Key Concepts

After completing this chapter, you will be able to

1. Identify the major biomes and outline their characteristics.
2. Describe the differences between natural and anthropogenic ecosystems.
3. Identify/understand the ecoregions and ecozones of North America.

Biomes: Global Ecosystems

A biome is a geographically extensive type of ecosystem. A particular biome occurs wherever environmental conditions are suitable for its development, anywhere in the world. Biomes are characterized by the life forms of their dominant organisms, but not necessarily by their particular species. On land, biomes are generally identified by their mature or older-growth vegetation. In contrast, aquatic biomes are usually distinguished by their dominant animals. Biomes are classified using a system that is used at an international level—that is, by ecologists working in many countries.

Figure 8.1 shows a map of the distribution of the most extensive terrestrial biomes. The distribution of biomes is determined by environmental conditions, which must be appropriate to support the dominant species. Moisture and temperature are usually the most important environmental influences on the distribution of terrestrial biomes (Figure 8.2). The distribution of various types of wetlands within terrestrial biomes is mostly influenced by the amount and permanence of surface water and the availability of nutrients. Marine biomes are most strongly influenced by water depth and upwellings, which affect the amounts of light and nutrients that are available to support primary productivity.

Figure 8.1. Distribution of the Major Terrestrial Biomes. Note that the spatial complexity is greatest in regions with mountainous terrain, such as the western Americas and southern Asia. Source: “The main biomes of the world” by Ville Koistinen is licensed under CC BY-SA 3.0.

Figure 8.2. Environmental Influences on the Distribution of Terrestrial Biomes. This diagram suggests the reasons why temperature and moisture are believed to be the most important environmental factors affecting the distributions of terrestrial biomes. Source: “Climate influence on terrestrial biome” by Navarras is licensed under CC0 1.0.

As long as environmental conditions are suitable for its development, a particular biome may occur in widely divergent regions, even on different continents. Although widely separate regions of a biome may be dominated by different species, their life forms are typically convergent. In other words, the different species are comparable in their form and function, because the regimes of natural selection occurring in similar environments result in parallel (or convergent) evolutionary responses. Therefore, biomes are defined primarily by the structure and function of their ecosystem, but not necessarily their species composition.

This context is illustrated by the boreal forest, an extensive biome that occurs in northern regions of Canada, Alaska, and Eurasia. The boreal forest occurs in regions with a cold and long winter, short but warm summer, and generally moist soil. This biome is situated between the more northern tundra, and temperate forest or prairie to the south. The dominant vegetation of boreal forest is typically coniferous trees, especially species of fir, larch, pine, or spruce. However, the particular species vary from region to region, and angiosperm (hardwood) trees may also be present.

Over much of northern Canada the boreal coniferous forest is dominated by stands of black spruce (Picea mariana). However, in some regions, stands of white spruce (Picea glauca), jack pine (Pinus banksiana), balsam fir (Abies balsamea), or tamarack (Larix laricina) are dominant. In the boreal forest of northern Europe, Siberia, and northern parts of Japan, Korea, and China, there are other species of coniferous trees. In some cases, there may be stands dominated by hardwood trees, such as trembling aspen (Populus tremuloides) in parts of northern Canada. However, all of these different forest types occurring on several continents are structurally and functionally convergent ecosystems within the same biome—the coniferous forest.

We should also note that any particular biome is described on the basis of its dominant, most extensive kind of ecological communities. For the boreal forest, this is usually stands of coniferous trees. However, biomes are not homogeneous, and they contain other kinds of less-widespread communities. For instance, parts of the boreal forest are dominated by persistent areas of shrubs such as species of alder, dwarf birch, and willow, and there may also be wetlands, such as bogs and fens as well as distinctive communities associated with streams and rivers.

In addition, local areas may be subjected to occasional catastrophic disturbances, which may result in a landscape being composed of a mosaic of stands of various stages (and ages) of ecological recovery, called succession. In the case of boreal forest, disturbances are typically caused by wildfire or by epidemics of insects that kill trees (see Chapter 26).

The Major Biomes

Natural biomes are characterized by their dominant ecological communities, which are composed of particular assemblages of plants, animals, and microorganisms. There are also anthropogenic ecosystems that are strongly influenced by humans and their activities, such as cities and agricultural land. In fact, all of the modern biomes have been influenced by people to some degree—at the very least, all organisms in even the most remote places now contain trace contaminations of organochlorine chemicals (such as DDT and PCBs) that humans have manufactured and dispersed into the environment (see Chapter 26).

Ecologists have used a number of systems to divide the biosphere into major biomes, one of which is illustrated in Figure 8.1. The classification of global biomes described here is modified from a system proposed by the ecologist E.P. Odum. In the following sections, the world’s biomes are examined within global and ecoregional contexts. This is appropriate because biomes are widespread ecological units whose boundaries and species do not respect political boundaries.

Terrestrial Biomes

Tundra is a treeless biome that occurs in environments with a long, cold winter and a short, cool growing season (Image 8.1; Figure 8.3). There are two types of tundra: alpine and arctic. Alpine tundra occurs at higher elevations in mountainous regions, even in tropical countries. Arctic tundra occurs at high latitudes—that is, in northern regions of the Northern Hemisphere and southern parts of the Southern Hemisphere. Most tundra ecosystems are a meteorological desert because they receive sparse precipitation. Nevertheless, the soil may be moist or wet because the cold environment restricts the amount of evaporation that occurs, and frozen soil may prevent deep drainage of water. The coldest, most northerly, high-Arctic tundra is extremely unproductive and dominated by short, long-lived plants, generally growing less than 5-10 cm above the surface. In the less-cold environments of the lower Arctic, well-drained tundra may be dominated by shrubs growing as tall as 1-2 m, while wetter habitats support productive meadows of sedge, cottongrass, and grass.

