chapter 41. key concepts 41.1 ecological systems vary over space and time 41.2 solar energy input...

The Distribution of Earth’sEcological Systems

Chapter 41

Chapter 41 The Distribution of Earth’s Ecological Systems

Key Concepts41.1 Ecological Systems Vary over Space and Time

41.2 Solar Energy Input and Topography Shape Earth’s Physical Environments

41.3 Biogeography Reflects Physical Geography

41.4 Biogeography Also Reflects Geological History

41.5 Human Activities Affect Ecological Systems on a Global Scale

Chapter 41 Opening QuestionCan basic ecological principles suggest why removing cattle has not restored grasses to the Borderlands?

Concept 41.1 Ecological Systems Vary over Space and Time

Physical geography—study of the spatial distribution of Earth’s climates and surface featuresBiogeography—study of the spatial distributions of speciesThe explorer/scientists in the 18th and 19th centuries began to realize that the distributions of species and environments are linked.


Abiotic components of the environment are nonliving.Biotic components—living organismsAn ecological system—one or more organisms plus the external environment with which they exchange energy and materials.


A system is defined by the interacting parts it contains.Ecological systems can include any part of the biological hierarchy from the individual to the biosphere.Each level brings in new interacting parts at progressively larger spatial scales.


At the smallest scale is the individual organism and its immediate environment.Individuals remove materials and energy from the environment, convert them into forms that can be used by other organisms, and, by their presence and activities, modify the environment.


Population—group of individuals of the same species that live, interact, and reproduce in a particular geographic areaCommunity—assemblage of interacting populations of different species in one areaEcosystem-An ecosystem is the relationships between and among the populations and the abiotic (environmental) factors Biosphere—all the organisms and environments of the planet


Ecologists replace the term “ecological system” with ecosystem when they are explicitly including the abiotic components of the environment;

And in particular when considering communities and their environmental context.


Generally, large ecological systems tend to be more complex because they have more interacting parts, and larger spatial scale.But small systems can also be complex:

The human gut is densely populated with hundreds of microbial species. These cells far outnumber the trillion or so human cells in the body.


The mammalian gut environment provides stable conditions and ample nutrients.Gut microbes metabolize foods, including some the host cannot digest, and excrete waste products that provide nutrition to the host or to other microbes.Microbial species interact with one another and with host cells by forming biofilms that coat the gut lining.


Biotic and abiotic components of ecosystems are distributed unevenly in space, and ecosystems can change over time.The human gut illustrates this variation—each person has a unique gut community.But patterns do exist: gut communities of genetically related people are more similar that those of unrelated people.


Gut communities in lean people and obese people vary in the ratio of two bacterial phyla.When obese people lose weight, their gut community becomes more similar to that of a lean person.Bacteria in the phylum Firmicutes are good at breaking down indigestible polysaccharides and extracting more energy from food than Bacteroidetes.

Figure 41.1 Genetics and Diet Affect the Composition of the Gut Microbial Community


In experiments with mice, it has been shown that the gut community contributes to obesity, along with diet and genetic factors.

Figure 41.2 The Microbial Communities of Genetically Obese Mice Contribute to Their Obesity (Part 1)

Concept 41.2 Solar Energy Input and Topography Shape Earth’s Physical Environments

Earth’s environments vary greatly from place to place and also through time.On long time scales, the coming and going of oceans, ice ages, and other geologic events shape environments.On short time scales, physical conditions depend largely on solar energy input, which drives the circulation of the atmosphere and the oceans.


Weather is the state of atmospheric conditions in a particular place at a particular time.Climate is the average conditions and patterns of variation over longer periods.Climate is what you expect; weather is what you get.Adaptations to climate prepare organisms for expected weather patterns.


Earth receives uneven inputs of solar radiation due to its spherical shape and the tilt of its axis as it orbits the sun.• It is colder at the poles because there is less solar input: the sun’s rays

are spread over a larger area and pass through more atmosphere. • High latitudes experience more seasonality—greater fluctuation over

the course of a year.

Figure 41.3 Solar Energy Input Varies with Latitude

Figure 41.4 The Tilt of Earth’s Axis of Rotation Causes the Seasons


Solar energy inputs are always greatest in the tropics and decrease poleward.This latitudinal gradient drives global circulation patterns in the atmosphere.


Hadley cells:The tropical air is warmed, rises, and then cools adiabatically (an expanding gas cools).The rising warm air is replaced by surface air flowing in from the north and south.The cooling air sinks at 30°N and 30°S.

Figure 41.5 Tropical Solar Energy Input Sets the Atmosphere in Motion


Hadley cell circulation produces latitudinal precipitation patterns:Rising warm tropical air releases lots of moisture as rainfall. The sinking air at 30°N and 30°S is dry—most of the great deserts are at these latitudes.


Some of the descending air in the Hadley cells flows towards the poles, overriding cold, dense polar air that is flowing equatorward. The interaction of these warm and cold air masses generates winter storms that sweep from west to east through the middle latitudes.Earth’s rotation adds an east–west component to the north–south movement of the air masses—the Coriolis effect.

