Earth contains an astonishing diversity of ecosystems. The interdependence of these biomes sustain the planet as a whole.
How is Biodiversity Distributed Across the Earth?
Watch this video on geography and climate.
Because of the curvature of the Earth and the fact that it is tilted slightly on its axis relative to the sun, different regions of the planet receive different amounts of sunlight energy throughout the year. This impacts the length of warm, cold, wet, and dry seasons in these different regions, as well as the temperature, humidity, and other environmental factors that define the region. A biodiversity hotspot is a region containing an exceptional concentration of endemic species, but is threatened by human-induced loss of habitat. These hot spots support nearly 60% of the world’s plant, bird, mammal, reptile, and amphibian species. Many global organizations are working to conserve these biodiversity hotspots, such as the World Wildlife Foundation’s Global 200 and the Critical Ecosystem Partnership Fund. A relatively small number of countries (17) have less than 10% of the global surface, but support more than 70% of the biological diversity on earth. Countries rich in biological diversity and associated traditional knowledge belong to a group known as the Like Minded Megadiverse Countries (LMMC). Read more about LMMC.
Another consequence of the Earth's curvature and rotation is that the hydrologic cycle distributes water differently among these different regions. The result is striking differences in the global distribution of rain and snow (for an explanation of the Hydrologic Cycle, see the Natural Resources chapter). As a result, different regions on the planet have specific sets of environmental conditions, which results in differences in predominant vegetation. Species residing in different regions are characterized by specific adaptations that allow success under the particular set of environmental conditions of the region. Regions can be broadly divided into terrestrial biomes and aquatic ecosystems (Figures 7 & 8).
The terrestrial biomes can be divided into four broad categories: forest, desert, savanna/grassland, and tundra (Figures 7, 8 and 9).
Forest biomes can be divided into three distinct types based primarily on the types of organisms that populate them and seasonal changes in temperature and/or precipitation. These three types are tropical, temperate, and boreal (Figure 7a.). See examples of vegetation in Figure 9 e., f., and g., respectively.
In the upcoming Biodiversity and Action section you will learn about actions some farmers are taking to protect the Amazon rainforest from deforestation.
Tropical forests support the highest biodiversity of all biomes. They occur near the equator where 1) day lengths are long and vary little from 12 hours, 2) rainfall is higher than any other biome, and 3) temperatures are high, averaging around 20-25° C, with little seasonal variation (see Figures 7a & 9e).
Deforestation is a significant problem in this biome and is occurring rapidly for a number of reasons: logging of particular tree species, like teak and mahogany for fine furniture; clearing land for farming or cattle production; oil drilling and mining; and establishment of plantations such as those for palm oil or sugar cane. We saw an example of this in the Kakadu and the Mirrar case study that began this chapter.
Temperate forests (Figures 7a & 9f) are typically dominated by deciduous tree species, which lose their leaves every autumn. These forests also support pines, hemlocks, and other conifers. The location of temperate forests is in the mid-latitudes (between 30°N and 45°N and latitudes 30°S and 45°S). In these latitudes, forests experience four well-defined seasons. Precipitation (75-150 cm) is distributed evenly throughout the year.
Deforestation is a problem in this biome as well, with much of the primary, old-growth temperate forests cleared by humans for fuel wood, building materials, and as a source of wood pulp to make paper. As such, many of the land covered in deciduous forest in the United States and elsewhere today contain regrown, secondary forests.
Human-induced biome degradation by unsustainable deforestation violates the moral precept of environmental sustainability. This issue is discussed in the Biodiversity and Ethics section below.
Boreal forests, also called Taiga, are dominated by coniferous, cone-bearing trees. The needles on these trees remain green throughout the year. This biome (Figures 7a & 9g) covers extensive swaths of land between 50 - 60º north latitudes. In these latitudes, seasons are divided into short, and moderately warm summers and long, cold winters. Winter temperatures are very low, with snow contributing the most to annual precipitation, which is 40-100 cm annually.
