Conservation and Harvesting

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• Conservation and Harvesting
• Well consider these subjects again near the end
  of the semester. For now, well consider them in
  reverse order.
• How ecologists measure and estimate the effect of
  harvesting depends on the system under study. In
  plant ecology, since the objects of study are
  sessile, there are a number of techniques used
  that are not available in studies of animals
• Measures of net primary production by cutting,
  drying, and weighing plant biomass. Roots are
  only occasionally taken (remember the study of
  goldenrods shown as an indication of allocation).

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• (cont.) The above ground biomass is called AANP
  (Annual Aboveground Net Productivity). Sometimes,
  instead of biomass, the dry plants are
  incinerated, and the loss on ignition is carbon.
  The carbon is used as the measure of production.
• Another approach directly measures the rate at
  which CO2 is incorporated into leaves by
  measuring its loss from the atmosphere. The tool
  is an infrared gas analysis system (IRGA).
  Infrared radiation is absorbed by CO2, so
  measurement of the atmosphere around the leaf
  indicates how fast CO2 is disappearing, absorbed
  and fixed by photosynthesis. Knowing leaf area,
  the small scale measurement can be extrapolated
  to whole plants and even communities.

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Heres what the business end of one type of
IRGA looks like
Leaves inside the chamber
Tubes (and a pump) circulate gases from the
chamber to the analyzer
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3. A third method uses radioactive carbon in CO2
and measures the incorporation of C14 into leaf
tissues to assess photosynthesis and primary
production. Heres the approach
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The C14 approach can also be used to measure
animal (secondary or tertiary) productivity. The
food source is developed to contain radiocarbon.
Rates of uptake, assimilation, and incorporation
can then be measured. In animals, harvesting
measures are used to assess populations and their
dynamics much more frequently. The biomass you
would measure indicates net production, and
compared to food assimilated, measures net
production efficiency. If, instead, you use total
food intake, the measure you get is called gross
production efficiency.
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• The further up the food chain you move, typically
  the efficiency (whichever measure) decreases due
  to the need to cover more ground to find
  sufficient food to maintain a larger organism.
• Harvesting, as an economic as opposed to
  scientific tool, is one of the major ways that
  humans impact populations, communities, and
  ecosystems.
• Harvesting in tropical forests has severe impact
  on biodiversity in both the plant community and
  the huge diversity of birds, insects and others
  dependent on tropical forest trees. It also
  severely impacts soils there, leading to
  laterization. Sadly, we still estimate a loss of
  about 2 of remaining tropical forest each year.

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This is what has happened in Panama to create
pastureland to raise cattle, mostly for marketing
in highly developed countries (think MacDonalds
in the U.S. and Canada).
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We know about cod, but other fisheries have been
similarly overexploited. Examples Sardines
(Sardinops spp.) were, until the 1950s, a heavily
exploited fishery along the western coast of
North America. The fishery collapsed from
overexploitation and, like the cod, has not
recovered. With the loss of sardines, the west
coast fishery discovered that anchovies,
previously fished off the coast of South America,
increased in abundance. Anchovies are now fished
just as intensively off North America as off
South America. Any predictions of their fate?
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One final example the shifting whale fishery in
the second half of the 20th century. The favored
whale species for the fishery were humpback,
right, bowhead and gray whales. They had been
hunted to such low numbers that further hunting
was uneconomical. Soon after the beginning of the
20th century, hunting shifted to the blue whale.
You saw the shift in its survivorship it became
uneconomical to hunt by around 1950. Hunting then
shifted to fin whales their numbers declined to
unprofitability between 1965 and 1975. Next came
sperm and sei whales. By this time the
International Whaling Commission banned
commercial whale hunting.
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Heres what the shifting hunt looked like
Japan, while claiming to have accepted the
whaling convention, still has a limited
research hunt (though the meat and other
byproducts are sold).
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There are many aspects to conservation biology
(and an entire course eponymously titled). At
this point, we should be mostly interested in
population level aspects of the subject, and how
it relates to harvesting. How do you organize a
harvest to make use of a population without (in
either short or long term) destroying it? Weve
already considered maximum sustained yield. If a
population is growing logistically, then dN/dt is
at its maximum at K/2. Harvest the population at
or near that size, and those you harvest will be
replaced most quickly. Typically, the theoretical
strategy is to harvest from above back to K/2.
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What remains is which animals or plants to take
in the harvest? To optimally protect the
population, there is another demographic variable
you can calculate, called reproductive value.
This parameter indicates the relative
contribution of different age classes to the
future growth of the population. Heres the
formula (No! you wont be asked to calculate
it!) For a population stable in size For a
population changing in size
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To protect the health of the population, you
harvest age classes that have the lowest
reproductive values. Is there a pattern to
reproductive value with age? Yes! Reproductive
value typically rises from birth to a maximum at
age ?, then declines more-or-less slowly through
the reproductive age classes until reaching 0 at
age O. At Point Pelee it has become necessary to
cull white- tailed deer every few years.
Everytime culling is necessary, it is
controversial. Imagine you are managing the cull.
Use reproductive value to figure out which age
classes you should remove (kill).
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In conservation we want to preserve the
population, an opposite intention. To conserve a
species, the population size cannot be allowed to
become too small. If that should happen, the
species may enter an extinction vortex (the term
was coined by Primack, see references). Heres
how he described the process
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• The list at the lower right of the previous
  figure tells you how we make populations smaller,
  and put them in danger of entering an extinction
  vortex
• Habitat destruction or fragmentation. Destruction
  consequences are obvious. Fragmentation and
  isolation of populations makes each one smaller,
  and increases the likelihood of inbreeding.
• Environmental degradation. Even if we dont
  totally destroy an area, chemical and physical
  changes we cause may make it difficult or
  impossible for a population to persist there, or
  cause it to become smaller.
• Overexploitation. No further comment necessary.

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4. Introduction of exotic species. Exotic species
usually lack predators and other control
agents active in their native habitats. As a
result, they frequently outcompete and
displace native species having similar
niches. Populations of the native species
decrease markedly, or they are driven locally
extinct. Examples are numerous. The zebra
mussel it attaches to and eventually
smothers native bivalves in the Great Lakes.
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Nile perch introduced into Lake Victoria, one
of the African rift lakes to improve productivity
of harvestable protein for human populations
around the lake. Nile perch predated and drove
hundreds of unique species of endemic cichlids
extinct.
Nile perch Its
cichlid prey