Stable isotope evidence for the food web consequences of species invasions in lakes

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Stable isotope evidence for the food web
consequences of species invasions in lakes
Vander Zanden et al., 1999
Presentation by Amber Aspinall Biology 402
Aquatic Ecology
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Human impacts on aquatic ecosystems

• Alteration of nutrient levels

• Widespread introduction of exotic species

• Extinction of native species

• Collapse of native fisheries

• A loss of ecosystem integrity and function

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Importance

• Major contributor to depletion and extinction of
  native species
• Competition for a common resource
• Prey species have not evolved defense mechanisms
  against new predators
• The invasive species is unchecked by natural
  predators or disease in the exploited habitat

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Historical incidence

• Sea Lamprey
• Introduced to the Great Lakes from Atlantic Ocean
  via man-made canals for ships in 1819
• Resulted in the total collapse of fisheries in
  1950s
• Near extinction of lake trout and 15 million/yr
  is spent on control

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Vander Zanden et al. (1999)

• Study the effect of recent non-native fish
  species invasions in Canadian lakes
• 5 unimpacted and 5 impacted lakes
• Similar latitude/longitude and Secchi depth
• A range of lake size

• Document the change in food-web interactions
  through the use of N and C stable isotope ratios

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Species involved

• Endemic
• Lake trout

• Invasive
• Smallmouth bass
• Rock bass

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Characters of invasion

• Many, small eggs
• 2,000-7,000 eggs/lb
• Fast growth rate
• 5-6 days to leave spawning ground
• Tolerates salinity
• Effective predator
• Cryptic colouration, streamlined, big mouth

Jackson Mandrak (2003)
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How are they able to spread?

• Intentional introductions
• Dumping of unused live bait
• Dispersal through drainage systems
• Endemic to areas N. America
• Are now found on every continent!

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Prey-fish abundance and diversity

• A significantly higher abundance and diversity
  in
• unimpacted lakes

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The use of stable isotopes

• Greater proportion of 13C and 15N in benthic
  algae relative to phytoplankton

fish
phyto- plankton
benthic algae
Percent 13C and/or 15N in tissues
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Theory and method

• 3.4 times enrichment of 15N per trophic level
• Trophic position
  (15Nconsumer- 15Nbaseline)/3.4 2
• 15Nconsumer fish tissue samples
• 15Nbaseline zooplankton and mussels present in
  each lake and habitat

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Unimpacted food chain

• Cold-water, pelagic piscivore
• Eats littoral fish when low prey availability
• 15N 62 fish
• 13C 62 fish
• Trophic position 3.9

Trophic Position
Pelagic Littoral
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Impacted food chain
13C

• Bass consume large amount of littoral fish
• Trout must eat more pelagic plankton
• 15N 22 fish
• 13C 27 fish
• Trophic position 3.3

Trophic Position
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The effects of invasion

• Lake Trout and Prey Fish trophic positions are
  more similar
• Lake Trout 13C content of tissues is more similar
  to zooplankton

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Long-term effects on food web
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Conclusions

• Although Lake Trout is pelagic, it consumes a lot
  of littoral prey
• The two habitats are closely linked

• The invasion of bass produces strong top-down
  effects on the food web

• Structure of food web will determine the effect
  of bass invasion
• High pelagic prey-fish small effect
• Low pelagic prey-fish large effect

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Importance for Canadian Lakes

• High implications for fishery management
• Bass are often introduced to enhance fisheries
• Neglect to consider negative impact of
  competition with native fish

• Protection of native species
• Halt introduction of bass and other predators
• Impose limitations on live bait use