Exotic Fish and Food Webs

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Exotic Fish and Food Webs

• e.g., Lake Victoria, Africa

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Worlds largest tropical fresh water
lake Relatively shallow mean depth of 40
m Shoreline 3440 km (2138 mi). Millions of
people live on its shore. Territorial
administration by Kenya, Tanzania, and Uganda
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• Relatively young (formed c. 400,000 ybp)
• Has completely dried up three times
• Last dry stage 17,300 ybp
• Began refilling 14,700 ybp
• Astonishing radiation of cichlid fishes

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Impacts of exotic species on communities

• Effects typically dramatic and negative.
• Basis exotic species were not experienced in the
  evolutionary histories of native populations.
• Nile Perch (Lates nilotica) and Nile Tilapia
  Introduced into Lake Victoria beginning 1954,
  extending through the 1960s.
• Population explosion in the mid-1980s.
• Quickly dominated the ecology of the lake.
• Fish an export commodity
• peak years, hundreds of millions of /yr.
• Then, overfishing produced a mining industry.

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Before introduction
Many guilds
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After introduction of exotic predators
6 ft 130 lbs.
Confounding factors in extirpation of native
fishes Pollution from deforestation and human
overpopulation Cultural eutrophication. O2
concentrations significantly decreased.
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Humans as a Keystone Species
Humans and food species
Annual subsistence hunting, Brazilian natives
14 million mammals, 5 million
reptiles ( birds) Annual Commercial Hunting
4 million animals wounded that escape to
die 37 million
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Herbivory algae/caddisflies
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Influence of elevating tiles on colonization
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Excluding Helicopsyche borealis
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Plant Defenses Against Herbivory

• Algae none
• Others secondary compounds
• Not part of metabolic pathways used for
  maintenance
• Why is energy being expended in their synthesis?
• Suspect defense.

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• Plant defense strategies
• 1. Quantitative defenses
• Secondary compounds are ingested in large amounts
  by the herbivore.
• Interfere with food digestion
• Ex. tannins and resins
• Amount synthesized keyed to herbivory intensity.

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• 2. Qualitative defenses
• Secondary compounds are highly poisonous
• Very small doses can kill herbivores
• Ex. Atropine

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• Plants can be classified according to defense
  strategy.
• 1. Apparent plants
• Long-lived
• Apparent to herbivores (mainly insects)
• Defenses are mainly quantitative
• Effective against specialist and generalist
• Long history of association of herbivores with
  these K-selected plants

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• 2. Unapparent plants
• Ephemeral food source not reliable in long term
• E.g., weeds
• Defenses mainly qualitative
• Herbivores are mainly generalists

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