Chapter: Ecology and the Biosphere

Ecology is the study of organisms and their environment. Ecologists study life at several different fields of organization, ranging from individual organisms and populations to entire biomes and the biosphere itself.

Fields of ecology

Organismal ecology

Organismal ecology is the study of how individual organisms interact with their environment. It focuses on the morphological, physiological, and behavioral adaptations that allow individuals to survive and reproduce. Organismal ecologists examine how differences among individuals affect fitness and reproductive success. In many ways, Charles Darwin could be considered an organismal ecologist because of his focus on adaptation and natural selection.

Behavioral ecology

Behavioral ecology is a branch of organismal ecology that studies the ecological and evolutionary basis of animal behavior and how behavior contributes to survival and reproduction. Traits that provide a selective advantage in a particular environment are said to have adaptive significance. If a trait improves survival or reproductive success, natural selection is more likely to favor it over time.

Population ecology

Population ecology is the study of how populations change over time and space. It examines the dynamics of populations, including changes in population size, density, and distribution, as well as how populations interact with environmental factors. Population ecologists study birth rates, death rates, immigration, emigration, and the environmental conditions that influence population growth.

Community ecology

In ecology, a community consists of two or more populations of different species living in the same geographic area. Community ecology studies the interactions among species within these communities, including competition, predation, parasitism, and mutualism.

Ecosystem ecology

Ecosystem ecology moves beyond species (essentially ignoring the species level) and communities to examine how energy and nutrients move through ecosystems. It studies the interactions between biological organisms and the physical environment, including the cycling of matter and the flow of energy through ecosystems.

Abiotic and biotic factors

What determines where a species can live? Why can some organisms survive in one biome but not another? The geographic area where a species naturally occurs is known as its range. A species’ range is strongly influenced by abiotic factors, which are nonliving environmental conditions such as temperature, precipitation, sunlight, soil composition, and water availability.

Organisms are adapted to specific environmental conditions, and fitness trade-offs often limit the environments in which they can survive successfully. For example, early humans were well adapted to the warm climates of Africa. As humans migrated into colder regions, they developed clothing and other cultural adaptations to survive in colder environments.

Biotic factors also influence species distribution, although they are often less important than abiotic factors. Biotic factors include interactions with other organisms such as competition, predation, disease, and parasitism. Competition can strongly influence a species’ range. If a more competitive species enters an ecosystem, it may gradually reduce the range and abundance of less competitive species.

Global Climate Patterns

Nearly all of Earth’s biomes are shaped primarily by two factors: sunlight and water availability. The amount of sunlight received at different latitudes strongly influences temperature and precipitation patterns across the planet.

At the equator, sunlight strikes Earth most directly, producing the highest amount of solar energy per unit area. As latitude increases toward the poles, sunlight strikes Earth at lower angles, spreading the energy over a larger surface area and reducing heating. As a result, equatorial regions are warm, middle latitudes are moderate, and polar regions are cold.

Global light intensity

Because Earth is spherical, sunlight intensity varies with latitude. The equator receives much more direct sunlight than the poles, making latitude one of the major factors controlling the distribution of Earth’s biomes.

Seasons and Earth’s tilt

Seasons are caused by Earth’s axial tilt, also called the angle of incidence. In the Northern Hemisphere, Earth tilts toward the Sun during summer and away from the Sun during winter. This causes seasonal temperature variation. The opposite pattern occurs in the Southern Hemisphere, where summer and winter are reversed relative to the Northern Hemisphere.

Global Atmospheric Circulation

Earth’s global climate patterns are strongly influenced by the unequal heating of the planet by the Sun. Because Earth is spherical, sunlight strikes different latitudes at different angles. At the equator, sunlight strikes Earth most directly, producing intense heating. Toward the poles, sunlight strikes at lower angles, spreading the same amount of energy over a larger surface area and producing much cooler temperatures. These differences in heating drive large-scale atmospheric circulation patterns that distribute heat and moisture across the globe. Global atmospheric circulation is organized into three major circulation cells in each hemisphere: the Hadley cells, Ferrel cells, and Polar cells. Together, these circulation systems regulate temperature, precipitation, wind patterns, and the distribution of Earth’s major biomes.

