Invasive Species

1
Invasive Species

• Organisms introduced to areas where they did not
  formerly occur
• Usually rapidly expire
• Some maintain local populations
• Some are extremely successful (invade)
• Costs estimated at 120 billion per year in US
  alone all natural disasters combined!

2
Success of Introduced Species

• Odds are against introduced species - number of
  individuals is usually small, habitat is usually
  unfamiliar.
• Even if the population gets established, random
  forces can cause extinction when population size
  is small.
• Successful invaders are likely to breed rapidly -
  get past critical window of opportunity

3
Why do some species take off and have very rapid
population growth?

• What determines whether invasion will be
  successful?
• Usually out compete or exploit (i.e. eat) native
  organisms

4
Invasive Species Seem to More Common on Islands

• Mosquitoes, introduced birds on Hawaii
• Deer, possums, etc., on New Zealand
• Prickly Pear, rabbits in Australia
• Pigs, goats, rats, cats, mongoose many islands

5
Plenty of Invasive Species on Continents

• Great Lakes zebra mussels, gobies, sea lamprey
• Terrestrial
• Starlings, house sparrows
• mosquito fish, gypsy moth
• Kudzu, water hyacinth
• garlic mustard, salt cedar, tumbleweed

6
Why Do Species Invade?

• Ecological Theory -
• Release from predation, competition
• Invading species have left these checks on
  population size back in their original habitat.

7
But is Ecological Release the Whole Story?

• On islands the lack of competitors, predators,
  and challenging prey are almost certainly
  important
• In other environments - ecological release may be
  important but the data are unclear
• Example - House Finch - native to western North
  America - introduced to New York - swept across
  eastern North America until met up with original
  range - now found across the continent.

8
Invasion may be evolutionary as well as ecological

• Adaptation to the new environment may be
  important. Successful invaders may be ones that
  can adapt (or are pre-adapted) to their new
  environment

9
Predictions (if adaptation is an important aspect
of species invasions)

• Genetic differences between original populations
  and new populations
• Common garden and/or reciprocal transplant
  experiments can test for adaptation (difficult to
  do in some organisms)

10
Transgenic Organisms

• New genotypes caused by inserting genes from
  other species
• Can introduce genes into other species (via
  interbreeding or bacteria)
• Issues much the same as with invasive species

11
Evolution of Resistance

• Antibiotic, Pesticide, or Herbicide Resistance
  (also evolution of resistance to other drugs)
• Fundamental Issues are the same - a few
  biological differences (e.g haploidy vs. diploidy)

12
How Does Resistance Evolve?

• Very strong selection for any alleles that confer
  resistance
• Addition of chemical is equivalent of major
  habitat shift
• The only individuals that survive are resistant.

13
Factors Influencing Evolution of Resistance

• Strength and duration of selection
• Does varying chemical help?
• Not very effective (requires cost of resistance)
• Does using more than one chemical help?
• Yes - only way that AIDS can be treated

14
Malarial Resistance to Drugs

• Plasmodium (organism that causes malaria) is a
  eukaryote - antibiotics dont work
• Most drugs have side effects
• Plasmodium can evolve resistance to the drugs
  faster than we can

15
Costs to Resistance

• Important in considering how to apply chemicals
• If resistance is costly then it will tend to be
  lost when chemical is removed
• Means that alternating chemicals may be an
  effective strategy.
• Data - bacteria and insects tend to have low cost
  of resistance, plants have a higher cost of
  resistance

16
Dominant or Resistant Alleles

• If an allele is recessive and it confers
  resistance (when homozygous) then selection will
  be ineffective in increasing it in frequency when
  the allele is rare.
• These types of alleles are more easily controlled
  than dominant or additive resistant alleles.

17
Refuges and Inbreeding

• Refuges where chemical is not used - allow
  susceptible alleles to persist. Prevents
  resistant alleles from becoming present in
  homozygous form (assumes resistance is rare and
  recessive)
• Inbreeding may increase occurrence of resistant
  individuals in this situation.