Image 8.1. Tundra. Tundra is a biome of short vegetation growing in climatically stressed environments of the Arctic, Antarctic, and on mountaintops. This is a view of the Alaskan tundra where on the vegetated beach ridges Muskox and Greater White-fronted Geese graze in front of the Igichuk Hills. Source:  “Muskox and Geese” by Western Arctic National Parklands is licensed under CC BY 2.0.

Figure 8.3. Climate Graphs. A climograph is a graphical representation of a location’s climate and display data for two variables: (a) monthly average temperature and (b) monthly average precipitation. Different biomes display distinct climographs, which explains the observed differences in p[lant community composition and structure. Source: NASA.

Boreal coniferous forest, or taiga, is an extensive biome of environments with a cold winter, short but warm growing season, and moist soil (Image 8.2; Figure 8.3). It is most extensive in the Northern Hemisphere. The boreal forest is dominated by coniferous trees, especially species of fir, larch, pine, and spruce. Some angiosperm trees may also be prominent, particularly aspen, birch, and poplar. Stands of boreal forest are poor in tree species, and may be dominated by only one or a few kinds. Most regions of boreal forest are subject to periodic disturbances, usually by wildfire, but sometimes by windstorms or insect epidemics.

Image 8.2. Taiga. The boreal coniferous forest (taiga) is extensive in subarctic regions of Canada, Alaska, and Eurasia. This photo shows a stand of black spruce. Source:  “Taiga Forest” by K. Adams is licensed under CC BY 2.0.

Montane forest occurs at sub-alpine altitudes on mountains in temperate latitudes. It is similar in structure to high-latitude boreal forest and is also dominated by conifers.

Temperate deciduous forest occurs in relatively moist, temperate climates with a short, moderately cold winter and warm summer (Image 8.3; Figure 8.3). This biome is dominated by a mixture of hardwood tree species. Most of the trees have seasonally deciduous foliage, meaning their leaves are shed each autumn and then regrown in the springtime. This is an adaptation to surviving the drought and other stresses of winter. Common trees of temperate deciduous forest in North America are species of ash, basswood, birch, cherry, chestnut, dogwood, elm, hickory, magnolia, maple, oak, sassafras, tulip-tree, and walnut. These trees occur in distinctive communities based on their preferences for particular qualities of soil moisture and fertility, soil and air temperature, and other environmental factors.

Image 8.3. Deciduous Forest. The temperate deciduous forest contains species of angiosperm trees, which drop their leaves in the autumn, plus some coniferous trees. This biome is widespread south of the boreal forest and is the most prominent biome of the Northeast United States. This stand in Binghamton University’s Nature Preserve is dominated by beech, maples, and hemlock. Source:“Binghamton University Nature Preserve” by JJEFFREY TSAI is licensed under CC BY-NC-SA 2.0.

Temperate rainforest develops in a climate in which the winter is mild and precipitation abundant year-round. Because this climate is too moist to allow frequent wildfires, old-growth forest often develops. The old-growth forest is dominated by coniferous trees of mixed age and species composition, but some individual trees are extremely large and can be centuries old, sometimes even exceeding a thousand years. Prominent tree species in temperate rainforest of the humid west coast of North America are Douglas-fir, hemlock, red cedar, redwood, Sitka spruce, and yellow cypress.

Temperate grassland occurs in temperate regions where the annual precipitation is 25-60 cm/y (Image 8.4; Figure 8.3). Under these conditions, soil moisture is adequate to prevent desert from developing, but insufficient to support forest. Temperate grassland is called prairie in North America and steppe in Eurasia, and this biome occupies vast regions in the interiors of both continents. Prairie is commonly divided into three types according to the height of the dominant vegetation: tall-grass, mixed-grass, and short-grass. Tall-grass prairie is dominated by various grasses and herbaceous angiosperm plants, such as blazing stars and sunflowers, some as tall as 2-3 m. Fire is an important factor that prevents tall-grass prairie from developing into an open forest. Tall-grass prairie is a critically endangered ecosystem because almost all of it has been converted into agricultural land. Mixed-grass prairie occurs where there is less rainfall and the habitat is characterized by shorter species of grasses and herbaceous angiosperms. Short-grass prairie develops where precipitation is even less, and it can be subject to unpredictable, severe drought.

Image 8.4. Grassland. Temperate grassland is widespread in the dry interior of North America and other continents, and is dominated by species of grasses and other herbaceous plants. Source: “Buffalo Gap National Grassland” by Encyclopædia Britannica.

Chaparral develops in south-temperate environments with a so-called Mediterranean climate, with winter rains and summer drought. Typical chaparral is characterized by dwarfed trees and shrubs with interspersed herbaceous vegetation. Periodic fires are characteristic. In North America, chaparral is best developed in coastal southern California.

Desert can be temperate or tropical, and it most commonly occurs in continental interiors or in the rain shadow of mountains (Image 8.5; Figure 8.3). The distribution of desert is determined by the amount of soil moisture, which in the temperate zones is generally associated with an annual precipitation of less than about 25 cm. The driest desert supports almost no plant productivity, but less-dry conditions may support communities of herbaceous and succulent plants, both annual and perennial. Occasional moist places with springs of groundwater develop a relatively lush vegetation of shrubs or trees and are known as oases.

