Natural Selection Image Map

Too Much of a Good Thing

Over-fertilization

Chemical fertilizers contain mostly nitrogen, phosphorous, and potassium. They are used on crops because they make crops grow better. Indeed, new genetic strains of crops were developed because they could grow better if given extra fertilizer that is not normally present in most soils. The "Green Revolution" has saved millions of people from starvation because fertilization of genetically improved plants has allowed us to increase food production in order to keep up with the demands of the rapid growth in human population to 6 billion people. The argument has been made that we should do even more fertilization, so that we can grow more food on the same amount of land and thus stop clearing of forests and farming on land that is subject to washing away when it rains.

But what happens to extra fertilizer that is not taken up by plants?  For example, fertilizer can be washed away in the runoff when it rains. Where does this water go?  It flows into creeks, ponds, rivers, and lakes. What happens then? The plant life in the water explodes, especially in response to the nitrogen in fertilizer. So why is that a problem? For one thing, it can choke the waterway so that fish and other animal life can no longer live there. In cooperation with University of California scientists and others, ARS plant pathologist Lars Anderson researches treatments that will stem the growth of aquatic weeds such as Eurasian watermilfoil. Photo by Brian Prechtel. Source: USDA-ARS. 

Green algae, water lilies, and sea "weeds" can grow so fast that they cannot rot as fast as they die. The decay of this massive growth robs the water of oxygen, so fish cannot live. Ponds and lakes can become really stinky. The overgrowth robs other living things in the water of light and nutrients. It changes the niches and breeding grounds for other species and typically drives them out. Click here to see examples of these problems.

We should also mention that fossil fuels are used to make fertilizers, and we are already running out of fossil fuels. Extra growth of plants also makes demands for more water. This is a real problem when farmland has to be irrigated. There may not be enough water to go around.

Antibiotic Use

Have you ever had a doctor prescribe antibiotics? Did the doctor tell you to make certain that you skip no doses and continue taking the medicine until all pills are taken?  Why do you suppose this is? If you stopped taking an antibiotic as soon as you felt better, would there still be some  bacteria alive? Which ones would be most likely to have survived up to this point? the ones that had so far resisted the antibiotic. In other words, you could be creating a population of bacteria in your body that had developed resistance to the antibiotic.  

What would happen if you took the same antibiotic all the time, that is, every time you got a cold or felt bad. A few bacteria would survive, and at some point, they might even start to thrive in the presence of the antibiotic. The constant exposure to the same antibiotic would become a natural selection force that could select the creation of a new species or strain of bacteria that is resistant to that antibiotic. Now when you get sick again with these resistant bacteria, your antibiotic will no longer work.  

This is exactly what has happened with penicillin. Penicillin, the first antibiotic, was discovered in the 1940s and was very effective against many kinds of bacteria.  Now, because of overuse of antibiotics, many of these bacterial strains  are no longer affected by penicillin.

The same problem has developed with several other antibiotics. The more popular and widely used an antibiotic is, the more likely it will lead to resistant strains of bacteria. Eventually, we may run out of effective antibiotics. Then what?  

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