Figure 41.6 Global Atmospheric Circulation and Prevailing Winds


These atmospheric circulation patterns affect climate patterns by transferring heat energy from the hot tropics to the cold poles.Without this transfer, the poles would sink toward absolute zero in winter, and the equator would reach fantastically high temperatures throughout the year.


Prevailing surface winds drive the major ocean surface currents, which carry materials, organisms, and heat with them.

Example: In the northern tropics, the trade winds drag water to the west; when it reaches a continent, it is deflected northward until the westerlies drive the water back to the east. The result is a clockwise gyre.

Figure 41.7 Ocean Surface Currents

American Mediterranean


As they move poleward, tropical surface waters transfer heat from low to high latitudes, adding to the heat transfer by atmospheric circulation.The Gulf Stream and North Atlantic Drift bring warm water towards northern Europe, warming the air there. The same latitudes in Canada are much colder.


Deep ocean currents are driven by water density differences.Colder, saltier water is more dense and sinks to form deep currents.Deep currents regain the surface in areas of upwelling, completing a vertical ocean circulation.


Oceans and large lakes moderate terrestrial climates because water has a high heat capacity:

Temperature of water changes slowly as it exchanges heat with the air.Water temperatures fluctuate less than land temperatures, and the air over land close to oceans or lakes also shows less seasonal and daily temperature fluctuation.


Topography (variation in elevation) also affects the physical environment.As you go up a mountain, air temperature drops by about 1°C for each 220 m of elevation because rising air expands and cools adiabatically.When prevailing winds bump into mountain ranges, the air rises, cools, and releases moisture. The now-dry air descends on the leeward side, creating a rain shadow.

Figure 41.8 Orographic Rain Shadow


Topography also influences aquatic environments:Flow velocity depends on slope.Water depth determines gradients of many abiotic factors, including temperature, pressure, light penetration, and water movement.


Climate diagram—superimposed graphs of average monthly temperature and precipitation over a year. The axes are scaled so that it is easy to see the growing season:

When temperatures are above freezing and the precipitation line is above the temperature line

Figure 41.9 Climate Diagrams Summarize Climate in an Ecologically Relevant Way

Concept 41.3 Biogeography Reflects Physical Geography

An organism’s physiology, morphology, and behavior affect how well it can tolerate a particular physical environment. Thus, the physical environment greatly influences what species can live there. We expect species that occur in similar environments to have evolved similar phenotypic adaptations.


Early scientist–explorers began to understand how the distribution of Earth’s physical environments shapes the distribution of organisms.Their observations revealed a convergence in characteristics of vegetation found in similar climates around the world.


Biome—a distinct physical environment inhabited by ecologically similar organisms with similar adaptations.Species in the same biome in geographically separate regions display convergent evolution of morphological, physiological, or behavioral traits.


Terrestrial biomes are distinguished by their characteristic vegetation.Distribution of terrestrial biomes is broadly determined by annual patterns of temperature and precipitation.

These factors vary along both latitudinal and elevational gradients.

Figure 41.10 Terrestrial Biomes Reflect Average Annual Temperature and Precipitation

Figure 41.11 Global Terrestrial Biomes


Other factors, especially soil characteristics, interact with climate to influence vegetation.

Example: Southwestern Australia has Mediterranean climate with hot, dry summers and cool, moist winters. The vegetation is woodland/shrubland, with no succulent plants.

The soils are nutrient-poor, and there are frequent fires. Succulents are easily killed by fires.


Grasslands normally occur where there is not enough precipitation to support forests, but is more plentiful than is typical of deserts.But some grasslands occur in unexpected places, demonstrating that biome boundaries are not perfectly predicted by temperature and precipitation—other factors also affect the vegetation.

Figure 41.12 Similar Vegetation Types, Different Conditions


Fire is a significant factor affecting vegetation.Fire rarely kills grasses but often kills shrubs and trees; fires help maintain grasslands.Humans have probably used fires for millennia to manipulate their environment and maintain grasslands.


The biome concept is also applied to aquatic environments.Aquatic biomes are determined by physical factors such as water depth and current, temperature, pressure, salinity, and substrate characteristics.

Table 41.1 (Part 1)

Table 41.1 (Part 2)


The primary distinction for aquatic biomes is salinity: freshwater, saltwater, and estuarine biomes.Salinity determines what species can live in the biome, depending on their ability to osmoregulate.


In streams, current velocity is important. Organisms must have adaptations to withstand flow.Current also impacts the substrate—whether rocky, sandy, silty, etc. Substrate also determines what species are present.


Lakes and oceans are divided into water-depth zones.Nearshore regions (littoral or intertidal) are shallow, impacted by waves and fluctuating water levels. Distinct zonation of species is common.Photic zone—depth to which light penetrates; photosynthetic organisms are restricted to this zone


In the open-water limnetic zone of lakes and the pelagic zone of oceans beyond the continental shelf, the prominent photosynthesizers are phytoplankton (free-floating photosynthetic organisms).