Image 8.5. Desert. Desert is a sparsely vegetated biome of extremely dry environments. This view is of arid habitat in California, Death Valley National Park has sand dunes and is the lowest elevation in the United States and one of the hottest places on Earth. Source: “Sand Dunes in Death Valley National Park” by Brocken Inaglory is licensed under CC BY-SA 4.0.

Tropical grassland and savanna occur in regions with as much as 120 cm of annual rainfall, but a pronounced dry season. Savanna is dominated by grasses and herbaceous angiosperms, with scattered shrubs and tree-sized plants that provide an open canopy. Some tropical grasslands and savannas support large populations of big animals, including migratory ones. This is particularly true of Africa, where this biome supports a diverse community of large mammals, such as elephant, gazelle and other antelopes, hippopotamus, rhinoceros, water buffalo, and predators of these herbivores, such as cheetah, hyena, leopard, lion, and wild dog.

Semi-evergreen tropical forest develops in a warm climate with pronounced wet and dry seasons. Most trees and shrubs are seasonally deciduous, shedding their foliage in anticipation of the dry season. This biome supports a great richness of biodiversity, though less than tropical rainforest.

Evergreen tropical rainforest occurs in tropical climates with copious precipitation throughout the year. Tropical rainforest often develops into an old-growth condition because wildfire and other catastrophes are uncommon. Old-growth tropical rainforest supports a tremendous richness of tree species of many sizes and ages, most of which retain their foliage throughout the year. This forest also sustains an extraordinary diversity of other plants, animals, and microorganisms. Tropical rainforest represents the peak of development of terrestrial ecosystems because the biome supports huge biomass, high productivity, and rich biodiversity under relatively benign climatic conditions. Within the continental U.S., it can be found in southern Florida, but can also be observed in Hawaii, as well as U.S. territories Puerto Rico, Guam, and American Samoa.

Image 8.6. Tropical Rainforest. Tropical rainforests are among the most productive ecosystems in the world, and home to more than half of the world’s biodiversity. Source: “Tropical Rainforest – West Indies” by pali_nalu is licensed under CC BY-NC 2.0.

Aquatic Biomes

Like terrestrial biomes, aquatic biomes are influenced by a series of abiotic factors. The aquatic medium—water— has different physical and chemical properties than air. Even if the water in a pond or other body of water is perfectly clear (there are no suspended particles), water still absorbs light. As one descends into a deep body of water, there will eventually be a depth which the sunlight cannot reach. While there are some abiotic and biotic factors in a terrestrial ecosystem that might obscure light (like fog, dust, or insect swarms), usually these are not permanent features of the environment. The importance of light in aquatic biomes is central to the communities of organisms found in both freshwater and marine ecosystems. In freshwater systems, stratification due to differences in density is perhaps the most critical abiotic factor and is related to the energy aspects of light. The thermal properties of water (rates of heating and cooling) are significant to the function of marine systems and have major impacts on global climate and weather patterns. Marine systems are also influenced by large-scale physical water movements, such as currents; these are less important in most freshwater lakes.

Freshwater Biomes

Freshwater biomes include lakes and ponds (standing water) as well as rivers and streams (flowing water). They also include wetlands, which will be discussed later. Humans rely on freshwater biomes to provide aquatic resources for drinking water, crop irrigation, sanitation, and industry. These various roles and human benefits are referred to as ecosystem services. Lakes and ponds are found in terrestrial landscapes and are, therefore, connected with abiotic and biotic factors influencing these terrestrial biomes.

Lakes and Ponds

Lentic ecosystems contain standing or very slowly flowing water, as occurs in lakes and ponds. The ecological character of lentic systems is most strongly influenced by water chemistry, particularly its transparency and nutrient concentration. Waters that are well supplied with nutrients are highly productive (eutrophic), while infertile waters are unproductive (oligotrophic). In general, shallow waterbodies are much more productive than deeper ones of a comparable surface area. However, water bodies with poor transparency are much less productive than might be predicted on the basis of their nutrient supply. Waters that are brown-colored because of dissolved organic matter have poor transparency, as do turbid waters with fine suspended particulates. Lentic ecosystems are characterized by zonation in two dimensions. Horizontal zonation is due to changes in water depth and is usually related to the slope and length of the shore. Vertical zonation occurs in deeper water and is related to the amount of light, water temperature, and nutrient and oxygen concentrations. Lentic ecosystems often develop distinct communities along their shore (known as the littoral zone), in their deeper open water (the well-lit, upper limnetic zone, and the deeper, darker profundal zone), and on their sediment (the benthic zone; Figure 8.3).

Figure 8.3. Lentic Ecosystem Zonation. Freshwater lakes exhibit horizontal and vertical zonation as environmental conditions change with distance away from the shore and depth. Source: Encyclopædia Britannica.

Rivers and Streams

Lotic ecosystems are characterized by flowing water and include rivers and streams. The quantity, velocity, and seasonal variation of water flow are important environmental factors. Within streams or rivers, silt-sized particles are deposited in places with relatively calm water, leaving a fine-grained or muddy substrate. In contrast, the substrate of places with vigorous water flow is rocky because fine particles have been eroded away. For similar reasons, the turbidity is greatest during times of high water flow. Turbidity is an important factor because it interferes with light penetration and thereby restricts primary productivity. Lotic ecosystems sustain some productivity of algae and aquatic plants, but usually their primary production is not large. Most of the productivity of aquatic invertebrates and fish in lotic ecosystems is sustained by inputs of organic matter from upstream lakes and from the terrestrial watershed in the form of plant debris.

