Habitats and biomes

Life in the form of individual organisms composed of one or more living cells is found in a vast array of places on Earth, each with its own distinctive types of organisms. Life on Earth has been classified by scientists into units called species, whose individuals appear similar, have the same role in the environment, and breed only among themselves. The space in which each species lives is called its habitat.

Habitats, Communities, Ecosystems, and Biomes

The term “habitat” can refer to specific places with varying degrees of accuracy. For example, rainbow trout can be found in North America from Canada to Mexico, but more specifically, they are found in freshwater streams and lakes with an average temperature below 70 degrees Fahrenheit and a large oxygen supply. The former example describes the macrohabitat of the rainbow trout, which is a broad, easily recognized area. The latter example describes its microhabitat—the specific part of its macrohabitat in which it is found. Similarly, the macrohabitat of one species can refer to small or large areas of its habitat. The macro habitat of rainbow trout may refer to the habitat of a local population, the entire range of the species, or (most often) an area intermediate to those extremes. While “habitat,” therefore, refers to the place an organism lives, it is not a precise term unless a well-defined microhabitat is intended. The total population of each species has one or more local populations, which are all the individuals in a specific geographic area that share a common gene pool; that is, they commonly interbreed. For example, rainbow trout of two adjacent states will not normally interbreed unless they are part of local populations that are very close to one another. The entire geographic distribution of a species, its range, may be composed of many local populations.

On a larger organizational scale, there is more than only one local population of one species in any habitat. Indeed, it is natural and necessary for many species to live together in an area, each with its own micro- and macrohabitat. The habitat of each local population of each species overlaps the habitat of many others. This collective association of populations in one general area is termed a community, which may consist of thousands of species of animals, plants, fungi, bacteria, and other one-celled organisms.

Groups of communities that are relatively self-sufficient in terms of both recycling nutrients and the flow of energy among them are called ecosystems. An example of an ecosystem could be a broad region of forest community interspersed with meadows and stream communities that share a common geographic area. Some ecosystems are widely distributed across the surface of Earth and are easily recognizable as similar ecosystems known as biomes—deserts, for example. Biomes are usually named for the dominant plant types, which have very similar shapes and macrohabitats. Thus, similar types of organisms inhabit them, though not necessarily ones of the same species. These biomes are easily mapped on the continental scale and represent a broad approach to the distribution of organisms on the face of Earth. One of the more consistent biomes is the northern coniferous forest, which stretches across Canada and northern Eurasia in a latitudinal belt. Here are found needle-leaved evergreen conifer trees adapted to dry, cold, windy conditions in which the soil is frozen during the long winter.

North American Biomes

The biomes of North America, from north to south, are the polar ice cap, the Arctic tundra, the northern coniferous forest; then, at similar middle latitudes, eastern deciduous forest, prairie grassland, or desert; and last, subtropical rainforest near the equator. Complicating factors that determine the actual distribution of the biomes are altitude, annual rainfall, topography, and major weather patterns. These latter factors, which influence the survival of the living, or biotic, parts of the biome, are called abiotic factors. These are the physical components of the environment for a community of organisms.

The polar ice cap is a hostile place with little evidence of life on the surface except for polar bears and sea mammals that depend on marine animals for food. A distinctive characteristic of the Arctic tundra, just south of the polar ice cap, is its flat topography and permafrost, or permanently frozen soil. Only the top meter or so thaws during the brief Arctic summer to support low-growing mosses, grasses, and the dominant lichens known as reindeer moss. Well-known animals found there are the caribou, musk-ox, lemming, snowy owl, and Arctic fox.

The northern coniferous forest is dominated by tall conifer trees. Familiar animals include the snowshoe hare, lynx, and porcupine. This biome stretches east to west across Canada and south into the Great Lakes region of the United States. It is also found at the higher elevations of the Rocky Mountains and the western coastal mountain ranges. Its upper elevation limit is the “treeline,” above which only low-growing grasses and herbaceous plants grow in an alpine tundra community similar to the Arctic tundra. In mountain ranges, the change in biomes with altitude mimics the biome changes with increasing latitude, with tundra being the highest or northernmost.

Approximately the eastern half of the United States was once covered with the eastern deciduous forest biome, named for the dominant broad-leaved trees that shed their leaves in the fall. This biome receives more than seventy-five centimeters of rainfall each year and has a rich diversity of bird species, such as the familiar warblers, chickadees, nuthatches, and woodpeckers. Familiar mammals include the white-tailed deer, cottontail rabbit, and wild turkey. The Great Plains, between the Mississippi River and the Rocky Mountains, receives twenty-five to seventy-five centimeters of rain annually to support an open grassland biome often called the prairie. The many grass species that dominate this biome once supported vast herds of bison and, in the western parts, pronghorn antelope. Seasonal drought and periodic fires are common features of grasslands.

