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11.4: Ecosystem Essentials

An Ecosystem is a self-sustaining association of living plants and animals. The basis of an ecosystem is the plants which harvest energy from the sun and convert it into sugars and starches through a process known as photosynthesis. Plants also depend on soils to harvest nutrients. Animals then consume the plants and use the stored energy from the structure of the plants. When the plants and the animals die, they are recycled by decomposers, such as microbes and fungi that convert the tissue from the plants or animals into organic matter that becomes a part of the soil. The cycle begins again when a new plant begins growing in the soil enriched by the decomposition of prior organisms. These interrelated groups of organisms are called communities. Within a community two concepts are important:

  • A habitat is the type of environment in which an organism resides or is biologically adapted to live.
  • A niche is the function of a life form within a given community. Every organism occupies a niche within its habitat.

Ecosystems are affected by temperature and precipitation. Recall from chapter 6 that a major effect on temperature of a region is its latitude and elevation.

Exercise \(\PageIndex{1}\)

The limiting factor in an ecosystem is the one physical or chemical component that most inhibits biotic function. Look up limiting factors on the internet, a text, or your library, and write down at least five limiting factors:

  1. ___________
  2. ___________
  3. ___________
  4. ___________
  5. ___________

As discussed above, ecosystems are comprised of communities that depend on each other. The first level of these communities are called producers. Producers are those organisms (plants) that harvest the sun's energy and convert it to sugars and starches. Producers are eaten by primary consumers, creatures that eat plants. For example, a primary consumer might be a grasshopper. Primary consumers are eaten by secondary consumers (such as a chicken). The secondary consumers are then eaten by a tertiary consumer (such as a human).

Figure 1: An energy pyramid is a presentation of the trophic levels in an ecosystem. Energy from the sun is transferred through the ecosystem by passing through various trophic levels. Roughly 10% of the energy is transferred from one trophic level to the next, thus preventing a large number of trophic levels. There must be higher amounts of biomass at the bottom of the pyramid to support the energy and biomass requirements of the higher trophic levels.

As you consider the food pyramids, consider what the effects would be if a producer took in one unit of a pesticide, and that pesticide was magnified in each level of the food chain. For example, DDT is an insecticide which is harmless in small doses. However, DDT is stored in the fats of insects and animals, and can be passed on to offspring. This process is called bioaccumulation.

We will focus on the bioaccumulation of DDT, a pesticide which was widely used because it could be safely handled by humans. It was extensively used shortly after its discovery just before WW II. During the war, it was used to reduce mosquito populations and thus control malaria in areas where US troops were fighting (particularly in the tropics). It was also used on civilian populations in Europe, to prevent the spread of lice and the diseases they carried. Refugee populations and those living in destroyed cities would have otherwise faced epidemics of louse-born diseases. After the war, DDT became popular not only to protect humans from insect-borne diseases, but to protect crops as well.

Figure 2: Ball-and-stick model of the DDT molecule, \(C_{14}H_9Cl_5\). Image used with permission (Public Domain; Ben Mills).

By the 1960's, global problems with DDT and other pesticides were becoming so pervasive that they began to attract much attention. Credit for sounding the warning about DDT and biomagnification usually goes to the scientist Rachel Carson who wrote the influential book Silent Spring (1962). The silent spring alluded to in the title describes a world in which all the songbirds have been poisoned. DDT stands for dichloro, diphenyl trichloroethane (Figure 1). It is a chlorinated hydrocarbon, a class of chemicals which often fit the characteristics necessary for biomagnification. DDT has a half-life of 15 years, which means if you use 100 kg of DDT, it will break down as follows:

Year
Amount Remaining
0
100 kg
15
50 kg
30
25 kg
45
12.5 kg
60
6.25 kg
75
3.13 kg
90
1.56 kg
105
0.78 kg
120
0.39 kg

This means that after 100 years, there will still be over a pound of DDT in the environment. If it does bioaccumulate and biomagnify, much of the DDT will be in the bodies of organisms. DDT actually has rather low toxicity to humans (but high toxicity to insects, hence its use as an insecticide).

As the first of the modern pesticides, DDT was overused, and soon led to the discovery of the phenomena of insect resistance to pesticides and bioaccumulation and biomagnification. One of the most bizarre events to accompany this early use of DDT occurred when it became necessary to parachute cats into remote jungle villages in what was then Burma. The following has taken on urban legend proportions.

Operation Cat Drop

In the early 1950s, the Dayak people in Borneo suffered from malaria. The World Health Organization had a solution: they sprayed large amounts of DDT to kill the mosquitoes which carried the malaria. The mosquitoes died, the malaria declined; so far, so good. But there were side-effects. Among the first was that the roofs of people's houses began to fall down on their heads. It seemed that the DDT was also killing a parasitic wasp which had previously controlled thatch-eating caterpillars. Worse, the DDT-poisoned insects were eaten by geckoes, which were eaten by cats. The cats started to die, the rats flourished, and the people were threatened by outbreaks of sylvatic plague and typhus. To cope with these problems, which it had itself created, the World Health Organization was obliged to parachute live cats into Borneo.

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