Deserts (Figures 7a & 9c, d) cover about one fifth of the Earth’s land surface and occur where rainfall is less than 50 cm a year. These are the driest landscapes on Earth and support the least amount of life. Biodiversity is lowest in these biomes. Most deserts occur along latitudes of 30○N and 30○S and therefore have generally hot climates. These regions receive little precipitation due to atmospheric circulation patterns (Hadley Cells are explained in Chapter 2).
Drought is extending the desert in Northern Africa. In the upcoming Biodiversity and Action section you will learn about the action of the Great Green Wall Project to prevent further desertification.
Deserts can also occur at other latitudes and are produced in different ways. Rain shadow deserts are found on the leeward side of large mountain ranges (Figure 12). In these cases, warm moist air coming off oceans with the prevailing winds hits a mountain range and is deflected upwards. As the air rises, it cools and drops its moisture on the windward side of the range, creating cool, dry air after passing over the mountains; dry enough to form a desert region. Good examples of deserts formed by rain shadows are the Great Basin Desert on the leeward side of the Sierra Nevada mountain range in the western U.S., and the Patagonian Desert on the leeward side of the Andes in South America.
Polar deserts occur at the north and south poles where dry cold air prevails. Northern Greenland and non-ice covered areas of Antarctica are examples of polar deserts. Most deserts have a considerable amount of specialized vegetation as well as specialized vertebrate and invertebrate animals (Figure 13).
Because water is a precious resource, the leaves of many desert plants have evolved into spines to reduce water loss from larger leaves and protect their moisture-filled stems from being eaten by animals. In the case of the teddy-bear cholla cactus (Figure 7a), the spines serve additional roles of reflecting intense sunlight and trapping moisture. The Kit Fox, native to the Sonoran Desert in the U.S. Southwest, uses its large ears for evaporative cooling (Figure 13b).
Savanna & Grassland
Vegetation in both savanna and grassland biomes (Figures 7a & 9 a, b respectively), is dominated by perennial grasses and non-woody forbs. Savannas obtain enough rainwater annually to support scattered trees, whereas grasslands do not. Grasslands occur in temperate climates with hot summers and cold, snowy winters and have deep soil rich in organic matter.
Savannas are generally found in more tropical climates where seasonality is characterized not by temperature changes but by precipitation patterns. The abundant grasses of savannas and grasslands support large herds of herbivores, like the wildebeest found on the African savanna (Figure 14a) and the bison on the North American Great Plains (Figure 14b). Wildebeest, zebras, gazelles, and other large African mammals must migrate seasonally, tracking moisture. Because of human settlement, however, many migration routes are blocked by fences or other types of development. The American bison (Figure 14b) used to occur in vast herds across the U.S. grasslands but was nearly hunted to extinction in the 1800s. A concerted conservation effort has put this species back on track to recovery.
Because of the deep roots and quick growth of these grasses, the soils have become rich with organic carbon making them valuable for agriculture. Much of the planet’s natural grassland biome has been converted to farmland, which has caused a loss of these rich, valuable soils and a decrease in biodiversity.
Tundra (Figures 7a & 9h) is among the coldest biomes, with average winter temperatures of -34˚ C and summer temperatures between 3-12° C. The warmer growing season lasts only 50 – 60 days, but this is adequate to supply sustenance to its multitudes of migrating birds and caribou (Figure 15). Tundra soil is high in organic matter and lies atop permanently frozen soil called permafrost. With the increased temperatures induced by climate change, however, shallower layers of permafrost are beginning to thaw, allowing the organic content to decay, releasing methane (CH4) into the atmosphere that had been sequestered in organic form for millennia. This enormous release of methane from thawing tundra contributes considerably to greenhouse gas emissions (the Global Climate Change chapter explains how methane is a greenhouse gas 20X more potent than CO2).