Hadley cells

The Hadley cells are tropical circulation patterns that occur between the equator and approximately 30° north and south latitude. These cells are driven by intense solar heating at the equator. Because equatorial regions receive the greatest amount of solar energy, the air there becomes very warm. Warm air expands, becomes less dense, and rises into the atmosphere. Warm air also has a high capacity to hold water vapor, so the rising equatorial air is extremely moist. As this warm, moist air rises, it cools at higher elevations. Cooling air causes water to condense into clouds and precipitation. This process creates the consistently warm and wet conditions found in tropical rainforests near the equator. Regions influenced by rising equatorial air masses experience some of the highest levels of rainfall and biodiversity on Earth.

After reaching the upper atmosphere, the air spreads northward and southward away from the equator. As the air moves toward approximately 30° latitude, it cools and begins to descend back toward Earth’s surface. Descending air becomes compressed and warms as atmospheric pressure increases. As the air warms, its ability to hold moisture increases, making clouds and precipitation less likely to form. This creates dry conditions and contributes to the formation of many of the world’s major deserts, including the Sahara Desert, Sonoran Desert, Arabian Desert, and Australian deserts. The descending air at 30° latitude also creates regions of relatively high atmospheric pressure. Some of this air returns toward the equator near Earth’s surface, completing the Hadley cell circulation loop. These surface winds are deflected by Earth’s rotation through the Coriolis effect, producing the trade winds that blow from east to west in tropical regions.

Ferrel cells

Between approximately 30° and 60° latitude are the Ferrel cells. Unlike the Hadley and Polar cells, which are driven primarily by rising and sinking air, Ferrel cells are more complex and are strongly influenced by interactions between the adjacent circulation cells. At around 30° latitude, some of the descending air from the Hadley cells moves poleward toward higher latitudes. As this relatively warm air travels toward 60° latitude, it encounters colder polar air moving equatorward from the poles.

The collision of these contrasting air masses forces warm air upward near 60° north and south latitude. As the warm air rises, it cools and releases precipitation. This process creates relatively wet climates in many temperate regions. Areas influenced by rising air at these latitudes include much of northern Europe, parts of Canada, the Pacific Northwest, England, and portions of the northeastern United States. These regions often experience frequent cloud cover, precipitation, and changing weather patterns. Surface winds within the Ferrel cells are also affected by the Coriolis effect, producing the prevailing westerlies. These winds generally move from west to east and strongly influence weather systems across much of the temperate world.

Polar cells

The Polar cells occur between approximately 60° latitude and the poles. Near the poles, sunlight strikes Earth at extremely low angles, producing very cold temperatures. Cold air is dense and tends to sink, creating regions of high atmospheric pressure at the poles. This cold, dry air moves outward from the poles toward lower latitudes near Earth’s surface. Around 60° latitude, the cold polar air collides with the warmer air moving poleward from the Ferrel cells. Because warm air is less dense, it rises above the cold polar air. As the warm air rises and cools, precipitation often forms.

This interaction between Ferrel cells and Polar cells helps create the cool, moist climates commonly found around 60° north and south latitude. These regions are often characterized by forests, frequent storms, and relatively high precipitation compared with neighboring latitudes. After rising near 60° latitude, the air cools and moves poleward in the upper atmosphere, where eventually it sinks again and completes the Polar cell circulation pattern.

