Photo of American bison skulls from Wikipedia

Overexploitation

N

Time

Exponential growth∆N

∆t= r • N

Occurs when growth rate is proportional to population size;

Requires unlimited resources

Population Dynamics

N

b

d

r

Equilibrium (= carrying capacity, K)

or d

b

Notice that per capita fitness increases with

decreases in population size from K

Population Dynamics

Density-dependent per capita birth (b) and death (d) rates

N

Time

Logistic growth∆N

∆t= r • N • (1 – )

N

K

K = carrying capacity

∆N

∆tis maximized

∆N

∆t= 0

∆N

∆t= 0

Population Dynamics

Yield(Y)

N

oroverexploitation (overharvesting)

Sustainable yield

K

Rate of net production

of new individuals

Exploitation (harvesting)

Population Dynamics & MaximumSustainable Yield (MSY)

∆N

∆t

or

∆N

∆t=

N

K½ K

MSY

Sustainable yield

Constant quota exploitation

Yield(Y)

Population Dynamics & MaximumSustainable Yield (MSY)

N

Constant quota exploitation

MSY quota

Low quota

High quota

This is very rarely truly sustainable (since it is

difficult to obtain demographic information to make the predictions and half of the possible

intersections on the curve are unstable)

Yield(Y)

Population Dynamics & MaximumSustainable Yield (MSY)

N

Proportional or “constant effort”

exploitation

Removing a constant fraction of the pop. is more

often truly sustainable (since most yields intersect

the curve stably), even if there is error in estimating

demographic rates Yield(Y)

MSY exploitation

Lowexploitation

Very highexploitation

Population Dynamics & MaximumSustainable Yield (MSY)

Photo of lekking Attwater’s prairie chickens in Texas from www.nationalgeographic.com

Population Dynamics & Allee Effects

Allee Effects occur when per capita fitness declines as a population becomes smaller

NK

Zone of Allee Effects

b

d

Population Dynamics & Allee Effects

Allee Effects occur when per capita fitness declines as a population becomes smaller

or d

b

?

?

Logging

Subsistence hunting

Sport hunting

Pest management

Fishing

Shelling

Non-timber forest products

Trapping

Pet trade

Photo of mahogany from www.cites.org; photo of coyotes on fence from www.life.com

Examples of Exploitation of Target Species

Photo of Giant ground sloth bones from www.corbis.com

Paleolithic human hunters may have caused the

extinction of many species of megafauna (large-

bodied birds and mammals)

Pleistocene Overkill Hypothesis

Pleistocene = 1.8 million to 10,000 yr before present

Photo of at least one too many deer in Pennsylvania from www.nrcdeer.com

The Double-Edged Sword of Hunting

The Pleistocene Overkill Hypothesis illustrates the potential negative impacts of too much hunting

Remember trophic cascades (e.g., the consequences of removing top predators); now consider the potential positive impacts of

well-managed hunting

Policy as a Tool to Combat Overexploitatione.g., legislation on trade in endangered species

Photo from Greg Dimijian

Policy as a Tool to Combat Overexploitatione.g., legislation on trade in endangered species

Tim Wright et al. (2001) Cons. Biol.

This also indicated that legal and illegal trade in parrots are positively linked

(as opposed to speculation that the two are inversely proportional to one another)

Poaching rates of Neotropical parrots were significantly lower after U.S. Wild Bird

Conservation Act of 1992 banned trade in wild-caught CITES-listed birds

Image of domestic water buffalo from Wikipedia; case study from Ted Groves (UCSD), pers. comm.

Economic Incentives to Combat Overexploitatione.g., conservation agreement between Conservation International (CI)

& Chumnoab Commune, Cambodia

Commune members agreed to several specific measures of

good land stewardship in their patchwork landscape of

agriculture (rice) & protected forests (wildlife & timber)

CI agreed to provide economic incentives, including 8 water

buffalo after 1 year (5/25/06 – 5/24/07)

Threat: overharvesting of wildlife (e.g., crocodiles) & timber

Image of domestic water buffalo from Wikipedia; case study from Ted Groves (UCSD), pers. comm.

Economic Incentives to Combat Overexploitatione.g., conservation agreement between Conservation International (CI)

& Chumnoab Commune, Cambodia

12 ha cleared illegally by commune members; after

renegotiation, 6 water buffalo were provided by CI

Collectively, the commune eagerly entered into new

contracts in subsequent years

Compliance monitoring conducted by Cambodia

Forestry Administration, an independent party

Threat: overharvesting of wildlife (e.g., crocodiles) & timber

Economic Incentives to Combat Overexploitatione.g., conservation agreement between Tetepare Descendants’

Association (TDA) & villagers in the Solomon Islands

Image of leatherback turtle digging a nest from Wikipedia; case study from Ted Groves (UCSD), pers. comm.

Villagers agreed to protect sea turtles by reporting to sea

turtle monitors

Threat: overharvesting leatherback sea turtle females & eggs

Villagers & monitors receive compensation upon initial report & upon successful

hatching

So successful that TDA seeks to build an endowment to fund

future payments

Photo of Northern bluefin tuna from Wikipedia

Seafood Watch

Use the pocket guides to gauge the status of

seafood:

Web site

Marine Example

~75% of marine fisheries are considered to be fully fished or overexploited (FAO 2002)

Photo of Dall porpoise entangled in a fishing net from Wikipedia

Marine Example of Collateral Damage

~30% of marine fisheries “landings” are by-catch

Image from http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm

Marine Examples of “Fishing Down Food Webs”

“The Perils of Overfishing – Part 1” - NPR interview with Daniel Pauly

Time

Tro

phic

leve

l

Image of Orange Roughy, a.k.a. Slimehead (Hoplostethus atlanticus) from Wikipedia

Marine Examples

Image of Hagfish (family Myxinidae) from http://www.mnn.com

Marine Examples

Image of Chilean Sea Bass, a.k.a. Patagonian Toothfish (Dissostichus eleginoides) from Wikipedia

Marine Examples

Image of Monkfish, a.k.a. Headfish (genus Lophius) from Wikipedia

Marine Examples

Image of Alaska Pollock, a.k.a. Walleye Pollock (Theragra chalcogramma) from Wikipedia

Marine Examples

Image of Atlantic Cod (Gadus morhua) from Wikipedia

Marine Examples

Image of Peruvian anchovy, a.k.a. Anchoveta (Engraulis ringens) from Wikipedia

Marine Examples

Image from http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm

Marine Examples

“The Perils of Overfishing – Part 2” - NPR interview with Daniel Pauly

Time

Tro

phic

leve

l

Image of salmon from Wikipedia

Marine Examples

Salmon are carnivores, so farmed salmon are fed fish meal (ground up fish)

Pauly encourages us to eat wild-caught salmon(if you live close to a salmon fishery)

Image of a catfish from Wikipedia

Marine Examples

Catfish are often fed soy meal and are often farmed sustainably

Pauly encourages us to eat farmed catfish

Image of a fried tilapia (one of several members of the fish tribe Tilapiini) from Wikipedia

Marine Examples

Tilapia are often farmed sustainably (but also often become invasive)

Pauly recommends Tilapia for the table

Image of an oyster from Wikipedia

Marine Examples

Shellfish (clams, mussels, oysters) are often managed sustainably

Pauly also recommends shellfish

Image of a herring from Wikipedia

Marine Examples

Small shoaling fish generally feed low on the food chain

Pauly also recommends anchovies (Anchoa etc.), herring (Clupea), sardines (Sardina)