Water is the common link among the aquatic ecosystems and it makes up the largest portion of the biosphere (Figure 7b). This is where life began billions of years ago. Without water, organisms would be unable to sustain themselves (see the role of water for life in Chapter 2). Aquatic ecosystems support highly diverse groups of organisms and are classified into two broad categories: freshwater and saltwater or marine.
Freshwater ecosystems are characterized by having a very low salt (NaCl) content (less than 0.5 parts salt per 1,000 parts H20, ppt) and include streams/rivers, groundwater, lakes, ponds, reservoirs, and wetlands (such as fens, marshes, swamps, and bogs). Each presents unique conditions to which different kinds of organisms are adapted. Life in flowing water (called lotic systems), for example, requires different adaptations than life in ponds, lakes, reservoirs, and wetlands (still water or lentic systems).
In the forthcoming Biodiversity and Action section you will learn what students have done in Joondalup, Australia to save the freshwater frog.
Because climatic conditions vary across different latitudes, the species diversity in freshwater aquatic ecosystems differs geographically. Like terrestrial biomes, aquatic ecosystems in the tropics support many more species than those in latitudes further from the equator. This is particularly true for fish and amphibians. The Amazon River, for example, which runs on or near the equator supports 2,000 to greater than 5,000 species of fish. This is a very high fish diversity. By contrast, the Mississippi River basin in the United States, which runs from approximately 45º N to 30º N latitude (from the headwaters to the mouth of the river), harbors only about 375 species of fish. Collectively, approximately 15,000 of the earth’s species of fish, nearly 45% of all fish species, rely on either fresh or brackish water habitats. The other 55% are marine species.
Microalgae (Figure 16 e. through h.), both benthic (living on the bottom) and planktonic (living in the water column), are the major primary producers in most aquatic ecosystems and thus serve as the base of the food chain. As with all organismal groups, the number and identity of algal species in a community can influence ecosystem processes at a much larger scale. 1When the base of a food chain is diverse, higher trophic levels tend to be diverse as well.
View a mural of some of the species of fish that live in the Amazon River.
Marine ecosystems contain salt that eroded from land and eventually washed into the oceans. The average ocean salinity is 35 ppt worldwide. Marine ecosystems cover about three-fourths of the Earth’s surface and include oceans, seas, coral reefs, and estuaries. Estuaries are wetlands at the oceans’ shore that contain a mix of freshwater from rivers and saltwater from the ocean to produce brackish water, characterized by possessing a salinity between 0.5 ppt and 17 ppt. Marine phytoplankton are critical to all life on Earth because they supply much of the world’s atmospheric oxygen and take in a huge amount of atmospheric carbon dioxide for photosynthesis, acting as a “sink” for the greenhouse gas CO2.
There are six distinct marine eco-regions. All of these, like terrestrial biomes, are characterized by specific flora and fauna (Figure 17).
Estuaries (Figure 17a) are formed at the mouths of freshwater streams or rivers flowing into the ocean. Depending on the elevation gradient of the land and the ratio of water flow from river to ocean versus intrusion from ocean to river, estuaries can range in salinity from 0.5 ppt to 17 ppt. This mixing of waters with such different salt and nutrient concentrations creates a very rich and unique ecosystem at the edge of two very different aquatic systems. The blending of two distinct systems at their border is called ecotone and is often a zone of high biodiversity because it harbors species from both systems. Estuaries have higher diversity and productivity than either the river or stream alone. Microflora like algae, and macroflora, such as seaweeds, marsh grasses, and mangrove trees (only in the tropics), can be found here. Estuaries support a diverse fauna, including a variety of worms, oysters, crabs, and waterfowl, and are often important nursery grounds for fish and important feeding stops for migratory birds.