Abiotic features of rivers and streams vary along the length of the river or stream. Streams begin at a point of origin referred to as source water. The source water is usually cold, low in nutrients, and clear. The channel (the width of the river or stream) is narrower than at any other place along the length of the river or stream. Because of this, the current is often faster here than at any other point of the river or stream.

The fast-moving water results in minimal silt accumulation at the bottom of the river or stream, therefore the water is clear. Photosynthesis here is mostly attributed to algae that are growing on rocks; the swift current inhibits the growth of phytoplankton. An additional input of energy can come from leaves or other organic material that falls into the river or stream from trees and other plants that border the water. When the leaves decompose, the organic material and nutrients in the leaves are returned to the water. Plants and animals have adapted to this fast-moving water. For instance, leeches (phylum Annelida) have elongated bodies and suckers on both ends. These suckers attach to the substrate, keeping the leech anchored in place. Freshwater trout species (phylum Chordata) are an important predator in these fast-moving rivers and streams.

As the river or stream flows away from the source, the width of the channel gradually widens and the current slows. This slow-moving water, caused by the gradient decrease and the volume increase as tributaries unite, has more sedimentation. Phytoplankton can also be suspended in slow-moving water. Therefore, the water will not be as clear as it is near the source. The water is also warmer. Worms (phylum Annelida) and insects (phylum Arthropoda) can be found burrowing into the mud. The higher order predator vertebrates (phylum Chordata) include waterfowl, frogs, and fishes. These predators must find food in these slow moving, sometimes murky, waters and, unlike the trout in the waters at the source, these vertebrates may not be able to use vision as their primary sense to find food. Instead, they are more likely to use taste or chemical cues to find prey.

Wetlands

Wetlands occur in shallow, flooded places on land. There are four major types: marsh, swamp, bog, and fen. Marshes are the most productive; they are dominated by plants that are rooted in sediment but grow as tall as several meters above the water surface, such as reed, cattail, and bulrush (Image 8.7). Open-water areas of marshes have floating-leaved plants, such as water lily and lotus. Swamps are forested wetlands that may be flooded seasonally or permanently. Swamps are often dominated by such trees as silver maple (Acer saccharinum), white elm (Ulmus americana), or bald cypress (Taxodium distichum). Bogs are acidic, relatively unproductive wetlands that develop in a cool, wet climate. Their supply of nutrients is sparse because bogs are fertilized only by atmospheric inputs of dust and chemicals dissolved in precipitation. Bogs are typically dominated by species of Sphagnum moss (also known as peat moss). Fens also develop in a cool and wet climate, but since they have a better nutrient supply than bogs, they are less acidic and more productive.

Image 8.7. Wetland. A marsh is a fertile wetland dominated by taller herbaceous plants, such as bulrush and cattail. This image depicts the western marsh of Binghamton University’s Nature Preserve in Vestal, NY, where in addition to cattails, invasive Phragmites has begun to take over. Source: Michael-Luca Natt

 

Image 8.8. Swamp. A swamp is a forested wetland. This example is dominated by Bald Cypress (Taxodium distichum) in Okefenokee Swamp and National Wildlife Refuge, Georgia. Source: “Swamp” by JamesDeMers is licensed under CC0 1.0.

Marine Biomes

The ocean is the largest marine biome. It is a continuous body of salt water that is relatively uniform in chemical composition; it is a weak solution of mineral salts and decayed biological matter. Within the ocean, coral reefs are a second kind of marine biome. Estuaries, coastal areas where salt water and fresh water mix, form a third unique marine biome. The ocean is categorized by several areas or zones (Figure 8.4). All of the ocean’s open water is referred to as the pelagic zone. The benthic zone extends along the ocean bottom from the shoreline to the deepest parts of the ocean floor. Within the pelagic realm is the photic zone, which is the portion of the ocean that light can penetrate (approximately 200 m or 650 ft). At depths greater than 200 m, light cannot penetrate; thus, this is referred to as the aphotic zone. The majority of the ocean is aphotic and lacks sufficient light for photosynthesis. The deepest part of the ocean, the Challenger Deep (in the Mariana Trench, located in the western Pacific Ocean), is about 11,000 m (about 6.8 mi) deep. To give some perspective on the depth of this trench, the ocean is, on average, 4267 m.

Ocean

The physical diversity of the ocean is a significant influence on plants, animals, and other organisms. The ocean is categorized into different zones based on how far light reaches into the water. Each zone has a distinct group of species adapted to the biotic and abiotic conditions particular to that zone. The open ocean consists of pelagic and benthic ecosystems (Figure 8.4). The pelagic (open-water) ecosystem is strongly influenced by physical and chemical factors, particularly waves, tides, currents, salinity, temperature, light intensity, and nutrient concentration. The rate of productivity is small, and comparable to that of terrestrial desert. The primary production is associated with phytoplankton, which range in size from extremely small photosynthetic bacteria to larger (but still microscopic) unicellular and colonial algae. The phytoplankton are grazed by tiny animals known as zooplankton (most of which are crustaceans), which are eaten in turn by larger zooplankton and small fish. Large predators such as bluefin tuna, sharks, squid, and whales are at the top of the pelagic food web. The benthic ecosystem of the open ocean biome is supported by a sparse rain of dead biomass from the surface. The benthic ecosystem of the deep oceans is not yet well described, but it appears to be somewhat rich in species, low in productivity, and extremely stable over time. Some large regions of the open ocean have an enormous rotating surface current known as a gyre, which is caused by the Coriolis effect associated with the rotation of Earth. Gyres in the Northern Hemisphere rotate in a clockwise direction, while those in the Southern Hemisphere are counter-clockwise. Gyres collect floating material, such as floating seaweeds like Sargassum, as well as garbage from coastal dumping and debris from fishing fleets. One example is the North Pacific gyre, which covers most of that oceanic basin, and another is the North Atlantic gyre, also known as the Sargasso Sea.