Figure 41.13 Water-Depth Zones (Part 1)

Figure 41.13 Water-Depth Zones (Part 2)


The aphotic zone is too deep for light penetration and so is sparsely populated.Benthic zone—lake or ocean bottomOrganisms in the deepest oceans (abyssal zone) must have adaptations to deal with high pressure, low oxygen, and cold temperatures.

Concept 41.4 Biogeography Also Reflects Geological History

Alfred Russel Wallace advanced the idea of natural selection along with Darwin.Wallace studied species distributions in the Malay Archipelago and observed dramatically different bird faunas on two neighboring islands, Bali and Lombok.The differences could not be explained by soil or climate.


He suggested that the deep channel between the islands would have remained full of water (and a barrier to movement of terrestrial animals) during the Pleistocene glaciations when sea level dropped.Thus, the faunas on either side of the channel evolved mostly in isolation over a long period of time.


Wallace’s observations led him to divide the world into six biogeographic regions. Each region encompasses multiple biomes and contains a distinct assemblage of species, many of which are phylogenetically related.Many of the boundaries correspond to geographic barriers to movement: bodies of water, deserts, mountain ranges.

Figure 41.14 Earth’s Terrestrial Biogeographic Regions


Boundaries of some biogeographic regions are related to continental drift.

Example: Southern beeches (Nothofagus) are found in South America, New Zealand, Australia, and some south Pacific islands.

The genus originated on the supercontinent Gondwana during the Cretaceous and was carried along when Gondwana broke apart.

Figure 41.15 Distribution of Nothofagus (Part 1)

Figure 41.15 Distribution of Nothofagus (Part 2)


The biogeographic regions occupy land masses that have been isolated from one another long enough to allow the organisms to undergo independent evolutionary radiations.The biotas developed in isolation throughout the Tertiary (65 to 2.6 mya), when extensive radiations of flowering plants and vertebrates took place.


Continental movement can also eliminate barriers, allowing biotic interchange.

Examples: when India collided with Asia about 45 mya; when a land bridge formed between North and South America about 6 mya.


Biogeographers use phylogenetic information, the fossil record, and geological history to study modern distributions of species.Geographic areas are superimposed on phylogenetic trees. The sequence and timing of splits in the phylogenetic tree are compared with sequence and timing of geographic separations or connections.

Concept 41.5 Human Activities Affect Ecological Systemson a Global Scale

Human activities are altering ecological systems on a global scale.Some have suggested we are entering a new geological period, the “Anthropocene” or Age of Humans.We are changing the distributions of organisms, vegetation, and topography, as well as Earth’s climate.


Others suggest the new age should be called the “Homogenocene,” or Homogeneous Age, because the net effect of our activities is to make ecological systems less complex and more uniform.


When we use natural ecosystems for hunting, fishing, grazing, or logging, we remove particular species and change their abundances. If we remove too many, we can even cause some species to go extinct.This can change the patterns of interaction among species, and thereby change how entire ecosystems function.


Human-dominated ecosystems, such as croplands, pasturelands, and urban settlements now cover about half of Earth’s land area.These ecosystems have fewer interacting species and are less complex.


In agricultural lands, monocultures replace species-rich natural communities.Diversity of crops planted is also very low: 19 crops comprise 95% of total global food production.Agricultural systems are more spatially and physically uniform than natural ecological systems.

Figure 41.16 Human Agricultural Practices Produce a Uniform Landscape


Human activities also reduce complexity in natural ecosystems:• Damming and channelization of rivers • Pollution and habitat fragmentation • Overexploitation of wild species • Introductions of new species


Humans move species throughout the globe, sometimes deliberately, sometimes inadvertently.Human-assisted biotic interchange is homogenizing the biota of the planet, blurring the spatial heterogeneity in species composition that evolved during long periods of continental isolation.


New subdisciplines of ecology address how we can preserve ecological systems and their ability to sustain life on our planet.Conservation ecology seeks to understand the process of extinction and ways to prevent extinction of vulnerable species.Restoration ecology seeks to restore the health of damaged ecosystems.


Natural systems are sometimes altered so strongly that extinction will occur unless the systems are restored.


Natural history—observation of nature outside of a formal, hypothesis-testing investigation—provides important knowledge about ecosystems

These observations are often the source of new questions and hypotheses and aid in design of ecological experiments.


Mathematical models and computer simulations are often needed to study the complexities of ecological systems.They must be based on natural history knowledge of the system.

Answer to Opening QuestionThe Borderlands are arid—they are at 32° N, where the warm, dry air of the Hadley cells descends.Other grasslands around the world occupy different physical environments, and have different histories.Why grasslands of the Borderlands have not recovered from overgrazing may be due to many factors:

Answer to Opening Question• Periodic burning may be needed. • It may be necessary to restore the original landscape, to return water

and nutrients to the soil that grasses require.• Interaction between fire and nutrients may influence which vegetation

type will be favored. • It may be necessary to remove the existing shrubs before other

measures will have a chance of success.

chapter 41. key concepts 41.1 ecological systems vary over space and time 41.2 solar energy input...

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small systems

term ecological system

immediate environment

external environment

human gut

human cells

host cells