ecological Applications 16: 1338 - 1350
Seabloom et al. (2006) Ecological Applications 16: 1338 - 1350
Seabloom et al. (2006) Ecological Applications 16: 1338 - 1350
Ecological Impacts: Extinctions
• Question: How have 8 endangered plants been
impacted by introduced species?
• Method: Literature and field surveys
• galopagos
Ecological impacts: Extinctions
How much of a role do invasive species
really play?
Extinctions are caused by multiple factors:
1. Habitat destruction
2. Invasive species
3. Pollution
4. Disease
Most ‘documented’ extinctions involve speculation
Ecological impacts: Extinctions
• Animals more impacted than plants
• Mainland less impacted than islands
Sax & Gains (2008) PNAS 105: 11490-11497
Sax et al. (2002) American Naturalist 160: 766-783
Ecological Impacts
• Do invasions always negatively impact
ecosystem properties and community
structure?
Positive impacts
• All of the factors that have a negative
impact on native plants can also be
positive under certain conditions
– Competitive release
– Facilitation by acting as nurse plants
– Trophic subsidy
– Pollination
– Predatory release
Ecological Impacts
Summary
• Ecological impacts typically involve:
(1) nutrients/water flow
(2) primary production impacts
(3) alterations of disturbance regimes
(4) changes in community dynamics
Ecological Impacts
Summary
• Ecological impacts typically involve:
(1) nutrients/water flow
(2) primary production impacts
(3) alterations of disturbance regimes
(4) changes in community dynamics
•
Ecosystem functions (C sequestration, N cycling, water,
fire frequency/intensity)
Ecological Impacts
Summary
• Ecological impacts typically involve:
(1) nutrients/water flow
(2) primary production impacts
(3) alterations of disturbance regimes
(4) changes in community dynamics
•
Ecosystem functions (C sequestration, N cycling, water,
fire frequency/intensity)
• Effects observed as:
Species replacements (direct/individual or large scale)
Ecological Impacts
Summary
• Ecological impacts typically involve:
(1) nutrients/water flow
(2) primary production impacts
(3) alterations of disturbance regimes
(4) changes in community dynamics
•
Ecosystem functions (C sequestration, N cycling, water,
fire frequency/intensity)
• Effects observed as:
Species replacements (direct/individual or large scale)
• Loss of native species (threatened or endangered species)
• Often in conjunction with human-caused habitat change
• Especially on islands
• Especially rare/specialized species
• More evidence for population reduction than for extinction
(e.g. Harrison et al 2006)