Terminology The scope of the problem Economic impacts Questions, hypotheses, examples.
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Transcript of Terminology The scope of the problem Economic impacts Questions, hypotheses, examples.
weed ruderal
invasive
Can a native plant be invasive?
Some terminology:
exotic introduced alien non-indigenous
•Little understory•Decreased forage
•increased soil erosion
•Decreased soil fertility
•Decreased range value
Pinyon-juniper expansion…
•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)
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 scope of the problem, or How many sp have been introduced?
The “tens” rule:
1000 plants introduced100 escape10 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
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)
Costs due to invasive exotic species:
Pimental et al., 2000
Why do some species become invasive?
•pines in S. Africa (Rejmanek and Richardson, 1996) –
24 pine species
10 life history traits
•Woody plants in US (Reichard & Hamilton, 1996) –
Predictive models:
Why do some species become invasive?
Taxonomic analysis (à la Daehler, 1998)
•N-fixing•Aquatic•Clonal (trees)•Grasses•Climbers
Natural Enemies Hypothesis
Evolutionary increased competitive ability (Blossey, Callaway, Buckley et al., 2003)
Why do some species become invasive?
Given sufficient propagule supply, few communities are likely to remain free of invasion
Upshot on community invasibility:
Some particular issues for forest and range systems:
Next lecture – range weeds (cheatgrass, among others)
Today: some specific forest/ forestry concerns
Exotic tree plantations
•Dominate forest productivity in many areas
Sitka spruce in BritainCaribean pine and slash pine in AustraliaLoblolly pine in various areas
Problems?
•They exhaust soil water and nutrient reserves,•Decreased understory growth (with decreases in overall diversity)•Soil erosion•Loss of soil fertility
•where plantations are adjacent to native forests
Exotic pines are considered invasive in Australia
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 Camphor laurel
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)