Chapter 9
Species Invasions
Introduced Species
• Anything released outside its native range
• Synonyms:
– Invasive species
– Nonnative species
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
Conservation Implications
• Economics
– Introduced species are commercially important
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
Conservation Implications
• Biodiversity
– Initial increase in alpha-richness
– Decrease in beta-richness
– Introduced species associated with extinctions of
native species in habitats worldwide
Impacts of Invasions
Multiple levels of consideration
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
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
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
Morphological impacts
• Introduced species can cause changes in
physical structure of native species to prevent
reproduction, reduce reproductive rate
Behavioral impacts
• Introduced species can affect the behavior or
preferences of native species
Genetic/evolutionary impacts
• Opportunity for hybridization between
nonnative and native species
• Reduced fitness for native strains
Ecosystem impacts
• Invasives that play key roles in their native
habitats may greatly affect new habitats
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
Do all species have the potential
to be nonnative invaders?
What character(s) allow species to be
“better” at invading new areas?
9.9 The series of events leading to a successful invasion can be pictured as a series of bottlenecks
Invaders’ “right stuff”
• Successful “invasions” need some match
between biology and habitats
• Three levels of consideration
– Propagule pressure
– Invader characteristics
– Community characteristics
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
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
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
Invading species
• General conclusions based on invasive species
case studies:
– Habitat/diet generalists
– High reproductive rate
– Simple reproductive system
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?
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
9.12 Invader incidence and successful germinations vs. species richness (Part 1)
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
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
How are species introduced?
We didn’t build it,
but they came anyway!
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
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
Managing Species Invasions
Species-based Control
Invasion Prevention
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
Species-based Control
• Biological control
– Releasing a predator to reduce the invader
population
• Mixed results in literature
• Ethical considerations as well
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
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)
Pop Quiz
Definitions
• Overexploitation
• Bycatch
• Density dependence
• Surplus production
• Constant quota