Overexploitation 鄭先祐 (Ayo) 台南大學 環境與生態學院 院長

2 Introduction The term harvesting is often used synonymously with exploitation. Empty forest Empty sea

3 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

4 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)

5 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

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

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

8 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

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

10 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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12 Fig. 8.9 (A) Logistic population growth of a population up to maximum population size, Nmax

13 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

14 Constant quota exploitation

15 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.

16 Fig Equilibria and population stability under proportional exploitation.

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18 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)

19 Bio- economics The tragedy of the commons (Hardin, 1968)

20 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

21 Yield per recruit models Dynamic pool concept in fisheries book (1957) Older fish provide more meat and older trees provide 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.

22 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.

23 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.

24 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.

25 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 forests for timber

26 問題與討論