ECOSYSTEMS 25 latest
ECOSYSTEMS
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ECOSYSTEMS
Study objectives
When you complete this chapter you should be able to:
Define the four major components of an ecosystem, and explain their interdependence.
Recognize that other environmental controls may be more important on a local scale, but that climate has the greatest influence over ecosystems on a worldwide basis.
Explain how vegetation becomes established on barren or devastated areas, and cite an example of the steps in plant succession.
Provide examples of how plants, animals, and the environments in which they live are interdependent, each affecting the others.
Outline the climatic factors that have the greatest effect on plants and animals, and summarize the nature of those climatic impacts.
Describe the major components of a soil and how they vary to produce different soil types.
Discuss the role of water in soil processes and how different amounts of available soil water can affect the vegetation growing on a soil.
Understand the factors that determine a soil’s formation, development, and fertility, including the role of vegetation.
Explain why soils are among the world’s most critical resources, and the need for effective soil conservation practices.
Cite a few reasons why humans affect ecosystems and soils more than all other life forms, and some examples of major impacts.
Biogeography is the study of how environmental factors affect the locations, distributions, and life processes of plants and animals.
Basically, this discipline seeks explanations for the geography of life forms. Biogeographers delineate the spatial boundaries of ecosystems, and investigate how and why environmental characteristics change spatially and over time.
Soils are intimately related to the factors that also influence the biogeography of an area.
The characteristics of a soil reflect the interactions among the climate, vegetation, rocks, minerals, and fauna at its location.
Soils are also environments that teem with organisms living on and beneath the surface.
Ecosystems
The term ecosystem refers to a community of organisms that occupy a given area, and the interdependent relationships—with each other and the environment—that allow the organisms to thrive.
Generally, ecosystems are studied on a local or regional basis, but the entire Earth system (the ecosphere) also functions as an ecosystem. When farmers plant crops, spread fertilizer, control weeds, and spray insecticides, new ecosystems (although artificial) are created.
Despite environmental alterations by human activities, plants and animals that can adapt will still live together in interdependent relationships with soil, rainfall, temperatures, sunshine, and other characteristics of the physical environment.
Ecosystems are open systems, with movement of both energy and materials into and out of these systems.
Ecosystems are not isolated in nature, they are usually closely related to nearby ecosystems and integrated with the larger ecosystems of which they are a part.
The ecosystem concept is a valuable model for examining the structure and function of life on Earth.
Major Components
Despite their great variety on Earth, the typical ecosystem has four basic components (Fig. 9.2).
The first of these is the nonliving, or abiotic, part of the system. This is the physical environment in which the plants and animals of the system live.
In a terrestrial ecosystem, the abiotic component provides life-supporting elements and compounds in the soil, groundwater, and atmosphere.
The second component of an ecosystem consists of the basic producers, or autotrophs (meaning “self-nourished”). Plants are important autotrophs, because they can use solar energy to convert water and carbon dioxide into organic molecules through photosynthesis.
The sugars, fats, and proteins produced by plants through photosynthesis supply the food that supports other forms of life.
Some bacteria are also capable of photosynthesis, and sulfur-dependent organisms that dwell at thermal vents on the sea floor are also classified as autotrophs.
A third component of most ecosystems consists of consumers, or heterotrophs (meaning “other-nourished”). These are animals that survive by eating plants or other animals.
Herbivores eat only plant material, carnivores eat other animals, and omnivores feed on both plants and animals. Animals contribute to the Earth ecosystem in many ways.
They use oxygen in respiration and exhale carbon dioxide that is required for photosynthesis by plants.
Without the fourth component of ecosystems, the decomposers, plant growth could soon come to a halt.
The decomposers, or detritivores, feed on dead plants and animals, promoting decay and returning mineral nutrients that plants can use to the soil and bodies of water.
Animals also influence soil development through digging and trampling, and those activities also affect local plant distributions.
Trophic Structure
The living components of an ecosystem are organized in a sequence by their eating habits.
Herbivores eat plants, carnivores may eat herbivores or other carnivores, and decomposers feed on dead plants and animals and their waste products.