B. Intertidal & Sub-Tidal Zones
Marine ecosystems along the coasts of land masses, but not influenced by infusion of freshwater like estuaries, include the intertidal and sub-tidal zones. Intertidal ecosystems are alternately exposed to the air and submerged as ocean tides wax and wane. Most species that live in this ecosystem are tolerant to and often thrive on periodic exposure to air (Figure 17b), like mussels, crabs, starfish, sea anemones, and seaweeds. Tide pools, small shoreline depressions that retain permanent water, can even support a diversity of fish.
Sub-tidal zones occur further offshore and are permanently submerged but still strongly influenced by tidal surges. Dense kelp forests (Figure 17c) or sea-grass beds can grow in these areas, serving as habitat for multitudes of fish, shrimp, and other marine organisms.
C. Coral Reefs, Sea Grass Beds, & Mangroves
Coral reefs are truly awe-inspiring natural formations. In the upcoming Biodiversity and Spirituality section you will explore more about this experience of awe in nature.
Coral reefs and adjacent sea-grass beds and mangrove forests (Figure 19) are of high economic and ecological value to tropical countries,2 but at the same time are very sensitive to environmental changes, both natural and anthropogenic.
Coral reefs, for example, act as barriers, protecting sea grasses and mangroves from oceanic swell and storms but are vulnerable to harm by tourists diving and collecting coral and by the aquarium industry that collects millions of colorful fish to sell on the market.3
Watch a video about Peru's vanishing fish.
Major groups of marine species on the International Union for Conservation of Nature (IUCN) Red List of threatened species include the following:
View the complete list of marine species on the IUCN Red List of threatened species.
- all the world’s known species of reef-building corals (845 species)
- sharks, rays and chimaera (1,046 species)
- groupers (161 species)
- seabirds (349 species)
- marine mammals, which include whales, dolphins, porpoises, seals, sea lions, walruses, sea otter, marine otter, manatees, dugong and the polar bear (134 species)
- marine turtles (7 species)
- seagrasses and mangrove.5
D. Pelagic Zone
The Pelagic Zone comprises all far off-shore 'open water' habitat extending from the ocean surface to the depth limits of light penetration. This zone supports the massive schools of planktivorous forage fish like anchovies, smelt, and sardines, which serve as the primary diet of salmon, swordfish, tuna, and many other larger fish (Figure 17e). Stocks of many of the large predatory fish are declining due to over fishing of both the forage fish and larger predatory fish themselves. Forage fish are subject to overfishing (Figure 21) in areas where they are used to produce feed for farmed fish or commercial production of pet food.
E. Abyssal Zone
The abyssal zone is the deepest region of oceans that lies below the pelagic zone, its upper limit at the depth where sunlight can no longer penetrate (Figure 28). Because these deep waters are in constant darkness, no photosynthetic organisms live there, yet a diversity of unique life still thrives comprising an unusually complex food web with bacteria, rather than microalgae, serving as the food web base.
In the complete darkness of the abyssal zone, predators, like the angler fish have developed evolutionary adaptations to allow them to capture prey. The angler fish uses a fluorescent lure extending off its head to attract prey (Figure 23).
The 'base of the food chain' or food that supports abyssal zone life comes from the bacteria that feeds on feces and the bodies of dead organisms raining down from the pelagic zone. In addition to these decomposer bacteria, other sea floor habitats promote life. At locations where molten magna emerges through the sea floor, creating new crust and pushing crustal plates apart, warm and nutrient rich water emerges from hydrothermal vents (Figure 22).
Watch this exciting documentaryabout scientists exploring the deepest sea vents on the planet, with its little known and highly diverse biodiversity.
These provide unique habitats for chemosynthetic bacteria to grow. Both decomposers and chemosynthetic bacteria provide a rich food source for a multitude of invertebrate fish species that are unique to the ocean’s abyss.
Questions to Consider
Imagine you were an astronaut circling the Earth, like the astronauts discussed in the Healing Earth Introduction. Which of the 13 terrestrial and aquatic biomes discussed above do you think you could see when looking down at the Earth?
- Check your answer with photographs from this Earth from Space website.