Invasive Species

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Invasive Species

• relocation of organisms across geographical
  boundaries occurs naturally by various means.
• Since humans began exploring the globe, however,
  the rate of new species being introduced into
  regions has greatly increased.
• humans have dispersed species on purpose
• plants transported from Europe to North America
  for agricultural and ornamental purposes
• Others were transported accidentally by ship,
  train, airplane - even on the shoes of hikers.

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Local Examples

• Asian shore crab, Hemigrapsus sanguineus
• Pathway introduced from Asia to eastern seaboard
  most likely by ballast water
• reported in Long Island Sound in 1993
• Current Distribution Maine to North Carolina
  prefers rocky cobble
• Impact dominant rocky intertidal crab consumes
  juvenile mussels and oysters, green crabs,
  snails, polychaetes, algae, hydroids, barnacles

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• Green crab, Carcinus maenas
• Omnivore - consumes barnacles, clams, hermit
  crabs, worms, algae, organic debris
• Pathway introduced by end of 18th century from
  Europe, probably via ship hulls reported in LIS
  in 1817.
• Current Distribution Gulf of St. Lawrence to
  Delaware lives on rocky shores, quiet
  backwaters, marshes, estuaries, brackish waters
  or coastal waters
• Impact voracious predator and scavenger
  consumes juvenile shellfish (may have caused
  decline of softshell clam industry in Maine and
  even Long Island Sound) competes with native
  crabs.

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• Mute swan, Cygnus olor
• Pathway Introduced from Europe as decorative
  waterfowl in late 1800s or early 1900s
• Current Distribution southern Ontario to North
  Carolina or Florida reported in LIS by 1920s
• Impact using long necks, the swans graze on
  vegetation by ripping important submerged aquatic
  vegetation such as eelgrass out by the roots,
  damaging marsh and shallow water habitats
  overpopulated, displacing native swan

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Invasive Species

• Some introduced species are not able to survive
  in their new habitat.
• Others may find optimal conditions for growing,
  reproducing, and adapting to the new environment,
  and their populations soar.
• lack of predators
• ability to outcompete other species (for ex, by
  surviving and growing on fewer resources)
• Alter ecosystem function and services

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Terminology

• "invasive", "exotic", "introduced, nonnative,
  alien
• Introduced species that have profound effects on
  their new ecosystems have been termed invasive
  species.
• These effects include outcompeting native
  species, sometimes causing their extinction, and
  altering ecosystem functioning upon which we
  depend!

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Growth of invaders in a new range
Log phase
Lag phase

• Phases of proliferation and spread
• Lag phase makes early detection difficult
• Often no lag phase

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Invasive Species Why such a problem?

• Very hard to predict
• Difficult to identify common characteristics
  shared among invasive species
• Difficult to ID characteristics among invaded
  communities
• Above problems could be partly solved if we knew
  more about EARLY STAGES of invasions
• Control sometimes feasible, often costly
• Are often hard/impossible to eradicate

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Community Vulnerability to Invasion

• Current hypotheses
• Vacant niches
• Escape from biotic constraints
• Community species richness
• Disturbance before or upon immigration

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Zebra Mussels

• Dressena polymorpha
• widely known example of an aquatic invader in the
  U.S.
• native to southern Russia, introduced to the
  Great Lakes in 1985 or 1986 via ballast water
• now exist in many aquatic systems in the eastern
  US and expected to invade freshwaters throughout
  the nation in about 20 years
• Progression USGS link

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Effects of Zebra Mussels

• filter feeds unusually large amounts of
  phytoplankton
• outcompetes other filter feeders (esp.
  zooplankton, an important food for fish)
• can reach densities of up to 700,000
  individuals/m2.
• smothers clams, native mussels and snails,
• clogs water intake and exit pipes for facilities
  such as electric generating plants.
• annual damage for U.S. utilities at about 100
  million

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Why are zebra mussels succesful?

• Lack of predators and parasites
• Availability of space attach to hard substrates
  with byssal threads (pipes, boat hulls and
  motors, trailers, docks, anchors, and rocky
  beaches)

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Why are zebra mussels succesful?

• free-swimming veliger larva can disperse widely
• Females are also extremely fecund (huge release
  of eggs spawning events several times a year).
• Have a relatively long lifespan for an
  invertebrate (2-5 y)
• early sexual maturity (can be within 1 year)
• eggs and sperm viable for several or more hours
• dispersal of all life stages
• tolerance of some salinity

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• Figure 2A. Changes in populations of animals that
  are thought to depend on phytoplankton for food
  in response to the arrival of zebra mussels in
  the Hudson River.
• (a) Macroplankton are microscopic floating
  animals that are visible to the eye
• (b) Unionids are clams
• (c) Shaeriids are clams.
• The dashed line shows the point at which the
  zebra mussel became abundant.
• Data are yearly averages at one station during
  June-August
• Strayer, et al. 1999

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• Figure 2B. Changes in concentrations of edible
  and inedible particles in water in response to
  the arrival of zebra mussels in the Hudson River.
  
• (a) phytoplankton measurement is concentration
  of the pigment chlorophyll a
• (b) biomass of microscopic zooplankton (tiny
  floating animals
• (c) solids suspended in the water, units are
  milligrams per liter.
• The dashed line show the point at which the zebra
  mussel became abundant.
• Data are yearly averages at one station during
  June-August.
• Unusually heavy summer rains happened in 1996
• (Strayer etal. 1999)

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• Figure 2C. Key variables in the Hudson River
  ecosystem.
• (a) freshwater discharge units are cubic meters
  per second. (b) water temperature
• (c) estimated filtration rate of zebra mussels
  (grey bars) and all other filter feeding animals
  (white bars) averaged for the river.
• The dashed line shows the point at which the
  zebra mussel became abundant.
• Data are yearly averages at one station during
  June-August.
• (Strayer et al. 1999)