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Overexploitation. ….when renewable resources aren’t. The economist argument….. Exploit the resource until the marginal costs equal the returns. Some resource will remain because it is not “economical” (rational) to continue the exploitation.
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Overexploitation ….when renewable resources aren’t
The economist argument….. Exploit the resource until the marginal costs equal the returns. Some resource will remain because it is not “economical” (rational) to continue the exploitation. Why this will protect oil but maybe not biodiversity??
Economics and exploitation of wild species: special problems….remember this table • Rate of exploitation must be less than rate of natural increase • Population must remain above minimal viable size • Population must retain its functional ecosystem role • Genetic diversity must not be degraded • Size distribution, age structure, sex ratios must not be permanently altered • Social behavior must be retained (the last three= Darwin’s debt)
Overexploitation--definition • Use is consumptive = lost • Rate of use exceeds rate of renewal • Excess rate of use leads to a) loss of functional role in population, community or ecosystem b) failure to renew unless intervention c) extinction for organisms more likely
For ecosystem exploitationwe would add • Exploitation is a form of disturbance • Overexploitation then occurs when the frequency, scale, and intensity of the disturbance exceeds the resiliency of the ecosystem • Different disturbances may have independent or additive effects • A new kind of ecosystem may result
Overexploitation occurs when • There are no substitutes so rarity equals high value and investment in new harvesting techniques drives population ever lower. • Regulation or disincentives fail to stop destructive use of ecosystems • Stewardship values are weak • Impoverished people have few alternatives • We don’t know when we are reaching limits
Tyranny of small decisions Big problem when small increase in harvest has a slow but serious consequence
Economic piece Cost per effort may reduce harvest below MSY or technology may raise harvest above MSY
Pine martin example Arboreal predator in weasel/mink family. Trapped in Canadian woods for their valuable fur.
Great whale protection began in late 1970s Blue whale, largest vertebrate that ever lived, has shown some recovery Fin whale populations have been growing at about 3% per year is some places Sperm whale populations remain very low Sei whale populations remain very low
Indirect exploitation: whale killed by entrapment in commercial fishing net
Tropical forest losses due to excessive timber practices (all tropical countries), forest land converted to cattle pasture and plantation agriculture (all tropical countries) and tree cutting for cooking fuel (Africa)
Percentage of forest cover loss between 1985-1990
Africa: relative cost of unit of energy. This is why fuel wood is heavily exploited. To produce one Mj of energy, electricity costs almost 34 times the cost of fuelwood.
Mahogany (Swietinia mainly) Valuable hardwood from Latin America and Caribbean. Single large tree produces more than $100,000 of lumber. Mahogany sold in US creates $100 million dollar market. Most large trees inside parks and protected land.
Mahogany: the most valuable Neotropical wood Swietenia Cedrella Carapa
Illegal mahogany tree cut in Madre de Dios “protected” indigenous reserve in southern Peru
Madre de Dios indigenous reserve in southern Peru. Large mahogany trees are only found in remote reserves. Ashenika women: one of several indigenous cultures.
So, why not grow mahogany plantations? Answer: an insect won’t let us
Mahogany: Is sustainableharvest possible? Positive traits: • wind pollinated and wind dispersed seeds • Seeds not an important food resource • Swietenia “caoba” wide-spread
Negative aspects • Over-exploited, everywhere uncommon therefore hard to get adequate volume • Slow growing, but rate can be improved by eliminating vines • Shoot borer moth (Hypsipylla) destroys shoots, hence one moth larva can ruin timber value of entire tree
Negative traits con’t • Can’t be grown in plantation (shoot borer) • No chemical or biological control for moth…so far
Plan for sustainable harvest of mahogany • Monitor growth of individual trees • Enhance growth rate by vine elimination and reduce competition for light generally • Plant seedlings in forest in low density • Harvest with minimal damage to forest • Use for high value products, e.g. doors and tables
Current plan • Encourage farmers to plant trees on their farm (shoot borer??) • Contract advance price • Provide silvaculture information • Provide harvesting Will it work????
Mahogany and some general lessons for sustainable harvests Problematic when… • Individuals grow slowly • Mature after many years • Susceptible to other sources of mortality • History of intense exploitation • Very high unit value
Bluefin tuna Over exploiting a top predator
Some important aspects of exploitation of Bluefin • Valuable, one fish sold for $180,000 on the Japanese market • Few spawning grounds (Med.and Gulf of Mexico where larvae concentrated in hurricane path) • Schooling behavior (catch large numbers) • Migratory (cross jurisdictional areas)
