a. Most of the biosphere is powered by energy from the sun, though some communities obtain energy by other means.

b. Figure 25.1 is a diagram illustrating the concept of an ecosystem in the biosphere.

c. An ecosystem is structured by the trophic, or feeding, relationships among its inhabitants.

d. A review of terms from Section 7.9 is provided:

e. Only a small part of the enormous amounts of energy striking the earth ever gets into the biota.

a. The air enveloping the earth constantly absorbs energy from sunlight, though it is not heated uniformly everywhere.

b. Warm air rising is cooled and deposits rainfall (Figure 25.2).

c. Rising tropical air moves away from the equator and is replaced by cooler air, forming rotating Hadley cells (Figure 25.2).

d. Air has momentum with regard to the earth's rotation toward the east; air moving toward the equator creates prevailing west winds, and air moving away from the equator creates prevailing east winds.

a. Ocean currents are created by the same factors that produce winds (Figure 25.3), though water, due to its mass, circulates more slowly than air.

b. Water striking the continents is thrown into circular patterns called gyres.

c. Due to its high specific heat, water circulating in currents greatly affects world climates.

d. Oceans also are characterized by massive vertical movement of water, due to differences in densities at different temperatures.

e. Overturn mixes ocean water vertically (Figure 25.4).

a. The oceans are generally cold and dark and cover almost 75% of the earth's surface.

b. However, only about 10 percent of the known species on earth live in the oceans, though the species that exist there are wonderfully diversified.

c. The neritic province is the area of ocean in shallow waters over continental shelves, and is occupied by a large diversity of organisms.

d. The oceanic province is the open seas, where habitats are generally defined by depth of water, the shape of the ocean floor, and the penetration of light (Figure 25.5).

e. Benthic organisms live on the bottom of the ocean.

f. The open water is divided into pelagic zones according to depth.

g. Plankton, consisting of organisms that are tossed about in the sea, and nekton, consisting of free-swimming organisms, occupy the pelagic zones (Figure 25.6).

h. Phytoplankton consists of algae, and carries out about two-fifths of all photosynthesis on earth.

i. Zooplankton consists of minute animals that live on the phytoplankton.

j. Detritus from the pelagic zones feeds the lower marine communities.

k. Explorations of the abyssopelagic zone, 4,000 to 6,000 meters deep, have discovered a variety of organisms (Figure 25.7).

l. An assortment of organisms has been discovered in the abyssal communities around hydrothermal vents (Figure 25.8).

m. Most marine species occupy the neritic province, including many species that are able to anchor or protect themselves around the rocks on shore (Figure 25.9).

n. The littoral or intertidal zone is the part of the community exposed at low tide; its occupants must be able to live out of the water for a time, and often are found in tide pools and depressions (Figure 25.10).

o. An estuary is a partly enclosed body of water where the rivers empty into the ocean (Figure 25.11).

p. The water is brackish, with intermediate salinity, but supports a large variety of specially adapted plants and animals.

q. Estuaries generally contain high concentrations of nutrients, from three major sources: the rivers feeding them, the currents from the ocean, and the estuarine mud.

r. Estuaries are of enormous economic value, but are highly subject to change from pollution.

s. Three types of coral reefs (Figure 25.12) occur in a narrow tropical zone.

t. A fringing reef runs along the edge of an island or continent.

u. A barrier reef exists at some distance from land and is separated from it by a lagoon.

v. An atoll is a circular formation around a sunken island.

w. Corals must grow on solid foundations, but live only in shallow water near the surface.

x. Reef communities include a large assortment of brilliant animals, including the corals and other colonial cnidarians (Figure 25.13).

y. Cyanobacteria in the tidal zone, green algae in the lagoons, and red algae are producers that live among the corals.

z. Zooxanthellae live symbiotically in the tissues of many cnidarians, and they photosynthesize and add oxygen to the surrounding water.

