Transcript Document
Terminology The scope of the problem Economic impacts Questions, hypotheses, examples Some terminology: exotic introduced alien non-indigenous Some terminology: exotic weed introduced ruderal alien non-indigenous Some terminology: exotic weed invasive introduced ruderal alien non-indigenous Some terminology: exotic weed introduced alien non-indigenous ruderal invasive Can a native plant be invasive? Pinyon-juniper expansion… •Little understory •Decreased forage •increased soil erosion •Decreased soil fertility •Decreased range value “If humans introduced the species and humans are natural, then aren’t introduced species natural?” •Most invaders have not reached their new ranges via dispersal mechanisms that have evolved in their specific lineage(s); •Biogeographic barriers to dispersal (oceans, mtns, etc) have been broken down by humans; •The rate at which we’re moving species around the globe is much higher than that at which species would move without human intervention. The scope of the problem May be the second largest cause of species declines after habitat loss: (Wilcove et al., 1998) The scope of the problem, or How many sp have been introduced? introduction invasion time The scope of the problem, or How many sp have been introduced? 5000 exotic plant species have become established in non-cultivated ecosystems in US (compared to 17,000 native species) The scope of the problem, or How many sp have been introduced? Florida: 25,000 introduced; 900 have become established in surrounding natural ecosystems (Simberloff et al., 1997) <4% The “tens” rule: 1000 plants introduced 100 escape 10 become naturalized 1 will become invasive Williamson and Fitter, 1996 Biodiversity has increased 20% in North America mainly due to introductions… No extinctions through plant-plant competition Davis, 2003 Costs due to invasive exotic species: FORESTS: ? RANGE systems: $1 billion in lost forage crops $5 billion spent by ranchers to control weeds (palatability issue – e.g., leafy spurge and some thistles) Pimental et al., 2000 ? Why do some species become invasive? What makes some communities invasible? Why do some species become invasive? Invasive plants have different strategies: stolons: To copious seed production: Why do some species become invasive? Predictive models: •pines in S. Africa (Rejmanek and Richardson, 1996) – 24 pine species 10 life history traits •Woody plants in US (Reichard & Hamilton, 1996) – Why do some species become invasive? Taxonomic analysis (à la Daehler, 1998) •N-fixing •Aquatic •Clonal (trees) •Grasses •Climbers Why do some species become invasive? Climate-matching… Why do some species become invasive? Some hypotheses… Why do some species become invasive? Natural Enemies Hypothesis Evolutionary increased competitive ability (Blossey, Callaway, Buckley et al., 2003) Why are some communities invasible? Biotic Resistance Hypothesis •Disturbance •Low diversity Why are some communities invasible? Biotic Resistance Hypothesis •Disturbance •Low diversity fire Very diverse communities are highly invaded Stohlgren et al., multiple times Upshot on community invasibility: Given sufficient propagule supply, few communities are likely to remain free of invasion Some particular issues for forest and range systems: Next lecture – range weeds (cheatgrass, among others) Today: some specific forest/ forestry concerns EXOTIC •Pests (insects, disease) •Trees •Understory •Exotic tree plantations •Exotic species in the understory Exotic tree plantations •Dominate forest productivity in many areas Sitka spruce in Britain Caribean pine and slash pine in Australia Loblolly pine in various areas Problems? •where plantations are adjacent to native forests Exotic pines are considered invasive in Australia •They exhaust soil water and nutrient reserves, •Decreased understory growth (with decreases in overall diversity) •Soil erosion •Loss of soil fertility On the other hand, On sites with degraded soils, fast-growing trees (like eucalypts) help regeneration of understory species from surrounding areas, thus increasing their biodiversity and fertility (Harrington and Ewel, 1998) Caveat: but abundance is constrained by presence of alien species Exotic understory species in native forests •How do invasive species change the understory component? •How do such changes affect ecosystem processes like productivity (NPP)? Example: Australian dry rainforests & subtropical rainforests Rubber vine Changing understory? Changing NPP? Camphor laurel Japanese barberry is a pest in US eastern forests: Changing understory? Changing NPP? How do management practices affect invasions? Are managed forests more invaded than protected ones? No: Kaufman et al., 2003 What are the mechanisms? •Competition? •Allelopathy? •Alteration of ecosystem variables? •Other processes? < 5% of “impact” studies actually document this (Levine et al., 2003) Why do only a small fraction of established exotic plants have significant impacts? Why do invaders have large impacts in some systems but not others?