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:

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: Dodonea viscosa A’ali’i Pipturus albidus Mamaki Sophora chrysophylla Mamane Acacia koa Koa

Denslow et al. (2006) Oecologia 148:

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: Ecological Impacts: Replacement Delairea odorata - Cape Ivy

Alvarez & Cushman (2002) Ecological Applications 12:

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

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 ~ 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)

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:

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:

Sax et al. (2002) American Naturalist 160:

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)