The intertidal zone, which is the zone between high and low tide, is the oceanic region that is closest to land. Generally, most people think of this portion of the ocean as a sandy beach. In some cases, the intertidal zone is indeed a sandy beach, but it can also be rocky or muddy. Organisms are exposed to air and sunlight at low tide and are underwater most of the time, especially during high tide. Therefore, living things that thrive in the intertidal zone are adapted to being dry for long periods of time. The shore of the intertidal zone is also repeatedly struck by waves, and the organisms found there are adapted to withstand damage from the pounding action of the waves. The exoskeletons of shoreline crustaceans (such as the shore crab, Carcinus maenas) are tough and protect them from desiccation (drying out) and wave damage. Another consequence of the pounding waves is that few algae and plants establish themselves in the constantly moving rocks, sand, or mud.

The neritic zone (Figure 8.4) extends from the intertidal zone to depths of about 200 m (or 650 ft) at the edge of the continental shelf. Because light can penetrate this depth, photosynthesis can occur. The water here contains silt and is well-oxygenated, low in pressure, and stable in temperature. Phytoplankton and floating Sargassum (a type of free-floating marine seaweed) provide a habitat for some sea life found in the neritic zone. Zooplankton, protists, small fishes, and shrimp are found in the neritic zone and are the base of the food chain for most of the world’s fisheries.

Beyond the neritic zone is the open ocean area known as the oceanic zone (Figure 8.4). Within the oceanic zone there is thermal stratification where warm and cold waters mix because of ocean currents. Abundant plankton serve as the base of the food chain for larger animals such as whales and dolphins. Nutrients are scarce and this is a relatively less productive part of the marine biome. When photosynthetic organisms and the protists and animals that feed on them die, their bodies fall to the bottom of the ocean where they remain. The majority of organisms in the aphotic zone include sea cucumbers (phylum Echinodermata) and other organisms that survive on the nutrients contained in the dead bodies of organisms in the photic zone.

The deepest part of the ocean is the abyssal zone, which is at depths of 4000 m or greater. The abyssal zone (Figure 8.4) is very cold and has very high pressure, high oxygen content, and low nutrient content. There are a variety of invertebrates and fishes found in this zone, but the abyssal zone does not have plants because of the lack of light. Cracks in the Earth’s crust called hydrothermal vents are found primarily in the abyssal zone. Around these vents chemosynthetic bacteria utilize the hydrogen sulfide and other minerals emitted as an energy source and serve as the base of the food chain found in the abyssal zone.

Beneath the water is the benthic zone (Figure 8.4), which is comprised of sand, silt, and dead organisms. This is a nutrient-rich portion of the ocean because of the dead organisms that fall from the upper layers of the ocean. Because of this high level of nutrients, a diversity of sponges, sea anemones, marine worms, sea stars, fishes, and bacteria exist.

Figure 8.4. Marine Zones. Ocean also display horizontal and vertical zonation, though the depths of the deepest places far exceed those of freshwater lakes. Source: “ocean zonation” by Encyclopædia Britannica.

Continental Shelf

Continental shelf waters occur near the shores of continents and are relatively shallow because they overlie an underwater projection of the landmass (a continental shelf). Compared with the open ocean, these nearshore waters are relatively warm and well supplied with nutrients. The nutrients come from inputs of rivers and from deeper, relatively fertile oceanic water that is occasionally stirred from the bottom by currents or turbulence caused by windstorms. Because the nutrient supply of coastal waters is relatively high, phytoplankton are productive and support a higher biomass of animals than occurs in the open ocean (Image 8.9). Some of the world’s most important fisheries are supported by the continental shelf biome – for example, those on the Grand Banks and other shallow waters of northeastern North America, in the nearshore waters of western North and South America, and in the Gulf of Mexico.

Image 8.9. Kelp Forest. The Pacific continental shelf waters are rich in marine life. Pictured here is a kelp forest (Nereocystis spp.) in La Jolla, California. They provide critical habitat for many animals, such as Sea Otters. Source: “Kelp Forest” by California Sea Grant is licensed under CC BY 2.0.

Regions with persistent upwelling occur where local oceanographic conditions favor the upwelling of relatively deep, nutrient-rich water to the surface (Figure 8.5). The increased nutrient supply allows these areas to sustain high rates of primary productivity. This ecological foundation supports large populations of animals, including big fish, sharks, marine mammals, and seabirds. Some of the most productive fisheries occur in upwelling areas, such as those off the west coast of South America and extensive regions of the Antarctic Ocean.

Figure 8.5. Upwelling. Surface winds displace relatively warm surface water, allowing cold, nutrient rich deeper water to rise to the surface, allowing for primary producers and their consumers to access nutrients and energy. Source: NOAA.

Estuaries

Estuaries are biomes that occur where a source of fresh water, such as a river, meets the ocean. Therefore, both fresh water and salt water are found in the same vicinity; mixing results in a diluted (brackish) saltwater. Estuaries form protected areas where many of the young offspring of crustaceans, mollusks, and fish begin their lives. Salinity is a very important factor that influences the organisms and the adaptations of the organisms found in estuaries. The salinity of estuaries varies and is based on the rate of flow of its freshwater sources. Once or twice a day, high tides bring salt water into the estuary. Low tides occurring at the same frequency reverse the current of salt water.

