Conservation and Harvesting
We’ll consider these subjects again near the end of the
semester. For now, we’ll 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:
1. 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).
1. (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.
2. 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.
Here’s 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
3. A third method uses radioactive carbon in CO2 and
measures the incorporation of C14 into leaf tissues to
assess photosynthesis and primary production. Here’s
the approach:
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.
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.
1. 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.
This is what has happened in Panama to create
pastureland to raise cattle, mostly for marketing in
highly developed countries (think MacDonald’s in the
U.S. and Canada).
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?
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.
Here’s 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).
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?
We’ve 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.
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. Here’s the formula (No!
you won’t be asked to calculate it!):

For a population stable in size:
vx 

i x
li mi
lx
rx

For a population changing in size:
e
vx 
lx
e
y x
 ry
l y my
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 Ω.
At Point Pelee it has become necessary to cull whitetailed 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).
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). Here’s how
he described the process:
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:
1. Habitat destruction or fragmentation. Destruction
consequences are obvious. Fragmentation and
isolation of populations makes each one smaller,
and increases the likelihood of inbreeding.
2. Environmental degradation. Even if we don’t 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.
3. Overexploitation. No further comment necessary.
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.
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