Overexploitation
Photo of American bison skulls from Wikipedia
Population Dynamics
∆N
Exponential growth
∆t
Occurs when growth rate
is proportional to
population size;
Requires unlimited
resources
=r•N
N
Time
Population Dynamics
Density-dependent per capita birth (b) and death (d) rates
Notice that per capita
fitness increases with
decreases in population
size from K
b
b
or
d
r
d
N
Equilibrium
(= carrying
capacity, K)
Population Dynamics
∆N
Logistic growth
∆t
= r • N • (1 –
N
K
K = carrying capacity
∆N
∆t
=0
N
∆N
∆t
∆N
∆t
is maximized
=0
Time
)
Population Dynamics & Maximum
Sustainable Yield (MSY)
Exploitation
(harvesting)
or
overexploitation
(overharvesting)
∆N
∆t
or
∆N
∆t
Rate of net
production
=
of new
individuals
Sustainable
yield
Yield
(Y)
K
N
Population Dynamics & Maximum
Sustainable Yield (MSY)
Constant quota
exploitation
MSY
Sustainable
yield
Yield
(Y)
½K
N
K
Population Dynamics & Maximum
Sustainable Yield (MSY)
Constant quota
exploitation
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)
MSY quota
Low quota
Yield
(Y)
N
Population Dynamics & Maximum
Sustainable Yield (MSY)
Proportional or
“constant effort”
exploitation
Very high
exploitation
MSY
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)
Low
exploitation
N
Population Dynamics & Allee Effects
Allee Effects occur when per capita fitness declines
as a population becomes smaller
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
b
?
b
or
d
?
d
K
N
Zone of Allee Effects
Examples of Exploitation of Target Species
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
Pleistocene Overkill Hypothesis
Pleistocene = 1.8 million to 10,000 yr before present
Paleolithic human hunters
may have caused the
extinction of many species
of megafauna (largebodied birds and
mammals)
Photo of Giant ground sloth bones from www.corbis.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
Photo of at least one too many deer in Pennsylvania from www.nrcdeer.com
Policy as a Tool to Combat Overexploitation
e.g., legislation on trade in endangered species
Policy as a Tool to Combat Overexploitation
e.g., legislation on trade in endangered species
Tim Wright et al. (2001) Cons. Biol.
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
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)
Photo from Greg Dimijian
Economic Incentives to Combat Overexploitation
e.g., conservation agreement between Conservation International (CI)
& Chumnoab Commune, Cambodia
Threat: overharvesting of wildlife (e.g., crocodiles) & timber
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)
Image of domestic water buffalo from Wikipedia; case study from Ted Groves (UCSD), pers. comm.
Economic Incentives to Combat Overexploitation
e.g., conservation agreement between Conservation International (CI)
& Chumnoab Commune, Cambodia
Threat: overharvesting of wildlife (e.g., crocodiles) & timber
Compliance monitoring
conducted by Cambodia
Forestry Administration, an
independent party
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
Image of domestic water buffalo from Wikipedia; case study from Ted Groves (UCSD), pers. comm.
Economic Incentives to Combat Overexploitation
e.g., conservation agreement between Tetepare Descendants’
Association (TDA) & villagers in the Solomon Islands
Threat: overharvesting leatherback sea turtle females & eggs
Villagers agreed to protect sea
turtles by reporting to sea
turtle monitors
Villagers & monitors receive
compensation upon initial
report & upon successful
hatching
So successful that TDA seeks
to build an endowment to fund
future payments
Image of leatherback turtle digging a nest from Wikipedia; case study from Ted Groves (UCSD), pers. comm.
Marine Example
~75% of marine fisheries are considered to be
fully fished or overexploited (FAO 2002)
Seafood Watch
Use the pocket guides to
gauge the status of
seafood:
Web site
Photo of Northern bluefin tuna from Wikipedia
Marine Example of Collateral Damage
~30% of marine fisheries “landings” are by-catch
Photo of Dall porpoise entangled in a fishing net from Wikipedia
Trophic level
Marine Examples of “Fishing Down Food Webs”
Time
“The Perils of Overfishing – Part 1” - NPR interview with Daniel Pauly
Image from http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm
Marine Examples
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
Trophic level
Marine Examples
Time
“The Perils of Overfishing – Part 2” - NPR interview with Daniel Pauly
Image from http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm
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 salmon 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 catfish from Wikipedia
Marine Examples
Tilapia are often farmed sustainably (but also often become invasive)
Pauly recommends Tilapia for the table
Image of a fried tilapia (one of several members of the fish tribe Tilapiini) from Wikipedia
Marine Examples
Shellfish (clams, mussels, oysters) are often managed sustainably
Pauly also recommends shellfish
Image of an oyster from Wikipedia
Marine Examples
Small shoaling fish generally feed low on the food chain
Pauly also recommends anchovies (Anchoa etc.),
herring (Clupea), sardines (Sardina)
Image of a herring from Wikipedia