Basic Ecology Questions Answered

Much of the energy and nutrients in plants is not fully utilized or realized as the trophic levels go up. As such, the higher up the trophic levels you eat, the less of the original available energy and nutrients are available. By eating at the lowest or near the lowest level you are getting the most from your food, you'll be able to obtain and use more of the energy and nutrients found within the original source instead of getting the "filtered" or worn down version that occurs in typical higher trophic feeding.

Would a population in which most members survive for a long time produce few or many offspring? Which would be most the advantageous to the population as a whole?
This would depend on the species and the mortality rate of births or offspring. For species that exhibit high mortality rates (such as sea turtles) a large number of offspring would have to be produced in order to assure that a "good" number of the offspring survived and lived to reproduce. This sort of filtering of survival balances out the population. As for humans, a low mortality rate is present in infancy in most countries (at least developed or developing countries). This low mortality rate creates a much smaller need for a large number of offspring since they are likely to grow up and reproduce themselves. As we've seen with countries such as China, over population can easily be the result of too many offspring in human communities. This does change slightly in places of Africa, and the like, in which infant mortality is high and parents are encouraged and accustomed to having more children since few will survive and reproduce.

Point and non-point pollution

Point pollution comes from a known source such as a factory, a car, or human waste, while non-point pollution comes from an unknown source spread through rainfall, water moving through the ground, etc...

Why are nitrates and phosphates removed by sewage plants before allowing the water to flow back into a river, lake, or the like?

Phosphates and nitrates are stimulants for plant growth, in the case of water, they stimulate algae growth. When algae builds up on the surface of water the amount of sunlight the body of water is exposed to drastically decreases. This also causes a decrease in oxygen availability for other water-residing organisms. Excesses of algae, and especially dead algae or other plant or animal life in the body of water can lead to a build-up of bacteria which can further inhibit oxygen flow to fish and other animals in the lake. Excesses of algae and bacteria can create such a hostile environment toward the natural water-residing organisms that they eventually die out and the lake turns into a swamp or bog.

How do pesticides and heavy metals become concentrated through biological magnification?

Plants and some animals lower in the food chain are highly susceptible to toxins and can become highly toxin when living in contaminated waters. When these lower level organisms are consumed, the higher level consumer becomes contaminated with extreme levels of toxins. Some fishing businesses such as shellfish businesses are shut down during times when the waters are toxic in order to prevent the poisoning of people seeking a shellfish meal. However, these high toxic levels can still affect other animals that feed on the shellfish.