The sequence of feeding levels is referred to as a food chain, and organisms are identified by their trophic level, the number of steps they are removed from the producers (Fig. 9.3).
Plants occupy the first trophic level, herbivores the second, carnivores feeding on herbivores the third, and so forth until the last level, the decomposers, is reached.
Omnivores may belong to several trophic levels because they eat both plants and animals.
The simplest food chain would include only plants and decomposers. More complex food chains may include six or more levels as carnivores feed on other carnivores—for example, zooplankton eat plants, small fish eat zooplankton, larger fish eat small fish, bears eat larger fish, and decomposers consume the bear after it dies.
In reality, most food chains do not operate in a simple linear sequence; they overlap and interact to form a feeding network within an ecosystem called a food web.
Food chains and food webs can be used to trace the movement of food and energy from one level to another in an ecosystem.
Some biologists and ecologists find it helpful to separate the trophic structure into specific nutrient cycles. There are several such cycles that help explain the routing of nutrients through ecosystems.
Particularly important cycles have been developed for water, carbon, nitrogen, and oxygen. Knowledge of chemical nutrient cycles is essential to an understanding of energy flow in ecosystems.
Figure 9.4 is a summary diagram that illustrates the major processes involved in these cycles.
Energy Flow and Biomass
Sunlight provides energy to an ecosystem, which is used by plants in photosynthesis, and the energy is stored in the organic materials of plants and animals.
The total amount of living material in an ecosystem is referred to as the biomass.
Because the energy of an ecosystem is stored in the biomass, scientists measure each trophic level’s biomass to trace energy flow through the system.
The second law of thermodynamics states that whenever energy is transformed from one state to another there will be a loss of energy through heat.
When an organism at one trophic level feeds on an organism, not all of the food energy is used, some is lost from the system (see Fig. 9.3).
Additional energy is lost through respiration and movement. As energy flows from one trophic level to the next, the biomass successively decreases because of this loss of energy between trophic levels (Fig. 9.5).
At each higher trophic level, a greater amount of energy is required. A deer may graze in a limited area, but the wolf that preys on the deer must hunt over a much larger territory.
As the flow of energy decreases with each successive trophic level, the biomass also decreases.
This principle also applies to agriculture. A great deal more biomass (and food energy) is available in a field of corn than there is in the cattle that eat the corn.
Let’s skip the discussion on productivity and move straight to Succession and climax communities (pp.215)
Succession and climax communities
At least for terrestrial ecosystems, vegetation associations, which typically reflect climatic conditions, most easily distinguish one ecosystem from another.
Vegetative associations are called plant communities. Plant communities are aggregations of vegetation species that have adapted to existing environmental conditions.
If the vegetation develops naturally without significant human modification, the resulting plant association is called natural vegetation.
The species within a community have different environmental living requirements in terms of factors such as light, moisture, and mineral nutrients.
If two species within a community were to compete for the exact same resources, one would eventually eliminate the other.
Succession
Once natural vegetation becomes established it often develops and changes further in a progressive sequence of different plant communities over time.
This process, called plant succession, usually begins with a relatively simple plant community. There are two major types of succession, primary and secondary.
In primary succession, no soil or seedbed exists at the beginning. A pioneer community invades a barren area (volcanic lava, an area previously covered by glaciers, or a barren beach, among others).
As pioneer plants become established, their growth processes alter the environmental conditions. In time, these changes become sufficient to allow a new plant community (which could not have survived under the original conditions) to appear, dominate, and eventually replace the original vegetation.
The process continues with each succeeding community, rendering further changes to the environment.
Primary succession can take centuries or even a few thousand years because of the barren conditions under which the process began.
Secondary succession begins when a natural process, such as a wildfire, tornado, or landslide, destroys or damages much of the existing vegetation.
Ecologists refer to this process as gap creation. Even after such damage, seeds lying dormant in the soil are ready to sprout and invade the newly opened gap.
Compared to primary succession, secondary succession can occur more quickly. A common form of secondary succession, associated with agriculture in the southeastern United States, is depicted in Fig. 9.8.
