Overexploitation 鄭先祐 (Ayo) 台南大學 環境與生態學院 院長 Japalura@hotmail.com

Overexploitation

鄭先祐 (Ayo)台南大學 環境與生態學院 院長

Japalura@hotmail.com

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Introduction

The term harvesting is often used synonymously with exploitation.

Empty forest Empty sea

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Contents

History of, and motivations for exploitation Impacts of exploitation on target-species Impacts of exploitation on nontarget species

and ecosystems Biological theory of sustainable exploitation Comparison of methods for calculating

sustainable yields Sustainable use meets biodiversity

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History of, and motivations for, exploitation

Hunting creates more than 700,000 jobs in the US and a nationwide economic impact of about $61 billion per year, supporting nearly 1% of the entire civilian labor force in all sectors of the US economy (LaBarbera, 2003)

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Impacts of exploitation on target species

Tropical terrestrial ecosystems Timber extraction Nontimber forest products

Temperate terrestrial ecosystems Forestry Hunting

Aquatic ecosystems Marine ecosystems Freshwater ecosystems

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Fig. 8.4 Trends in global fisheries. The gray portion of the bars indicate capture fisheries and the black portion indicate aquaculture.

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Fig. 8.5 Chinese bahaba caught as an incidental by-catch by a trawler west of Hong Kong.

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Impacts of Exploitation on nontarget species and ecosystems

Tropical terrestrial ecosystems Logging and forest flammability Hunting and loss of seed dispersal services

Temperate ecosystems Aquatic ecosystems

Marine ecosystems Freshwater ecosystems

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Fig. 8.7 Mekong giant catfish, Pangasianodon gigas, from the Mekong River. (湄公河 )

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Biological theory of sustainable exploitation

As populations are reduced by exploitation, there may be reduced competition for food, territories, shelter, and a lower transmission rate of diseases.

This can lead to greater birth rates or enhanced survival.

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Fig. 8.9 (A) Logistic population growth of a population up to maximum population size, Nmax

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Fig. 8.9 (B) sustainable yield, Y against population size for the logistic case shown in (A). The maximum sustainable yield occur at 50% of the maximum population size.

dN/dt =rN (1-N/K) – Y

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Constant quota exploitation

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Proportional exploitation

Y = EN E, the exploitation rate rN(1-N/K) = EN N=K(1-E/r) As long as the exploitation rate is below the

intrinsic rate of natural increase, r. then all equilibria are stable.

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Fig. 8.11 Equilibria and population stability under proportional exploitation.

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Stability of exploitation

Constant quota exploitation Proportional exploitation Threshold exploitation

只取超過 K 的部分 This would minimize the chances of collapse. However, while such low rates of exploitation are

excellent for conservation, it is difficult to convince people to accept such severe restrictions.

Bioeconomics (Essay 8.2)

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Bio- economics

The tragedy of the commons (Hardin, 1968)

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Comparison of methods for calculating sustainable yields Surplus production models

Often used in fisheries The simplest ones require very little data. How yields have responded to different levels of

exploitation effort over time, then you could estimate the dome-shaped yield curve shown in Figure 8.9B.

It treats each year as an independent replicate, which it will not be. (time lags)

Yield per recruit models Full demography models

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Yield per recruit models

Dynamic pool concept in fisheries book (1957) Older fish provide more meat and older trees pr

ovide more wood.( 倘若太老，則會自然老死 ) Yield-per-recruit models search for the level of

mortality that maximizes the yield under the tradeoff between numbers and value.

Here, a “recruit” is defined as an individual that has become big enough to be captured.

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Full demography models

For some species there has been sufficient economic value or conservation concern to lead to the production of full-blown population models.

These models combine information on vital rates such as births, juvenile survival, age at maturity, and adult survival.

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Demographic rules of thumb

Parameter-hungry models are useless for the vast majority of the world’s exploited species, because we usually lack even the most fundamental information .

Various demographic rules of thumb have been developed that seek to overcome this problem.

One of the best-known rules of thumb has come to be known as the Robinson and Redford model. (tropical mammals, sustainable exploitation?)

Pmax = (0.6D x λmax) – 0.6D D is an equilibrium population density estimate near K, and

λmax is the maximum finite rate of increase.

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Sustainable use meets biodiversity

Even well-meaning management prescriptions can be completely misguided, bringing once highly abundant target species to the brink of extinction. The 97% decline of Saiga antelopes (from >1million to <30,

000) in the steppes of Russia and Kazakhstan over a 10-year period.

In many instance failure to protect species from overexploitation has more to do with institutional short-comings than lack of scientific knowledge.

The adoption of precautionary principles in exploitation by many countries is certainly a move in the right direction.

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Supplements

Case study 8.1 overexploitation of highly vulnerable species (sharks)

Case study 8.2 the bushmeat crisis Case study 8.3 managing natural tropical fore

sts for timber

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http://mail.nutn.edu.tw/~hycheng/

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