A. Reefs also hold a variety of plants and large animals (Figure 25.13), including annelid worms, molluscs, echinoderms, crustaceans, and fish (Figure 25.14); reef communities tend to be well fossilized, and have left a 600 million year long fossil record that indicates that modern day communities are changed greatly from past ones.

a. Freshwater habitats support a variety of organisms, including fish, amphibians, insects, snails, clams, cnidarians, sponges, and a variety of plants.

b. The energy base of such a community consists of phytoplankton of many kinds of algae, along with larger plants.

c. Submersed plants remain below the water surface, floating-leaved plants float at the surface, and emergents are anchored in the bottom and extend above the surface (Figure 25.15).

d. Two kinds of ponds and lakes are distinguished by their nutrient levels.

e. Oligotrophic lakes are nutrient-poor, due to a low intake of nutrients from the streams that feed them; they are usually deep and oxygen-poor.

f. Eutrophic lakes have abundant nutrients and support many types of phytoplankton and plants.

g. Three zones exist in lake and pond habitats: a littoral zone near the shore, a limnetic zone where light can penetrate, and a profundal zone that is too deep for light (Figure 25.16).

h. Thermal stratification is very dynamic in large lakes, and causes spring overturn and fall overturn, along with thermoclines.

i. Most natural lakes are oligotrophic but become eutrophic over time (e.g. Lake Erie) as sediment builds in them.

j. Bogs are small, poorly drained wetlands that have acidic water and support few plants and animals.

k. The plants that are able to grow in bogs tend to be dominated by Sphagnum mosses (Figure 25.17), hardy evergreens, and sedges.

l. Marshes and swamps are superficially similar.

m. Marshes (Figure 25.18) have water flowing through them and are dominated by soft-stemmed herbaceous plants.

n. Swamps have little water circulation and the dominant plants are trees and shrubs (Figure 25.19).

o. A lot of the Earth's surface contained wetlands at one time and supported a large variety of plants and animals; the preservation of wetlands is a crucial ecological concern and also plays into economics in the areas of flood management, for example.

a. The flow of water evaporating and precipitating through the biosphere is a critical factor in determining several features of the Earth.

b. The continents (Figure 25.20) help determine the distribution of rainfall over the Earth and also distort bands of water flow to create a pattern of climates (Figure 25.20).

c. The topography of the land further modifies this flow (Figure 25.21).

d. Rain shadows exist on the leeward sides of mountains, for instance.

a. Most organisms on the Earth live on land, and terrestrial ecosystems are explored in detail here (Figure 25.22).

b. These areas are characterized by heavy rainfall, rather constant warm temperatures (Figure 25.23), and high humidity.

c. A large variety of trees creates three to five layers of canopies (Figure 25.23).

d. These forests are the richest biomes on Earth in terms of biomass, and support a far greater variety of organisms than do the temperate regions.

e. The soil of the rain forest, however, is poor, as litter is quickly degraded and used by the growing plants.

f. Rain forest plants live relatively short lives, and the entire biome is currently threatened by the invasion of humans.

g. These areas are farther from the equator than the rain forest, and experience periodic droughts.

h. These regions are warm all year and they have periods of extensive rainfall interrupted by dry seasons.

i. They are characterized by fewer layers of vegetation than those found in the rain forest; evergreens are typical trees, and monkeys, elephants, koalas, and cockatoos are typical animals.

j. This is tropical grassland that is comparable to the tropical seasonal forest because of its uniformly warm temperatures with alternating wet and dry seasons.

k. Long grasses with a few trees are the standard vegetation, and herding animals and their predators predominate.

l. These regions are also known as chaparral in North America, and occur at the Pacific coasts, along the Mediterranean Sea, and in southern Africa.

m. These areas receive cool, moist air from the ocean, but relatively little rainfall; their summers are hot and dry.

n. Major vegetation includes broad-leaved evergreens and some deciduous trees (Figure 25.24).

o. Animals include deer, boar, and others that feed on the vegetation.

p. The inland areas of most continents, except Europe, make up these regions, where winters are cold and summers are hot and dry.

q. Prairies and steppes occur here, with predominantly grasses (Figure 25.25) growing on flat, rolling land with rich soil.

r. Precipitation varies, and there are wet seasons interspersed with occasional droughts.

s. Six major types of grasses once grew in these regions in North America (Figure 25.25).

t. Grazing herbivores, their predators, and a variety of smaller animals inhabit these regions.

u. Deserts receive less than 25 cm of rainfall each year (Figure 25.26), and have large areas of open land with plants concentrated in low areas.

v. Dramatic shifts in temperature occur between day and night.

w. Desert plants include cacti, which are typical xerophytes that are adapted to dry conditions.

x. Many desert plants photosynthesize through crassulacean acid metabolism (Section 10.9).

y. Invertebrate herbivores and their mammalian and reptilian predators make up most of the animals living in the desert.

z. These areas are similar to deserts, but have more rainfall and heavier vegetation, with very hot summers.

