Overexploitation

1
Overexploitation
Photo of American bison skulls from Wikipedia
2
Population Dynamics
?N
Exponential growth
r N
?t
Occurs when growth rate is proportional to
population size Requires unlimited resources
N
Time
3
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
r
or d
d
Equilibrium ( carrying capacity, K)
N
4
Population Dynamics
?N
N
Logistic growth
r N (1 )
?t
K
K carrying capacity
0
N
is maximized
0
Time
5
Population Dynamics Maximum Sustainable Yield
(MSY)
Exploitation (harvesting)
or overexploitation (overharvesting)
Rate of prod-uction of new indivs. or
Sustainable yield
Yield (Y)
K
N
6
Population Dynamics Maximum Sustainable Yield
(MSY)
Constant quota exploitation
MSY
Sustainable yield
Yield (Y)
K
½ K
N
7
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
8
Population Dynamics Maximum Sustainable Yield
(MSY)
Proportional or constant effort exploitation
MSY exploitation
Very high 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
Low exploitation
Yield (Y)
N
9
Population Dynamics Allee Effects
Allee Effects occur when per capita fitness
declines as a population becomes smaller
Photo of lekking Attwaters prairie chickens in
Texas from www.nationalgeographic.com
10
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
11
Collateral Damage
30 of marine fisheries landings are by-catch
Photo of Dall porpoise entangled in a fishing net
from Wikipedia
12
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
(large-bodied birds and mammals)
Photo of Giant ground sloth bones from
www.corbis.com
13
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
14
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
15
Policy as a Tool to Combat Overexploitatione.g.,
legislation on trade in endangered species
16
Policy as a Tool to Combat Overexploitatione.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