Terminology The scope of the problem Economic impacts Questions, hypotheses, examples

Some terminology: exoticintroducedaliennon-indigenous

weedruderal Some terminology: exoticintroducedaliennon-indigenous

weedruderal invasive Some terminology: exoticintroducedaliennon-indigenous

weedruderal invasive Can a native plant be invasive? Some terminology: exoticintroducedaliennon-indigenous

Little understory Decreased forage increased soil erosion Decreased soil fertility Decreased range value Pinyon-juniper expansion…

“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? introductioninvasion time

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 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

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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

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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? 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

Why do some species become invasive? Climate-matching…

Why do some species become invasive? Some hypotheses…

Natural Enemies Hypothesis Evolutionary increased competitive ability (Blossey, Callaway, Buckley et al., 2003) Why do some species become invasive?

Why are some communities invasible? Disturbance Low diversity Biotic Resistance Hypothesis

Why are some communities invasible? Disturbance Low diversity Biotic Resistance Hypothesis fire

Very diverse communities are highly invaded Stohlgren et al., multiple times

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 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? 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?

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?