Species Invasions

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Chapter 9

• Species Invasions

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Introduced Species

• Anything released outside its native range
• Synonyms
• Invasive species
• Nonnative species

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Range expansions

• Naturally occurring increases in occurrence
• Historically took place at slower rates and over
  smaller areas
• Today, increases in
• Number of introduced species
• Rate of introduced species
• Extent of areas experiencing pressure from
  introduced species

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Conservation Implications

• Economics
• Introduced species are commercially important

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Conservation Implications

• Economics
• Introduced species are commercially important
• Control efforts against introduced pest species
  cost billions annually
• Public Health
• Spread of pathogens through nonnative hosts

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Conservation Implications

• Biodiversity
• Initial increase in alpha-richness
• Decrease in beta-richness
• Introduced species associated with extinctions of
  native species in habitats worldwide

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Impacts of Invasions

• Multiple levels of consideration

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Introduced Predators

• Introduced species functions as a new predator to
  a community
• Prey species naïve to tactics of new predator
• Prey species may not recognize new species as a
  predator
• Example Brown tree snake on Guam

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Introduced Competitors

• Introduced species may access resources more
  easily than native species
• More difficult to observe (and demonstrate
  experimentally) than predation
• Direct and indirect competition
• Results in population reduction or elimination of
  native species

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Introduced Competitors

• Introduced species can prevent native species
  from functioning (zebra vs native mussels)
• Introduced species may fill niche left by native
  species reduced by other means
• Newly introduced species may negatively impact
  nonnative species introduced earlier

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Morphological impacts

• Introduced species can cause changes in physical
  structure of native species to prevent
  reproduction, reduce reproductive rate

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Behavioral impacts

• Introduced species can affect the behavior or
  preferences of native species

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Genetic/evolutionary impacts

• Opportunity for hybridization between nonnative
  and native species
• Reduced fitness for native strains

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Ecosystem impacts

• Invasives that play key roles in their native
  habitats may greatly affect new habitats

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Impacts of Invasions

• Impacts of invasive species can start with
  population and move up through natural hierarchy
  to affect ecosystems
• Can also move the other direction changes to the
  ecosystem may in turn affect populations
  indirectly

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Do all species have the potential to be nonnative
invaders?

• What character(s) allow species to be better at
  invading new areas?

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9.9 The series of events leading to a successful
invasion can be pictured as a series of
bottlenecks
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Invaders right stuff

• Successful invasions need some match between
  biology and habitats
• Three levels of consideration
• Propagule pressure
• Invader characteristics
• Community characteristics

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Propagule Pressure

• The quantity, quality, and frequency of arriving
  invaders
• Most invasions begins with a small population
• Overcoming drawbacks of small population helps to
  ensure successful invasion

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Propagule Pressure

• Drawbacks for small population
• Allee effect
• Per capita birth rate lower at low populations,
  b/c of difficulty finding a mate, for example
• Demographic stochasticity
• Random population fluctuations through time

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Propagule Pressure

• For invasive species
• Successful invaders have high propagule pressure
• Effective at establishing once new habitat is
  reached
• For communities
• Proximity to one end of the invasion pathway
• Inherent qualities that lend the community to
  invasions

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Invading species

• General conclusions based on invasive species
  case studies
• Habitat/diet generalists
• High reproductive rate
• Simple reproductive system

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Invading species

• Reality many more characteristics than
  manageable for conservation
• Enemy release members of new population free
  of parasites/disease
• Combination of factors more likely
• Maybe some species are just lucky?

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Invaded community

• Two general factors
• 1) Hospitable climate/habitat
• 2) Community structure must be able to
  accommodate new species
• Biotic resistance hypothesis
• Species-rich systems are more stable and less
  susceptible to invasions

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9.12 Invader incidence and successful
germinations vs. species richness (Part 1)
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Invaded community

• Disturbance hypothesis
• Disturbance may make a community more invadable
  than without disturbance event
• Some types of disturbance can bring invasives to
  the area (increasing propagule pressure)
• Other types of disturbance can facilitate the
  establishment of invasive species

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Invaders right stuff

• Should consider both invading species and invaded
  community characters together
• Consider that the same species may behave
  differently in alternate environments
• Predicting who, what, when of invasives is more
  intricate

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How are species introduced?

• We didnt build it, but they came anyway!

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Unintentional Pathways for Invasives

• Any and all human modes of transport
• Ships provided first means of broad-scale
  transport
• Canals shortened transport times for ships
• Improving technologies for transportation allow
  people, cargo, and invasive species to cross
  continents in a day

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Intentional Pathways for Invasives

• Agriculture, biocontrol, recreation, and
  ornamentals
• Common for colonizers to take animals/plants to
  new settlements
• Increased awareness has led to reduction of
  intentional introductions

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Managing Species Invasions

• Species-based ControlInvasion Prevention

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Species-based Control

• Physical control of invasives
• Trapping, digging up, removal
• Good solution when population of introduced
  species and area occupied are small
• Chemical control of invasives
• Highly controversial b/c of non-target effects

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Species-based Control

• Biological control
• Releasing a predator to reduce the invader
  population
• Mixed results in literature
• Ethical considerations as well

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Invasion prevention

• Protect against invasions by identifying and
  regulating invasion pathways
• Policies and legislation governing transport and
  travel, both domestic and international
• Incorporate the precautionary principle in new
  policies

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NISMP

• National Invasive Species Management Plan
• Clinton signed into law in 2001
• Provided guidelines for which agencies should
  manage which invasive pathway
• Still no national, overarching law/legislation
  for prevention or control of invasive species
• Some state more effective (CA, HI)

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Pop Quiz
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Definitions

• Overexploitation
• Bycatch
• Density dependence
• Surplus production
• Constant quota