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 ImpactsEcosystem 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

Denslow et al. (2006) Oecologia 148: 118-128

Dodonea viscosaA’ali’i

Pipturus albidusMamaki

Sophora chrysophyllaMamane

Acacia koaKoa

Denslow et al. (2006) Oecologia 148: 118-128

Question: How has invasion by Cape ivy affected 3 coastal habitats in SF Bay Area?

Methods: Comparative and experimental field studies

Alvarez & Cushman (2002) Ecological Applications 12:1434-1444

Ecological Impacts: Replacement

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

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 ImpactsThreatened & endangered species~ 614 animals and 749 plants are federally listed in US

~ a quarter of species are believed to be threatened by direct effects of invasive species

~ half of federally listed species are believed to be associated with invasives (includes plants plus other organisms)

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

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

Ecological Impacts

• Ecosystem functions (C sequestration, N cycling, water, fire frequency/intensity)

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

• Ecosystem functions (C sequestration, N cycling, water, fire frequency/intensity)

• Effects observed as:Species replacements (direct/individual or large scale)

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

• 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)