The land between the Rockies and the western coastal mountain ranges is a cold type of desert biome; three types of hotter deserts are found from western Texas west to California and south into Mexico. Deserts receive fewer than twenty-five centimeters of rainfall annually. The hot deserts are dominated by many cactus species and short, thorny shrubs and trees, whereas sagebrush, grass, and small conifer trees dominate the cold desert. These deserts have many lizard and snake species, including poisonous rattlesnakes and the Gila monster. The animals often have nocturnal habits to avoid the hot, dry daytime.

Southern Mexico and the Yucatán Peninsula are covered by evergreen, broad-leaved trees in the tropical rainforest biome, which receives more than two hundred centimeters of rain per year. Many tree-dwelling animals, such as howler monkeys and tree frogs, spend most of their lives in the tree canopy, seldom reaching the ground.

Aquatic biomes can be broadly categorized into freshwater, marine, and estuarine biomes. Freshwater lakes, reservoirs, and other still-water environments are called lentic, in contrast to lotic, or running-water, environments. Lentic communities are often dominated by planktonic organisms, small, drifting (often transparent) microscopic algae, and the small animals which feed on them. These, in turn, support larger invertebrates and fish. Lotic environments depend more on algae that are attached to the bottoms of streams, but they support equally diverse animal communities. The marine biome is separated into coastal and pelagic, or open-water, environments, which have plant and animal communities somewhat similar to lotic and lentic freshwater environments, respectively. The estuarine biome is a mixing zone where rivers empty into the ocean. These areas have a diverse assemblage of freshwater and marine organisms.

The Biosphere

All biomes together, both terrestrial and aquatic, constitute the biosphere, which is all the places on Earth where life is found. Organisms that live in a biome must interact with one another and must successfully overcome and exploit their abiotic environment. The severity and moderation of the abiotic environments determine whether life can exist in that microhabitat. Such things as minimum and maximum daily and annual temperature, humidity, solar radiation, rainfall, and wind speed directly affect which types of organisms can survive. Few places on Earth are so hostile that no life exists there. An example would be the boiling geyser pools at Yellowstone National Park, but even there, as the water temperature cools at the edges to about 75 degrees Celsius, bacterial colonies begin to appear. There is abundant life in the top meter or two of soil, with plant roots penetrating to twenty-two meters or more in extreme cases. Similarly, the mud and sand bottoms of lakes and oceans contain a rich diversity of life. Birds, bats, and insects exploit the airspace above land and sea up to a height of about 1,200 meters, with bacterial and fungal spores being found much higher. Thus, the biosphere generally extends about 10 to 15 meters below the surface of Earth and about 1,200 meters above it. Beyond that, conditions are too hostile. A common analogy is that if Earth were a basketball, the biosphere would constitute only the thin outer layer.

Studying Habitats and Biomes

Abiotic habitat requirements for a local population or even for an entire species can be determined in the laboratory by testing its range of tolerance for each factor. For example, temperature can be regulated in a laboratory experiment to determine the minimum and maximum survival temperatures as well as an optimal range. The same can be done with humidity, light, shelter, and substrate type: “Substrate preference” refers to the solid or liquid matter in which an organism grows or moves—for example, soil or rock. The combination of all ranges of tolerance for abiotic factors should describe a population’s actual or potential microhabitat within a community. Furthermore, laboratory experiments can theoretically indicate how much environmental change each population can tolerate before it begins to migrate or die.

Methods to study the interaction of populations with one another or even the interaction of individuals within one local population are much more complicated and are difficult or impossible to bring into a laboratory setting. These studies most often require collecting field data on distribution, abundance, food habits or nutrient requirements, reproduction and death rates, and behavior to describe the relationships between individuals and populations within a community. Later stages of these field investigations could involve experimental manipulations in which scientists purposely change one factor, then observe the population or community response. Often, natural events such as a fire, drought, or flood can provide a disturbance in lieu of a manipulation caused by man.