After agriculture has ceased, pioneer plants such as weeds and grasses colonize the bare fields. These plants stabilize the topsoil, add organic matter, and produce favorable conditions for the growth of shrubs and brush such as sassafras, persimmon, and sweet gum.
During this stage, the soil is enriched with nutrients and organic matter, and increases its ability to retain moisture. These conditions encourage the development of pine forests, the next stage in this vegetative succession.
As pine forests thrive in this newly created environment, the pine trees eventually shade out and dominate the weeds, grasses, and brush.
Ironically, growth of a pine forest can also lead to its demise. Pine trees require much sunlight for their seeds to germinate. When competing with scattered brush, grasses, and weeds, there is adequate sunlight for germination, but once a pine forest develops, the shade and litter will not allow the pine seeds to germinate.
Hardwood trees, such as oak and hickory, whose seeds can germinate in shady conditions, begin to grow as an understory, but eventually will replace the pines. In the above US example, a complete succession from field to oak– hickory forest will take about 100–200 years if it continues unimpeded.
Through succession, the area can return to the natural oak–hickory forest that existed prior to agricultural clearing. In other ecosystems, such as tropical rain forests, succession from deforestation back to a natural forest may take many centuries.
The Climax Community
The concept of plant succession is defined as a process of predictable steps ending with a vegetative cover that would remain in environmental balance unless affected by major climatic or environmental change.
The final result in a succession is called a climax community. It was thought that climax communities would be self-perpetuating, and in a state of equilibrium or stability with the environment.
As seen in our illustration of plant succession in the southeastern US, the oak–hickory forest would be considered the climax community.
The tropical rainforests are also a good example of a climax community (Fig. 9.9). Succession remains as a useful model for studying ecosystems, but some of the original ideas have been challenged.
Early proponents emphasized a predictable sequence of succession. One plant community would follow another in regular order as the ecosystem changed over time.
But, many changes in ecosystems do not follow a rigid or completely predictable sequence. Many scientists today no longer believe that only one type of climax vegetation is possible for each of the world’s major climate regions.
One of several different climax communities might develop within a given area, influenced not only by climate but also by local conditions of drainage, nutrients, soil, or topography.
The dynamic nature of climate is also now better understood than it was when the original theories of succession and climax were developed.
It is possible the time it takes the species structure of a plant community to adjust to climatic conditions, the climate may change again. Also, because every habitat has a dynamic nature, no one climax community can exist in equilibrium with an environment indefinitely.
Today, many biogeographers and ecologists view plant communities and their ecosystems as a landscape that is the expression of all of its various environmental factors functioning together.
They view the landscape of an area as a vegetative mosaic of interlocking parts, much like the tiles in a mosaic artwork. In a pine forest, for example, other plants also exist, and some areas may not support pine trees.
The dominant area of the mosaic—in this case, the pine forest— is called the matrix.
Gaps within the matrix, resulting from different soil conditions or from human or natural processes, are called patches within the matrix.
Relatively linear features that cut across the mosaic, including natural features such as rivers and human-created structures such as roads, fence lines, and power lines, are termed corridors (Fig. 9.10).
Every habitat is unique and constantly changing, and resultant plant and animal communities must constantly adjust to these changes.
Climate, a dominant environmental influence, has changed throughout Earth history and is still changing.
Climate change may be subtle, or may be sufficiently drastic to create ice ages or warm periods between ice ages.
Plant and animal communities must be able to adapt to environmental changes, or they will not survive.
Biogeographers work to reconstruct the vegetation communities of past climate periods by examining evidence such as tree rings, pollen, and fossils.
By determining how past climate changes affected Earth’s ecosystems, biogeographers hope to forecast future impacts that may develop as climate continues to change.
Environmental Controls
The plants and animals that exist in a particular ecosystem are those that have been successful in adjusting to their habitat’s environmental conditions.
Every living organism requires certain environmental conditions to survive. Certain plants can exist under a wide range of temperature, whereas others have narrow temperature requirements.