The short-term and rapid variation in salinity due to the mixing of fresh water and salt water is a difficult physiological challenge for the plants and animals that inhabit estuaries. Many estuarine plant species are halophytes: plants that can tolerate salty conditions. Halophytic plants are adapted to deal with the salinity resulting from saltwater on their roots or from sea spray. In some halophytes, filters in the roots remove the salt from the water that the plant absorbs. Other plants are able to pump oxygen into their roots. Animals, such as mussels and clams (phylum Mollusca), have developed behavioral adaptations that expend a lot of energy to function in this rapidly changing environment. When these animals are exposed to low salinity, they stop feeding, close their shells, and switch from aerobic respiration (in which they use gills) to anaerobic respiration (a process that does not require oxygen). When high tide returns to the estuary, the salinity and oxygen content of the water increases, and these animals open their shells, begin feeding, and return to aerobic respiration.

Seashore

Seashores are an interface of terrestrial and oceanic biomes and they support a complex of coastal ecosystems. The seashore biome is locally influenced by physical environmental factors, especially bottom type, the intensity of wave action, and the frequency of major disturbances such as storms. Hard-rock and cobblestone bottoms in temperate regions usually develop communities dominated by large species of seaweeds or kelp. These are productive ecosystems and can maintain large amounts of algal biomass. Areas with softer bottoms of sand or mud develop communities supported by the productivity of benthic algae and inputs of organic detritus from elsewhere. These soft-bottom ecosystems are usually dominated by invertebrates, especially mollusks, echinoderms, crustaceans, and marine worms.

Human-Dominated Ecosystems

Immense areas that were once occupied by natural habitats have been converted into land-uses that serve the human economy in various ways. These human-dominated ecosystems are anthropogenic in the sense that their characteristics are a consequence of environmental conditions associated with the activities of people. The character of these ecosystems may be an intended result of management practices, as is the case of agroecosystems in which crops are grown, or horticultural ecosystems where the intent is more aesthetic. Less-deliberate anthropogenic influences, such as pollution and disturbance, also affect the character of human-dominated ecosystems, often by causing ecological damage.

Of course, human-dominated ecosystems are prevalent wherever people are living in dense populations, such as in cities and towns. But they are also widely prevalent in the countryside where resource-extraction industries such as forestry and mining are important, and in transportation corridors associated with highways and electricity-transmission lines. Because anthropogenic ecosystems are becoming so widespread, and they support relatively few native species, they are the leading cause of the biodiversity crisis, which is characterized by the extinction and endangerment of native species and even of kinds of natural ecosystems (see Chapter 14). There is a great diversity of human-dominated ecosystems, but they can be aggregated into three major categories: urban-industrial techno-ecosystems, rural techno-ecosystems, and agroecosystems.

Urban-industrial techno-ecosystems are typical of urbanized areas and are dominated by the dwellings, businesses, factories, and other infrastructure of society (see Chapter 28; Image 8.11). This anthropogenic biome supports many species in addition to humans, but they are mostly alien plants and animals that have been introduced from other regions. Typically, the non-native species cannot live locally outside this biome (other than the foreign biome to which they are indigenous).

Rural techno-ecosystems occur outside of urbanized areas and consist of the extensive technological infrastructure of civilization. These ecosystems include rural transportation corridors (highways, railways, and electricity-transmission corridors) as well as small towns supporting industries involved in the extraction and processing of natural resources. Rural techno-ecosystems support a blend of introduced species, plus those native species that are tolerant of stresses associated with human activities.

Agroecosystems are a complex of habitats that are managed to grow crops for use by humans. The most intensively managed kinds involve monocultures (single-species crops) of plants or animals that are cultivated in agriculture, aquaculture, or forestry. These valuable and necessary crops are grown under conditions that enhance their productivity, although intensive management systems may cause many ecological problems (see Chapter 13). Less-intensively managed agroecosystems may involve the cultivation of mixtures of species (polycultures), and they may provide habitat for some native species. Semi-natural habitats used for the grazing of livestock also support some indigenous biodiversity. When an agroecosystem is abandoned, it slowly reverts to a more natural condition, although it can take many decades before there are ecological communities that are similar to what was originally present, especially in forested regions.

Image 8.11. Urban Sprawl. Brooklyn and Manhattan of New York City depicts urban sprawl, an ecosystem created of human influence and dense settlement patterns. Such cities are examples of urban-industrial techno-ecosystems. There is minimal natural land and porous surfaces, relying on irrigation and infrastructure. Source:  “Aerial View of New York City” by Tom Thai is licensed under CC BY 2.0.

Ecozones and Ecoregions

As we have learned, biomes are geographically extensive ecosystems that occur anywhere in the world where environmental conditions are suitable for their development, and they are characterized by the life forms of their dominant organisms rather than by their particular species. Learning about biomes is important because it provides insight into the character and environmental influences on major kinds of ecosystems.

Nevertheless, in the practical context of identifying and conserving the species and natural ecosystems of the world, there are limitations to the concept of biomes, mostly because of the non-specificity of their biotic assemblages. If the biodiversity of the world is to be conserved, we need to understand how species are naturally aggregated into communities and larger ecosystems, and how these biotic assemblages are distributed over space and time—there must be enough biogeographic resolution (identification of distinct communities) to conserve the intricate fabric of life on Earth, and biomes do not provide this kind of information.

