Overexploitation

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Chapter 8

• Overexploitation

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Distinction

• Exploitation
• Reliance on animals/plants for uses
• Reduction associated with humans
• Overexploitation
• Rate of consumption exceeds rate of replenishment

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Recent examples
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Passenger Pigeon

• Pest species
• Hunting for food and as nuisance
• Habitat degradation also led to extinction

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Bison bison

• Hunting for food, skins, and sport
• Hunting primary cause for population declines
• Not extinct

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Caesalpinia echinata

• Pau-Brasil tree
• Popular wood for carving and instruments
• Natural stands severely limited
• Individual trees in botanical gardens

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Great Auk

• Easy prey for humans on land
• Hunting for food primary cause of extinction

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Dodo

• Flightless bird on island of Mauritius
• Humans colonized and used as food
• Habitat loss also negatively impacted
• Extinction occurred in 80 years

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Stellar Sea Cow

• Discovered (and eaten) by stranded ship crew
• Popular source of meat after rescue
• Extinct in 30 years

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Exploitation

• Activities
• Hunting, fishing, logging
• Species
• Target
• Non-target

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Tropical timber extraction

• Level of selection determines effect
• Creates canopy gaps
• Roads create additional gaps and contribute to
  habitat fragmentation

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Tropical timber extraction

• Changes to microclimate
• Benefits to some species
• Area more susceptible to fire disturbance

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Subsistence Hunting

• Tropical regions
• Hunting animals for food, primarily
• Increases as human population increases
• 522/km2 in SE Asia
• 99/km2 in Africa
• 46/km2 in Latin America

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Nontimber Forest Products

• Anything OTHER than timber or bushmeat
• Fruits, nuts, latexes, resins
• Removal rates do not threaten persistence?

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Hunting and Seed Dispersal

• Potentially negative non-target effect between
  bushmeat and NTFPs harvest
• Bushmeat removes primates and large mammals
• Seeds dispersed after consumption
• Hunted areas have high seed density at parent
  tree effect variable

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Forestry

• Temperate forests subject to logging more than
  any other
• Type, extent, and frequency of logging have
  variable effects
• Possible to minimize negative effects by allowing
  multiple stages of forest succession to occur

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Hunting

• More for recreation than subsistence hunting
• Large and small game
• Bag limits determined by several
  agencies/countries
• Migratory species consider both
  breeding/nonbreeding population losses

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Nontarget effects of hunting

• Exploitation of ecosystem engineers a problem for
  habitats
• Prey population explosions can overgraze and ruin
  plant communities

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Marine Exploitation

• 75 of worlds fish are overexploited
• Wild capture coupled with aquaculture
• Extinction risk for target species
• Nontarget species include other fish, marine
  mammals, and pelagic birds

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Freshwater Exploitation

• Commercial and recreational fishing activities
• Both fish and other taxa targeted
• Salmon also experience habitat degradation
• Loss of genetic diversity when reared fish mix
  with wild populations

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Nontarget effects in Freshwater

• Bycatch of mega-fish

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Nontarget effects in Freshwater

• Bycatch of megafish
• Other species

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Sustainable Exploitation

• Is this possible?

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Biological Theory

• Populations are renewable.

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Biological Theory

• Populations are renewable
• Exploitation can result in population increase
• Density dependence

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Density Dependence

• Components of fitness prevent a population from
  growing indefinitely
• Results in populations fluctuating around some
  level carrying capacity

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Rate of Removal

• Do survivors experience some benefit?
• Increased growth/survival rates
• Over boost in access to resources
• Are survivors unable to compensate for loss of
  individuals?

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To understand how exploitation can become
overexploitation, we need to understand
population growth curves.
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8.9 (A) Logistic population growth of a
population up to a maximum population size, Nmax
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Surplus Production

• The yield that can be removed sustainably
• The growth rate of a population (N) before
  exploitation
• Two important concepts
• No growth as population approaches carrying
  capacity (Nmax)
• No growth when N0

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8.9 (B) Sustainable yield, Y against population
size for the logistic case shown in (A)
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Calculating a sustainable yield easy?

• Just plug values into a formula.

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Exploitation Strategies

• Constant quota
• Proportional
• Threshold

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Constant quota exploitation

• Number of individuals removed is independent of
  population size
• Still used today
• High, MSY, and Low quotas

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8.10 Equilibria and population stability under
constant quota exploitation
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Proportional exploitation

• Exploitation targets correspond with population
  size of target species
• Occurs naturally
• Advantage
• If exploitation rate lt rate of increase, then all
  quotas are stable

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8.11 Equilibria and population stability under
proportional exploitation
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Threshold exploitation

• Examines both level and necessity of exploitation
• Focuses on exploitation of surplus population
• Anticipates other influences upon populations
  that may not be calculable

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What about economics?

• Where does the money fit?

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Bioeconomics

• Attempt to incorporate the costs and benefits of
  exploitation
• Exploitation intensity should be balanced with
  costs of hunting/harvest
• Harvest levels might depend on ownership
• Exploitation important in different ways to
  different people

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8.13 The economics of exploitation
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Pop Quiz
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Define and Explain

• Habitat loss
• Habitat degradation
• Habitat fragmentation