Interactions among the circulation cells

The Hadley, Ferrel, and Polar cells are interconnected components of Earth’s global atmospheric circulation system. Together, they redistribute heat from the equator toward the poles and help regulate global climate patterns. Rising air is generally associated with clouds, precipitation, and wetter climates because cooling air loses its ability to hold water vapor. Descending air is associated with dry climates because warming air gains the ability to hold moisture and suppresses cloud formation. As a result, the locations of rising and sinking air strongly influence the distribution of Earth’s major biomes.

Near the equator, rising air in the Hadley cells produces tropical rainforests with high rainfall and biodiversity. Around 30° latitude, descending air creates many of the world’s deserts. Around 60° latitude, rising air associated with interactions between the Ferrel and Polar cells produces cool, wet climates. Near the poles, descending cold air contributes to dry, polar conditions. These global circulation patterns, combined with regional effects such as mountain ranges, ocean currents, and elevation, play a major role in determining temperature, precipitation, and biome distribution across Earth.

Regional effects

Regional factors can strongly influence local temperature and precipitation patterns. Two important regional effects are the rain shadow effect and the ocean moderation effect.

Rain shadow effect

The rain shadow effect occurs when moist air masses encounter mountain ranges. As air rises over mountains, it cools, causing water vapor to condense and fall as precipitation. This often produces wet conditions on the windward side of mountains. Tall mountain ranges such as the Sierra Nevada and Cascade Mountains remove large amounts of moisture from air masses. As the now-dry air descends on the opposite side of the mountains, it creates dry environments known as rain shadows. This process contributes to the formation of deserts such as the Mojave Desert.

Ocean moderation effect

Water has a high heat capacity, meaning it can absorb and store large amounts of thermal energy. Coastal regions are therefore strongly influenced by nearby oceans. During summer, oceans absorb heat from the atmosphere and moderate temperatures on nearby land. During winter, oceans release stored heat, preventing coastal areas from becoming as cold as inland regions. As a result, coastal climates tend to be milder and less variable than continental climates.

Terrestrial biomes

Tropical rain forest

Tropical rainforests occur near the equator, where temperatures remain consistently warm throughout the year. Seasonal variation in temperature is minimal, although precipitation varies heavily between wet and dry seasons. Tropical rainforests receive abundant rainfall and contain the highest levels of biodiversity and biomass of any terrestrial biome. These forests are highly layered, often containing multiple canopy levels. Sunlight is abundant in the upper canopy but very limited at the forest floor.

Subtropical deserts

Subtropical deserts are commonly found near 30° north and south latitude, where descending dry air from Hadley cells creates arid conditions. Examples include the Sahara, Sonoran, and Australian deserts. These biomes are characterized by high temperatures, low precipitation, sparse vegetation, and low biomass. Plants are often widely spaced because competition for water is intense.

Temperate grasslands

Temperate grasslands experience warm summers, cold winters, and relatively low precipitation. Limited rainfall restricts tree growth, while frequent fires help maintain grass-dominated ecosystems. Without periodic fires, many grasslands would gradually transition into shrublands or forests.

Temperate forests

Temperate forests experience moderate temperatures and higher precipitation than grasslands. These forests usually have distinct seasons, including cold winters. They are dominated primarily by deciduous trees and contain moderate levels of biodiversity and productivity.

Taiga

The taiga, also called the boreal forest, lies north of the temperate forests. It is characterized by long, cold winters and short, cool summers. Although precipitation is relatively low, evaporation is also low because of cold temperatures. The taiga is dominated by coniferous gymnosperms such as pines, spruces, and firs. Biomass is high, but productivity is relatively low because trees grow slowly. Biodiversity is also relatively low because few species can tolerate the harsh conditions.

Tundra

The tundra occurs in Arctic regions where the ground is not permanently covered by ice. Temperatures are extremely low, precipitation is limited, and the growing season is very short. Only the upper layer of soil thaws during summer, while deeper layers remain permanently frozen as permafrost. Vegetation consists mainly of small shrubs, grasses, mosses, and lichens. Large herbivores such as reindeer depend heavily on lichens as a food source.