This problem is dealt with by identifying and mapping extensive units known as ecozones (or ecoregions). These units are large landscapes or seascapes (ecoscapes) that contain distinct groupings of naturally assembled species and their communities. Like biomes, their spatial boundaries reflect conditions that existed prior to major changes in land-use caused by anthropogenic influences. The distribution of terrestrial ecoregions of the world has been mapped by Olson et al. (2001) and is presented in Figure 8.6. Note that the identity and distribution of the freshwater and marine ecoregions of the world must also be known for the purposes of conservation.

Figure 8.6. Terrestrial Ecoregions of the World. This map recognizes 867 terrestrial ecoregions, with the greatest amount of diversity occurring in humid tropical realms. Note that this map only covers terrestrial environments—comparable results for freshwater and marine ecoregions are also needed for effective conservation planning, but are not yet available. Source: Modified from Olson et al. (2001).

An ecozone is the broadest biogeographic division of the Earth’s land surface, based on distributional patterns of terrestrial organisms. Ecozones delineate large areas of the Earth’s surface within which organisms have been evolving in relative isolation over long periods of time, separated from one another by geographic features, such as oceans, broad deserts, or high mountain ranges, that constitute barriers to migration. As such, ecozone designations are used to indicate general groupings of organisms based on their shared biogeography. Ecozones correspond to the floristic kingdoms of botany or zoogeographic regions of zoology. Ecozones are characterized by the evolutionary history of the organisms they contain. They are distinct from biomes, also known as major habitat types, which are divisions of the Earth’s surface based on life form, or the adaptation of plants and animals to climatic, soil, and other conditions. Biomes are characterized by similar climax vegetation. Each ecozone may include a number of different biomes. A tropical moist broadleaf forest in Central America, for example, may be similar to one in New Guinea in its vegetation type and structure, climate, soils, etc., but these forests are inhabited by plants and animals with very different evolutionary histories.

Terrestrial Ecoregions of North America

The ecosystems found in North America have been described in various ways, including a hierarchical classification of distinctive types. Ecoregions are sub-ecozone units, and are characterized by regional factors related to climate and landform and, to some degree, by soil, vegetation, fauna, and land-use. Of course, the boundaries of biomes and ecoregions rarely align with political borders. Consequently, all of the southern ecozones of Canada also extend into the U.S. Figure 8.6 shows the results of a collaborative ecosystem-mapping study that involved scientists from Canada, Mexico, and the U.S. (CEC, 1997). This map shows the distribution of the 15 level I ecological regions of North America (these are equivalent in scale and qualities to biomes). In addition, these level I ecoregions are further divided into 50 level II ecological regions (intended to provide a more detailed description of the large ecological areas nested within the level I regions), and 182 level III ecoregions (smaller ecological areas nested within level II regions; Figure 8.7). Because countries share ecoregions, they also have a mutual responsibility to steward their ecological values. Sometimes, this can lead to conflict if one country believes the other is damaging shared resources or natural ecosystems. For example, Canada and the U.S. (or particular provinces or states) have ongoing arguments related to such binational issues as the following:

• The effects of raw sewage discharged by the city of Victoria, BC, may be damaging water quality in nearby U.S. waters in Juan de Fuca Strait.
• During seasonal times of high water levels, some of the volume of Devil’s Lake, North Dakota, is released into the Sheyenne River, a tributary of the Red River that runs north into Manitoba. This is done to reduce the risks of flooding on shoreline properties on Devil’s Lake. However, the Government of Manitoba is concerned about down-river flooding as well as the release of alien invasive species into the ecosystem of the Red River.
• There are many environmental issues associated with the jointly managed ecosystems of the Great Lakes, including those related to the diversion of water out of the system to serve U.S. purposes to the south, the release of alien invasive species, and pollution by sewage, agricultural fertilizer and pesticides, and industrial chemicals.

Binational considerations are also relevant to the many species that migrate between their breeding and wintering grounds, which may involve the use of different ecoregions in separate countries. For example, many of the songbirds that breed in Canada spend much of the year in habitats in the U.S. or in Central or South America. Migratory species of economic value are also an issue, such as species of Pacific salmon that may breed in particular rivers in Canada or the U.S., but could be fished in waters of either country, or even in international waters of the high seas. Global Focus 8.1. examines one such example concerning the monarch butterfly, some of which may breed in southern Canada, and then migrate through the U.S. to reach their hibernating sites in central Mexico.

Figure 8.6. Distribution of the 15 Terrestrial Ecological Regions (level I) for North America. These regions are roughly comparable to global biomes. Source: Commission for Environmental Cooperation (2009).

Figure 8.7. Level III Ecoregions of North America. This map indicates the 182 level III ecoregions in North America. Source: Commission for Environmental Cooperation (2009).

It is beyond the scope of this book to describe the ecoregions of the U.S. in detail. Detailed information on ecoregions is available by state on the website of the U.S. EPA.

Conclusions

Biomes are geographically extensive ecosystems that occur throughout the world wherever environmental conditions are suitable for their development. The same biome may occur in far-flung places, even on different continents, and in such cases it will be similar in structure and function but will usually be dominated by different species. Temperature and moisture availability are the most critical environmental factors affecting the distribution of terrestrial biomes. Marine biomes are most influenced by depth, nutrient availability, and temperature.

