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

Impacts of Invasions Ecology –Species replacement –Ecosystem functions –Threatened & endangered species Economic –Total environmental damage estimates Social –Water quantity and quality –Human health

Conceptual model: Walker & Smith in Lukens & Thieret (1997) Invasive species affect different community & ecosystem processes Ecological Impacts

Types of impacts (Parker et al. 1999) –Impacts on ecosystem properties –Impacts on community structure –Impacts on individual species

Ecological Impacts Impacts on ecosystem properties 1. Disturbance regimes

D’Antonio in Mooney & Hobbs (2002)

Ecological Impacts Impacts on ecosystem properties 2. Resource dynamics

Ehrenfeld (2003) Ecosystems 6:

Question: How is N cycling affected by exotic grass invasion? Method: Examined rates of N mineralization & nitrification, and characterized soil microbial communities Ecological Impacts: Nitrogen Hawkes et al. (2005) Ecology Letters 8:

Ehrenfeld (2003) Ecosystems 6:

Ecological Impacts: Carbon Woody plant invasion into grasslands is thought to increase amount of C stored

Question: Does woody plant invasion increase C sequestration? Method: Examined 6 sites along precipitation gradient Ecological Impacts: Carbon Jackson et al. (2002) Nature 418:

Ehrenfeld (2003) Ecosystems 6:

Ladenburger et al. (2005) J. of Arid Env 65: Tamarix ramosissima

Kulmatiski et al. (2006) Plant & Soil 288: _______ ___ _____ ________ _____ ______ __ _____ ___ _____ ___ ______ _______ ______ __ ____ ___ _____ ______ Ecological Impacts: Water

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

Crooks (2002) OIKOS 97:

Ecosystem Impacts: Transformers Question: What is the long-term impact of C. pubescence invasion on Galapagos vegetation? Methods: Monitoring permanent plots over 7 years 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

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 –Effects can be by: Direct species replacement Indirect through effects on community structure or function

Ecological Impacts Threatened & endangered species ~ 409 animals and 598 plants are federally listed in US 294 (29%) believed threatened by direct effects of invasive species ~ 400 of 958 federally listed species (~42%) believed to be due to invasives (includes plants plus other organisms) 104 records of extinctions directly due to invasives 88 animals (many birds, NZ and HI) 16 plants

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)

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:

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

Ecosystem functions (C sequestration, N fixation, fire frequency/intensity)

Ecological Impacts Ecosystem functions (C sequestration, N fixation, fire frequency/intensity) 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)