Terminology

The scope of the problem

Economic impacts

Questions, hypotheses, examples

Some terminology:

exotic introduced alien non-indigenous

weed ruderal

Some terminology:

exotic introduced alien non-indigenous

weed ruderal

invasive

Some terminology:

exotic introduced alien non-indigenous

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…

“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

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?

What makes some communities invasible?

Why do some species become invasive?

Invasive plants havedifferent 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 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?

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?