A. Plants are typically thorny scrubs called sclerophylls, which have small leathery leaves.

B. Animals include snakes and lizards and small herbivores.

C. Warm summers and cold winters, with relatively uniform precipitation characterize these areas.

D. Several types of forests exist (Figure 25.27), with thick undergrowth including shrubs and herbaceous plants.

E. Humus is produced here as litter turns into a thick layer of decaying material.

F. A variety of herbivores and their carnivore predators exist in these areas.

G. Many areas in North America that are allowed to return to wilderness quickly grow into these woods, though several such areas are still being destroyed.

H. In Alaska, Canada, Russia, Siberia, and on the southeastern coast of Australia, the taiga exists as a circumpolar band of coniferous forest.

I. The trees are mostly pines, spruce, and fir (Figure 25.28), with a few broadleaved types (e.g. aspen).

J. The winters are long and the growing season is short and cool.

K. Deer and moose, hares and rodents constitute the majority of herbivores, with beavers, weasels, bears, lynx, and wolves as some of the carnivores.

L. This area exists north of the taiga and south of the polar ice caps.

M. The tundra is extremely cold and dry, and is characterized by permafrost, or constantly frozen soil beneath the surface.

N. Low-lying grasses, mosses, and lichens are the major plants (Figure 25.29), and caribou, wolves, polar bears, and arctic fox are some of the animals that exist in these areas.

O. A typical mountain range (Figure 25.30) exhibits the sequence of biomes from tropical forests to tundra, as altitude increases.

P. The fauna of each mountain region often mimics the fauna of the corresponding lowlands, though the animals are adapted to a low oxygen environment.

a The earth's biomes are defined mainly by their vegetation, which depends on climate.

b. Climate is a combination of temperature and precipitation, and the interactions between these determines different biomes (Figure 25.31).

c. Climates can be divided into roughly six types, shown here with their corresponding vegetation:

a. Few organisms live on bare rock; soil is necessary to sustain life.

b. As erosion removes overlying rock from underlying rock, the lower layers crack and weather (Figure 25.32) and soil is made.

c. Rock is a mixture of minerals, many of which are integrated into the soil to nourish plant growth.

d. Plants alter soil further; humus accumulates in soil supporting plants.

e. Humic acids are released into the soil by the decay of woody plants; these and clay minerals form clay-humus complexes that largely determine the soil's character.

f. Water percolating through the soil replaces the ions in these complexes (Figure 25.33) and minerals are thus removed from the layers that support plant growth.

g. Podzolization is an extreme case of leaching that occurs in regions such as mountains where heavy rainfall occurs.

h. In cross-section, soil shows distinct layers (Figure 25.34), and is called a podzol (Figure 25.34).

i. In the grasslands, the typical soil is black earth, or chernozem (Figure 24.34), where leaching is balanced by the release of minerals from the plants above.

j. Other major soil types include a latosol, or oxisol, a very deep, loose, red soil formed by high temperature and high precipitation.

k. The typical desert soil is a sierozem, or aridosol, formed by the breakdown of parent rock under high temperatures, but with little rain to leach away minerals.

l. Living organisms contribute to the character of soil; earthworms especially are important in changing the characteristics of soil.

m. A variety of bacteria and fungi also live in and change the soil (Figure 25.35).

a. Regions called biogeographic realms (Figure 25.36) have been described on earth, each having distinctive plants and animals.

b. Figure 25.37 shows representatives of several animal groups that occupy similar niches in each realm.

c. Convergent or parallel evolution is thought to be responsible for the similarities among several groups of these organisms.

a. Alfred Wegener proposed the theory of continental drift, and named the theoretically original supercontinent Pangea.

b. Plate tectonics studies have established that the earth's surface is divided into plates that are indeed moving.

c. The geological record shows that Pangea broke up about 250 million years ago, during the Permian period (Figure 25.39), to form a northern continent called Laurasia, and a southern one called Gondwana.

d. These theories are used to account somewhat for the similarities among ancestors of the earth's existing flora and fauna.