There are obvious limits to how much scientists should tinker with the biosphere merely to see how it works. Populations and even communities in a local area can be manipulated and observed, but it is not practical or advisable to manipulate whole ecosystems or biomes. To a limited extent, scientists can document apparent changes caused by civilization, pollution, and long-term climatic changes. This information, along with population- and community-level data, can be used to construct a mathematical model of a population or community. The model can then be used to predict the changes that would happen if a certain event were to occur. These predictions merely represent the “best guesses” of scientists, based on the knowledge available. Population ecologists often construct reasonably accurate population models that can predict population fluctuations based on changes in food supply, abiotic factors, or habitat. As models begin to encompass communities, ecosystems, and biomes, however, their knowledge bases and predictive powers decline rapidly. Perhaps the most complicating factor in building and testing these large-scale models is that natural changes seldom occur one at a time. Thus, scientists must attempt to build cumulative-effect models that can incorporate multiple changes into a predicted outcome.

The biosphere, then, can be studied at different levels of organization, from the individual level through populations, communities, ecosystems, and biomes to the all-encompassing biosphere. Each level has unique relationships that require different methods of inquiry; in fact, these levels describe many of the subdisciplines within the science of ecology.

Biosphere and Biodiversity

Understanding the organization of the natural world of which man is a part is essential to the continued success of humankind. By understanding the abiotic and biotic relationships within and between each ecological level, from microhabitat to biosphere, scientists can partially explain why so many species of organisms have evolved over the last 3.5 billion years. This study of biodiversity may eventually be a key to maintaining a stable biosphere, in which there would be no drastic changes in climate or community relationships. For example, compare the diversity of microhabitats and species in a natural grassland biome with the established monoculture practices of agriculture, with the latter’s emphasis on one species. In a wheat field, there are fewer microhabitats available, but those few are available in abundance. This leads to an increase in the population size of “pest” species that compete with man for an abundant food resource, wheat. Understanding the microhabitat requirements of pest species can lead to the reduction of crop losses.

The goal of studying habitats, biomes, and the biosphere is the construction of predictive models. Once scientists have a general understanding of natural ecosystem processes, mathematical models may be able to predict future changes in the environment caused by the activities of civilization or natural climatic changes. For example, they may indicate whether increased human population size or increased large-scale agriculture in or near desert biomes will lead to the spread of desertlike conditions. On a biosphere scale, they help scientists predict the extent of climate change expected in the twenty-first century caused by human activity.

Before the relatively recent growth and interest in the science of ecology, man had little concern about the impact of the exploitation of natural resources such as forests, of synthetic chemical pollutants such as dichloro-diphenyl-trichloroethane (DDT), or even of the rapid growth of the human population size. With increasing knowledge and understanding of habitat requirements, natural community interrelationships, and cycling of nutrients and pollutants within the biosphere, however, scientists have greater predictive power concerning the effects of economic development and human population growth. Ecologists are studying the effects of changing rainforests into agricultural land, the thinning and recovery of the ozone layer, the extinction of species, and many other phenomena that can potentially change the abiotic environment and, therefore, affect the stability of biotic communities. These results are incorporated into future predictive models, increasing accuracy.

In the first two decades of the twenty-first century, human activities have caused the destruction of 0.537 percent of all natural habitats. This does not seem significant, but considering the delicate nature of the environment, the consequences of such degradation are wide-reaching. Mangroves were the most negatively impacted biome. The damage to the resilience of Earth’s biomes is evident in increasing global temperatures and decreased biodiversity.

With better predictive capabilities and an improved understanding of the Earth’s previous eras of mass extinction from the fossil record, the general public is more informed than ever before about the dangers of climate change and the importance of preserving Earth’s biomes.

The challenge—both to scientists and to human civilization as a whole—is to use an understanding of the biosphere to maintain a level of economic growth that is ecologically sustainable. The study of communities and ecosystems may discover ways that civilization can better adapt to its current environment rather than attempt to mold the environment to fit its own preconceived, established ideas.

Principal Terms

Abiotic: The physical part of an ecosystem or biome, consisting of climate, soil, water, oxygen and carbon dioxide availability, and other physical components

Biome: One of the widespread types of ecosystems on Earth, such as the Arctic tundra or the desert

Biosphere: The sum of all the occupiable habitats for life on Earth

Biotic: The living part of an ecosystem or biome, consisting of all organisms

Community: A population of plants and animals that live together and make up the biotic part of an ecosystem

Ecosystem: A relatively self-sufficient group of communities and their abiotic environment

Environment: The habitat created by the interaction of the abiotic and biotic parts of an ecosystem

Habitat: The specific part of the environment occupied by the individuals of a species

Population: A group of all the individuals of one species

Species: A group of similar organisms that are capable of interbreeding and producing fertile offspring

Bibliography

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"What Makes a Biome?" National Geographic Society, 19 Oct. 2023, education.nationalgeographic.org/resource/what-makes-biome. Accessed 10 Sept. 2024.