The Laws of Ecology
An ecosystem is composed of biotic and abiotic factors. Biotic factors include all living organisms, from single-celled microbes to huge land and water mammals. Abiotic factors are the nonliving components of an ecosystem, including water, gases in the air, and minerals in rocks and soil. Ecologists are scientists who study the interactions between the biotic and abiotic factors within an ecosystem.
Decades ago, ecologist Barry Commoner described four basic laws of ecology that apply to all ecosystems.
Everything is connected to everything else. What happens to one organism within an ecosystem affects all organisms in some way.
Everything goes somewhere. Nature doesn’t create waste, and it doesn’t throw things away. What exists remains in existence in one form or another.
Nature knows more than humankind. Things are always changing within nature. When humans cause those changes, however, the changes are likely to be harmful to the system.
Everything has limits. That is, there is only so much nature that humans can exploit, or take advantage of. When humans use natural resources, those resources eventually change from useful to useless forms.
Responding to Change
As Commoner’s third law of ecology tells us, nature is always changing. And as his first law says, all living and nonliving elements within an ecosystem are connected. What happens to one affects all.
Within an ecosystem, organisms live in equilibrium, or balance, responding to the natural changes that are constantly occurring within any system. Some changes, however, are larger than others. All ecosystems respond to disruptions, which are breaks or interruptions in normal events. Nature causes some of these disruptions. Humans cause others.
Fire and Floods as Disruptions
Fire, both natural and caused by human behavior, is a common disruption. , Natural fires can both harm and help a forest ecosystem. For example, after a wildfire, soils absorb nutrients from the charcoal and ash left after vegetation burns. The soil is warmer, too, encouraging microbial activity. However, intense heat can also cause soil particles to repel, or shed, water instead of allowing the water to soak in. After a fire, the water-resistant soil causes soil erosion, as rainwater carries soil away.
Some animals are affected more than others during a fire. Small animals, insects, and sick or old organisms may die. Larger animals are normally able to flee to safety. The fires, which destroy food resources, make it impossible for these animals to return immediately after the fire. This, however, gives other organisms opportunities for survival. Scavengers, or organisms that feed on dead plant and animal matter, take advantage of new food resources. Also, areas once thick with trees are laid bare, making it easier for predators to find prey.
New plants begin to grow, and increased quantities of seeds on the forest floor encourage birds to feed. The burning of trees and other plants make more nutrients, light, and water available to survivors. With such resources suddenly available, new and surviving plants grow more quickly. Among the benefits of fire is the destruction of harmful plant parasites, like mistletoe, that rob trees of essential nutrients. Another benefit is seed production, as some plants require the heat of wildfires to open their seeds.
Like fire, flooding can cause both harmful and beneficial consequences. Those consequences depend on where floods occur, how long they last, how deep and swift the waters are, and how sensitive the environment is to such disruption.
Floods can lead to the loss of plant and animal life, as well as loss of soils. They can also lead to new life. Some plant seeds remain dormant, or inactive, until heavy rains occur. Then these seeds sprout, leading to new growth. Some insects and reptiles that remain inactive during periods of hot or dry weather emerge from their resting state.
Introduced Species as a Disruption
Since the 1700s, about 40 percent of the species on Earth have become extinct, meaning they have died out completely. Scientists think that the human introduction of foreign species into environments that are not natural to these species has probably contributed to the massive extinction over recent centuries.
An invasive species is an organism that humans move from its natural environment to a new, foreign environment in which it causes harm. Sometimes, the introduction of an invasive species is accidental. At other times, it is done for a purpose.
An example of an accidental introduction comes from shortly after World War II, when military troops shipped cargo from Papua New Guinea to the Pacific island of Guam. Until then, Guam had no snakes other than a kind of blind, worm-like snake that fed on termites and ants. However, it is likely that a brown tree snake hitched a ride with the cargo. By the early 1960s, tree snakes inhabited more than half of the island. After only a few more years, they had colonized the entire island. As the tree snake population grew, native bird populations shrank. By the time the US Fish and Wildlife Service began listing species as endangered or threatened in 1984, most of the native forest bird species were extinct.
In the late 1800s, a group of Americans organized a club for the purpose of bringing birds mentioned in the plays of William Shakespeare to the United States. In 1890 and 1891, the club’s members released about 100 starlings into Central Park. The species flourished. Today, their population exceeds 200 million, and the birds live across the country. Scientists continue to study their effect on native species.
In the 1930s, the US Department of Agriculture imported a Japanese plant called kudzu, and they paid southern farmers to plant it as a means of controlling soil erosion. At first, the plant’s rapid growth was considered a success story. However, today some people call kudzu “the plant that ate the South.” Able to grow as much as two feet per day, kudzu blankets other vegetation, blocking sunlight. The plants beneath the kudzu die, eliminating potential food and shelter for native animals. Kudzu roots also penetrate the soil, where they affect water levels throughout ecosystems. Kudzu is such a successful invasive species that it has been labeled as one of the 100 worst invasive species on Earth.
Habitat Loss as a Disruptive Force
Some scientists identify the disruptive effect of human activity on natural habitats as the greatest threat to Earth’s biodiversity. Diversity means “variety,” so biodiversity refers to various forms of life, including the variety of species and genes. Agriculture, forestry, mining, urban growth, and the pollution that comes with these human efforts have led to massive habitat loss. The International Union for Conservation reports that human interference has caused the rate of species extinction to increase 1,000 times its natural rate.
Habitat loss includes habitat degradation, or the loss of habitat due to pollution or the introduction of invasive species. In habitat fragmentation, remaining wildlife areas are separated and divided into sections by roads, dams, and other structures. The remaining sections are often too small or they restrict access to larger areas where species can find food and mates. Fragmentation also makes it less likely that migratory animals will have the places they need to rest and feed along their routes.
In habitat destruction, habitats are destroyed. People use machines to cut or knock down trees, to fill wetlands with soil in preparation for building, and to scoop sediments and soils from river bottoms for the purpose of building waterways and dams and reclaiming land. Some powerful examples of habitat destruction are found in the United States. According to the US Fish and Wildlife Service, more than 85 percent of forest habitats have been permanently destroyed or logged. More than 75 percent of forests growing along waterways such as streams have been destroyed. In Michigan, 99 percent of mature oak and beech-maple forests are gone. In Oregon, nearly all of the state’s temperate rain forest has been destroyed. Across the country’s prairies, 95 percent of grasslands have been planted with crops or destroyed. In the Southwest, where desert conditions exist, cattle have overgrazed more than 90 percent of sagebrush habitats.
Leading causes of habitat destruction include agriculture, the conversion of land to building sites and parking lots, and water-development projects, such as dams. They also include pollution, particularly of freshwater resources. In some places, untreated sewage and other human waste enter water resources. So do metals and acids from mines and fertilizers and pesticides from farms. increase 1,000 times its natural rate.