24.2 Human Intervention in the Natural World

24.2 Human Intervention in the Natural World

People change the birth rates and death rates of other species when they harvest, clear land, release wastes, or add a predator on purpose. Resource management and environmental pollution belong in this section as biology. The mechanisms are population replacement, habitat, nutrient cycles, and food chains. Which law a government passes is a separate civic question. The biology question is what the organism, the population, or the cycle does.

Renewable and nonrenewable resources

A renewable resource can regenerate on a human timescale if harvest stays below renewal. A forest can regrow after cutting when seeds, young trees, and soil remain. A fishery can persist when enough breeding adults are left to replace the fish removed. Forests and fisheries are the usual examples. Renewable means regeneration is possible. It does not mean the resource is impossible to exhaust.

A nonrenewable resource does not regenerate on that timescale. Fossil carbon, in coal, oil, and natural gas, was stored by ancient organisms and then set aside from the fast carbon cycle by burial. Burning it returns carbon dioxide to the air far faster than new fossil deposits form. Chapter 18 placed fossil fuels outside the fast loop until they are burned. Carbon atoms are not destroyed. The reservoir does not refill on a human timetable, so fossil carbon is the nonrenewable case in this section.

Overharvest and deforestation

Overharvest removes individuals faster than reproduction replaces them. A fishery can crash even though the species is renewable in principle. The adults that would have produced the next generation are gone, so recruitment fails. A large catch in one season is not proof that the same catch can continue. The test is whether new breeding animals replace the ones removed.

Deforestation removes a forest stand. Trees were habitat and food, so clearing shrinks the populations that lived there. Roots and leaf litter had held the soil. Without them, rain carries soil off the slope, and erosion increases. The stand can return if seeds, soil, and time remain. Repeated cutting before recovery, or a cut that strips the soil, pushes a renewable resource toward exhaustion.

Environmental pollution

Pollution is a harmful addition to air, water, or soil. Three mechanisms cover the usual cases, and each one uses a cycle or a food chain from earlier work.

Eutrophication begins with nutrient runoff. Fertilizer, sewage, or manure adds nitrogen or phosphorus to a lake, river, or coast. Those nutrients had been scarce, so algae and other producers bloom. When the bloom dies, decomposers respire as they break it down and consume dissolved oxygen. Water can become low in oxygen, and animals that need that oxygen die or leave. Extra nutrients do not oxygenate the deep water. They feed a bloom whose decay can take oxygen away.

Biomagnification fits fat-soluble toxins that organisms do not readily break down or excrete, such as mercury or persistent pesticides. Producers pick up a small amount from the water. A herbivore eats many producers and stores the toxin. A predator eats many herbivores. Concentration in tissue rises toward the top of the food chain, so top predators carry more than the water does. This is not dilution. The load is not spread thinner at each step. Each consumer gathers many meals, and the toxin stays in the body.

Greenhouse gases connect to the same carbon cycle. Carbon dioxide in the air is the pool photosynthesis takes in and the pool respiration and decay return. Burning fossil carbon, and clearing vegetation or disturbing soil carbon, adds carbon dioxide to that pool. The biological point is the shortcut from a slow reservoir into the atmosphere.

Biological control

Biological control introduces a natural enemy, such as a predator or parasite, to reduce a pest. The method uses a real feeding link instead of broadcasting a chemical. It can fail. The enemy may not establish, may ignore the pest, or may attack nontarget species. Mongooses brought to islands to eat rats have preyed on native birds. An introduction is not safe merely because the agent is alive. The control organism is still a population with its own diet and its own capacity to spread.

Name the mechanism before reaching for a label. A crashing fishery is overharvest of a population that could have renewed. Lost habitat and washing soil are deforestation and erosion. A bloom followed by animals dying for lack of oxygen is eutrophication. A toxin higher in a predator than in the water is biomagnification. Carbon dioxide added by burning coal, oil, or gas is the carbon cycle with a shortcut from fossil carbon. A predator that also wipes out native animals is the nontarget risk of biological control.