Exploitation Passenger pigeon hunted to extinction dye-hardwood - - PDF document

Exploitation

Species Comments

Bison

hunted almost to extinction

Passenger pigeon

hunted to extinction

Pau-Brazil

dye-hardwood tree

vigorously harvested 1500-1850 (coastal Brazilian forests)

Mahagony

• ngoing harvest driven by

economical and not conservation

biology thinking

Non-timber-forest products

It is commonly thought that forest products other than timber can be harvested in a sustainable manner. But even such moderate harvest that tries to leave the forests intact is disturbing the forests. Y-axis is in percent (tree diameter in percent of the largest class)

25 50 75 100 No harvest Light moderate persistent

Tree diameter Juveniles

Non-target species (Bycatch)

“Species” Threat Death

Albatross long line fisheries 10000/year (blackfooted albatross) Sea turtles long line fisheries In Mediterreanean sea 20000/year Sea snakes prawn trawlers 120000/y (Australia) skate prawn trawlers 1000/y

Biological theory of exploitation

Reproductive rate Mortality rate

Population density rate equilibrium

Biological theory of exploitation

dN dt = rN(1 − N K ) − Y

change of N per time t population growth rate N=population size K=Carrying capacity Yield (Surplus of population)

• Yield is the population surplus that can be

removed sustainably.

• The logistic growth function can be solved

for Y and so we can get the surplus for known K and r.

K 50% Population size Yield

maximum sustainable yield

Solving the equation yields this yield curve

Stability of yield in a constant quota system

• Red: yield is higher than maximum

sustainable yield: population goes extinct

• Yellow: maximum sustainable yield:

unstable equilibrium, when actual population is smaller than the 50% population will go extinct, if population is bigger than the 50% then it will approach 50%.

• Green: two equilibria, only the upper (to

the right) is stable and desirable. Harvest close to the carrying capacity can be sustainable.

K 50% Population size Yield

dN dt = rN(1 − N K ) − Y

N0 = 700 r = 0.4 K = 1000 Y = 100 Stable at K/2 = 500 N0 = 700 r = 0.4 K = 1000 Y = 50 Stable at K/2 = 853 N0 = 700 r = 0.4 K = 1000 Y = 105 UNSTABLE

maximal sustainable yield for this example is 100 (per generation)

Comparison of different yields Constant effort

Y = EN EN = rN(1 − N K ) N = K(1 − E r )

Effort Yield

Stability of yield in a constant effort system

• Red: very high effort, unstable
• Yellow: maximum sustainable yield with

medium effort, stable

• Green: low effort [right], high effort [left].

both are stable but low effort is preferrable with same yield, but population needs to be near carrying capacity.

K 50% Population size Yield

Tragedy of the commons

• When many share a resource, the resource is at strong risk

to get depleted (= species goes extinct) because the economical strategy is to get more than the others (=maximize gain). Of course this strategy does not work for conservation strategies. With protected or partition resources typically they are not depleted in the same manner.

• We see this problem in hunting/fishing but also on grazing on

public land.

Effort and Cost

• Yield (Ym) is maximized at

Em, but gain (the distance between the black and the red curve is no maximal.

• Gain is maximized with effort

Eb, much less effort but almost same yield as with Em .

• Yield (Yc) and cost are the

same there is no gain.

High Low Effort Yield Cost Ec Eb Em Yc Yb Ym Gain = Yield-Cost