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:

For a population changing in size:

vl m

lxi i

xi x

ve

le l mx

rx

x

ryy y

y x

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 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).

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