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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 Ecological Impacts 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: 503-523 Ecological Impacts: Nitrogen Question: How is N cycling affected by exotic grass invasion? Method: Examined rates of N mineralization & nitrification, and characterized soil microbial communities Hawkes et al. (2005) Ecology Letters 8:976-985 Hawkes et al. (2005) Ecology Letters 8:976-985 Ehrenfeld (2003) Ecosystems 6: 503-523 Ecological Impacts: Carbon Woody plant invasion into grasslands is thought to increase amount of C stored Ecological Impacts: Carbon Question: Does woody plant invasion increase C sequestration? Method: Examined 6 sites along precipitation gradient Jackson et al. (2002) Nature 418:623-626 Jackson et al. (2002) Nature 418:623-626 Jackson et al. (2002) Nature 418:623-626 Ehrenfeld (2003) Ecosystems 6: 503-523 Tamarix ramosissima Ladenburger et al. (2005) J. of Arid Env 65: 111-128. Ecological Impacts: Water Kulmatiski et al. (2006) Plant & Soil 288: 271-284 _______ ___ _____ ________ _____ ______ __ _____ ___ _____ ___ ______ _______ ______ __ ____ ___ _____ ______ 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: 153-166 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 97:1252-1263 Jager et al (2009) J of Ecology 97:1252-1263 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 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 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 – 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 ~ 400 of 958 federally listed species (~42%) believed to be due to invasives (includes plants plus other organisms) 294 (29%) believed threatened by direct effects of invasive species 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: 11490-11497 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 • 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)