Why do Invasive Species Successfully Establish & Invade?

Many are introduced, few become invasive

Hypotheses about why invaders succeed: 1. Invasive species have traits that favor establishment and spread 2. Invasive species are released from enemies 3. Invasive species exploit empty niches 4. Invasive species are favored by anthropogenic pressure on natives (disturbance)

What traits favor invasive species?

Traits favoring invasive plants Seed production More seeds, more likely to be invasive Brassica tournefortii

Traits favoring invasive plants Seed production Rapid growth Faster growth, more likely to be invasive Polygonum cuspidatum

Traits favoring invasive plants Seed production Rapid growth Phenological plasticity Willis et al., 2010

Traits vary by invasion stage Stage Plant Traits Theoharides & Dukes, 2007

Problem: We ve found very few traits common to invasive species, and none of them are universal You ll explore this topic more in team exercise #2 next week

Observation: Invasive plants perform better in their non-native range Non-native Range Native Range Mack et al., 2000

Enemy Release Hypothesis Release: fewer herbivores are eating me Advantage: non-native plants grow more leaves/roots/flowers and invasion succeeds

Enemy Release Hypothesis Release: fewer herbivores are eating me Advantage: non-native plants grow more leaves/roots/flowers and invasion succeeds Alternative: Biotic Resistance Resistance: new species are eating me and competing with me Disadvantage: non-native plants are less successful and invasion fails

Life of the plant

Specialists vs. Generalists

Enemy Release in Plants Study of 473 naturalized plants in the U.S. Mitchell & Power, 2003

Enemy Release in Plants Mitchell & Power, 2003

Enemy Release in Plants Noxiousness/Invasiveness: Proportion of states declared noxious/invasive Mitchell & Power, 2003

Enemy Release in Animals Study of 26 non-native species Torchin et al., 2003

Evolution of increased competitive ability (EICA) Genetic diversity creates opportunities for rapid phenotypic response

Optimal defense hypothesis Defense is costly for plant fitness Tradeoffs between growth and defense. Too little defense and you re eaten, too much defense and you can t compete.

Defense! Toxins (aka allelochemicals)

Defense! Toxins (aka allelochemicals) Structural defense

Defense! Toxins (aka allelochemicals) Structural defense Tolerance Keep a reserve of resources to regrow what s been eaten

Evolution of increased competitive ability (EICA) Start with advantage due to enemy release Optimal defenses evolve in response to enemy release With sufficient genetic diversity, species can evolve rapidly to spend more resources growing and fewer resources defending

Species of the day: Cane toad (Bufo marinus) Native to South & Central America Introduced to Australia for biocontrol of beetles in sugar cane Secretes a toxin (bufotoxin) when threatened poisonous to most animals

Gradient of toad arrival history Phillips et al., 2004

Snakes that eat toads are poisoned Red-bellied black snake Green tree snake Phillips et al., 2004

Snake evolution in response to toads 1. Greater exposure leads to increased resistance to toad toxins Resistance

Snake evolution in response to toads 1. Greater exposure leads to increased resistance to toad toxins 2. Greater exposure leads to snakes with larger bodies, but smaller heads Change in body size Change in head size Less resistant to toxin

Purple loosestrife and specialist enemies G. pusilla leaf feeder L. salicaria invasive plant H. transversovittatus root feeder