Aquatic biomes

Lakes and ponds

Lakes and ponds are standing bodies of freshwater that are not directly connected to oceans or seas. These ecosystems contain several distinct zones. The littoral zone is the shallow nearshore region where rooted aquatic plants grow because sunlight can reach the bottom. The limnetic zone is the open-water region that still receives enough sunlight for photosynthesis. Together, the littoral and limnetic zones make up the photic zone, the zone which there is enough light for photosynthesis. The benthic zone consists of the bottom sediments, where detritivores commonly feed on dead organic matter that sinks from upper layers.

Freshwater wetlands

Freshwater wetlands are shallow, water-saturated ecosystems characterized by emergent vegetation, which consists of plants that rise above the water surface. There are three major types of freshwater wetlands. Bogs are stagnant, acidic wetlands with little water flow. Marshes are dominated primarily by nonwoody plants such as grasses. Swamps are dominated by trees and shrubs, such as the cypress swamps of Florida.

Streams and rivers

Streams and rivers are freshwater ecosystems in which water flows continuously in one direction. Near the source, rivers are typically cold, narrow, and fast-moving, supporting relatively few organisms. As rivers approach their mouths, the water becomes warmer, wider, and slower-moving. These conditions allow more plant growth and support greater biodiversity.

Estuaries

Estuaries occur where rivers meet the ocean, creating ecosystems with a mixture of freshwater and saltwater. Because of tidal influence, estuaries contain brackish water with salinity levels that fluctuate over time. Salinity in estuaries depends on several factors, including tidal strength, freshwater input from rivers, and proximity to the ocean. During drought conditions, salinity increases because less freshwater enters the estuary. During periods of heavy rainfall, salinity decreases because increased freshwater dilutes the seawater.

Marine aquatic biomes

Intertidal zones

The intertidal zone is the marine ecosystem located between high tide and low tide along the shoreline. Organisms living there are periodically submerged underwater and then exposed to air. Intertidal zones differ from estuaries because estuaries contain brackish water formed from mixing freshwater and saltwater, while intertidal zones are fully marine. They also differ from coral reefs because coral reefs remain underwater in warm, stable environments, while intertidal zones experience constant exposure to waves, temperature changes, and drying conditions. Intertidal zones are physically harsh environments with strong wave action, changing salinity, and fluctuating temperatures. Common organisms include barnacles, mussels, sea stars, crabs, snails, algae, and sea anemones. Many organisms attach tightly to rocks or burrow into sand to avoid being washed away or drying out.

Coral reefs

Coral reefs are marine ecosystems built by coral animals and are restricted mainly to warm, shallow tropical oceans because corals require sunlight and warm water. Corals are colored because of symbiotic dinoflagellates called zooxanthellae that live inside their tissues. These algae perform photosynthesis and provide sugars to the coral, while the coral provides nutrients and protection in return. Coral reefs are considered the most diverse aquatic ecosystem in the world and support enormous numbers of fish, invertebrates, and other marine organisms. When corals lose their symbiotic algae due to stress, coral bleaching occurs and the reef turns white.

Pelagic zone

The pelagic zone is the open ocean away from shore and above the ocean floor. Near the surface, sunlight penetrates the water, allowing photosynthesis to occur. This upper region is called the photic zone and contains phytoplankton, zooplankton, fish, and many marine mammals. Large actively swimming organisms in the pelagic zone are called nekton. Examples include sharks, whales, dolphins, squid, sea turtles, and fish. Nekton can move independently through the water rather than drifting with currents.

Benthic zone

The benthic zone is the bottom region of oceans and other aquatic systems. Conditions are often dark, cold, and high pressure, especially in deep water. Organisms living there are called benthos and include sea stars, crabs, worms, clams, sponges, and bacteria. Many benthic organisms feed on detritus, which is dead organic material that sinks from the upper ocean layers. Some benthos attach to surfaces while others crawl or burrow into sediments.