The natural landscapes of North America are divided into biophysical regions known as ecoregions. In turn, the ecozones are divided into smaller units known as ecoregions. Ecozones and ecoregions are characterized by their natural landforms, climate, species, and ecological communities. The natural biomes of the world, and the ecoregions of United States, are being rapidly modified by human activities, and many of their inherent biodiversity values are becoming increasingly at risk. These damaging changes are most intensive in regions where people live and work in high population densities, such as in the coastal regions of the U.S.

Questions for Review

1. List five biomes. What are the essential characteristics of each of them?
2. What are the characteristics of the ecozones that occur in the province where you live? For detailed information, visit  https://www.epa.gov/eco-research/level-iii-and-iv-ecoregions-epa-region
3. Select any American ecozone. What are the most important environmental factors affecting the species and ecological communities of that ecozone? Have these factors changed much over the past century or during the past decade? For detailed information on the ecozone, visit the website noted in the previous question.

Questions for Discussion

1. Why is it useful to know about the species of plants and animals that live in some defined area, such as a park, county, or province? Is it useful to know about the ecological communities? How does this kind of information assist in planning for conservation and sustainable development?
2. Ecologists usually consider native species to have greater “value” than non-native ones. Why do they think this way? Is the rationalization only scientific, or does it include an element of non-objectivity?
3. Select any one of the more southerly American ecozones, where human activities have become dominant influences affecting species and ecological communities. Describe any damage that you think human activities might have caused to the native species and natural ecosystems of that ecozone, and consider whether it might be possible to repair any of those effects. For detailed information on the ecozone, visit  https://www.epa.gov/environmental-topics.

Exploring Issues

1. You have been asked to characterize and map the various ecosystems occurring in a national park (choose one near where you live). How would you determine the distribution and characteristics of the various kinds of terrestrial, wetland, and aquatic ecosystems present in the park?

References Cited and Further Reading

Barbour, M.G. and W.D. Billings. 2000. North American Terrestrial Vegetation,2nd ed. Cambridge University Press, New York, NY.

Begon, M., R.W. Howorth, and C.R. Townsend. 2014. Essentials of Ecology. 4th ed. Wiley, Cambridge, UK.

Bolen, E.G. 1998. Ecology of North America. John Wiley & Sons, New York, NY.

Breckle, S.W. 2002. Walter’s Vegetation of the Earth. The Ecological Systems of the Geo-Sphere, 4th ed. Springer-Verlag, Berlin, Germany.

Commission for Environmental Cooperation (CEC). 1997. Ecological Regions of North America. Toward A Common Perspective. CEC, Montreal, PQ.

Commission for Environmental Cooperation (CEC). 2009a. Terrestrial Ecoregions, 2007. CEC, Montreal. http://www.cec.org/naatlas/maps/index.cfm?catId=7&mapId=15&varlan=english

Commission for Environmental Cooperation (CEC). 2009b. Marine Ecoregions, 2008. CEC, Montreal. http://www.cec.org/naatlas/maps/index.cfm?catId=7&varlan=english

Crabtree, P. (ed.). 1970. The Illustrated Natural History of Canada (9 vol.). NSL Natural Science of Canada, Toronto, ON.

Ecological Stratification Working Group. 1995. A National Ecological Framework for Canada. Environment Canada, Ottawa, ON.

Freedman, B., J. Hutchings, D. Gwynne, J. Smol, R. Suffling, R. Turkington, R. Walker, and D. Bazeley. 2014. Ecology: A Canadian Context. 2nd ed. Nelson Canada, Toronto, ON.

Heywood, V.H. (ed.). 1995. Global Biodiversity Assessment. Cambridge University Press, Cambridge, UK.

National Wetlands Working Group. 1988. Wetlands of Canada. Ecological Land Classification Series No. 24. Environment Canada, Ottawa, ON.

Odum, E.P. 1993. Ecology and Our Endangered Life-Support Systems. Sinauer, Sunderland, MA.

Odum, E.P. and G.W. Barrett. 2004. Fundamentals of Ecology. Brooks Cole, Florence, KY.

Phillips, D. 1990. The Climates of Canada. Environment Canada, Ottawa, ON.

Rowe, J.S. 1972. Forest Regions of Canada. Forestry Canada, Ottawa, ON.

Schultz, J. 2004. Ecozones of the World: The Ecological Divisions of the Geosphere. 2nd ed. Springer Verlag, Berlin, Germany.

Scott, G.A.J. 1995. Canada’s Vegetation: A World Perspective. McGill-Queen’s University Press, Montreal, PQ.

Shelford, V.E. 1974. The Ecology of North America. University of Illinois Press, Urbana, IL. USDA. 2009. Major Biomes Map. United States Department of Agriculture, Natural Resources Conservation Service. Washington, DC. http://soils.usda.gov/use/worldsoils/mapindex/biomes.html

Walter, H. 1977. Vegetation of the Earth. Springer, New York, NY.

Wilkinson, T., J. Bezaury-Creel, T. Hourigan, E. Wiken, C. Madden, M. Padilla, T. Agardy, H. Herrmann, L. Janishevski, and L. Morgan. 2007. Marine Ecoregions Of North America. Commission on Environmental Cooperation, Montreal, PQ

Wiken, E., D. Gauthier, I. Marshall, K. Lawton, and H. Hirvonen. 1996. A Perspective on Canada’s Ecosystems: An Overview of the Terrestrial and Marine Ecozones. Occ. Pap. No. 14. Canadian Council on Ecological Areas, Ottawa, ON.

Woodward, S.L. 2003. Biomes of the Earth: Terrestrial, Aquatic, and Human-Dominated. Greenwood Press, Oxford, UK.