This refers to an organism’s range of tolerance for certain environmental conditions. The ranges of tolerance will determine where a species may exist, and species with wide ranges of tolerance will be the most widely distributed.
The ecological optimum refers to environmental conditions under which a species will thrive. The farther away a species is from its ecological optimum or from the geographic core of its plant or animal community, the conditions will become increasingly difficult for that species or the community to survive.
However, those same conditions may be more amenable for another species or community. An ecotone is the overlap or the zone of transition between two plants or animal communities (Fig. 9.11).
On a global basis, climate has the greatest influence over natural vegetation. The major types of terrestrial ecosystems, or biomes, are associated with certain temperature ranges as well as critical annual or seasonal precipitation and evaporation characteristics.
Climate influences the sizes and shapes of tree leaves and determines whether trees can exist in a region, but, at the local scale, other environmental factors can be as important.
A plant’s range of tolerance for acidity, moisture, or salinity in the soil may also be a critical environmental determinant of whether a plant will grow, flourish, or die.
The discussion that follows illustrates how major environmental factors influence the organization and structure of ecosystems.
Climatic Factors
Sunlight is one of the most critical climatic factors that influence an ecosystem.
Sunlight is the energy source for plant photosynthesis, and it also strongly influences the behavior of both plants and animals.
Competition for light can make forest trees grow taller, thereby limiting plant growth on the forest floor to shade tolerant species such as ferns.
The sizes, shapes, and colors of leaves may result from variations in light reception, with large leaves developing in areas of limited light.
The intense sunlight of the low latitudes produces a greater biomass in the tropical forests compared to the much lower light intensity that reaches the high latitude Arctic regions.
Duration of daylight, which varies seasonally and with latitude, has a profound effect on the flowering of plants as well as animal mating and migration.
Many plants can tolerate a wide range of temperatures, although every species has optimum conditions for growth.
Vegetation, however, can be adversely affected by temperature extremes (unusual hot spells or cold temperatures) in climate regions where they rarely occur.
Temperatures may also affect vegetation indirectly. For example, high temperatures lower the relative humidity, thus increasing transpiration.
If a plant’s root system cannot extract enough moisture from the soil to meet this increase in transpiration, the plant will wilt and eventually die.
Virtually all organisms require water. Plants need water for germination, growth, and reproduction, and most plant nutrients must be dissolved in soil water to be absorbed by plants.
Marine and aquatic plants are adapted to living in water. Some trees, such as mangroves (Fig. 9.12a) and bald cypresses (Fig. 9.12b), rise from marshes and swamps.
Certain tropical plants become dormant during dry seasons, dropping their leaves, and others store water received during wet periods for surviving the drought seasons.
Desert plants, such as cacti, are well adapted to storing water when it is available while minimizing water loss from transpiration.
Luxuriant forests tower above the well-watered windward sides of mountain ranges such as the Sierra Nevada and Cascades, but semiarid grasslands, shrublands, and sparse forests cover the leeward sides.
Orographic precipitation, rain shadow effects, and elevation changes produce variations in temperature, precipitation, drainage, and evapotranspiration that directly affect vegetation types and distributions.
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Life zones change progressively with elevation; upper zones are dominated by hardy plant communities that can tolerate the lower temperatures and the precipitation regimes found at higher elevations.
Animals, because of their mobility, are not as dependent as plants are on climatic conditions, although animals are subject to climatic stresses. In arid regions, animals employ adaptations to heat and aridity.
Many become inactive during the hottest and driest seasons, and most leave their burrows or the shade only at night.
The geographic distribution of some animal groups reflects their degree of sensitivity to climate.
Cold-blooded animals, for example, are more widespread in warm climates and more restricted in cold climates.
Some warm blooded animals develop layers of fat or fur for protection from the cold. During hot periods, they may sweat, shed fur, or lick their fur in an attempt to stay cool.
Certain animals hibernate to survive in cold or arid regions. Cold-blooded animals such as the rattlesnake move into and out of shade in response to temperature changes.
Seasonally, warm-blooded animals may migrate great distances out of environmentally harsh areas.
Some warm-blooded animals exhibit a link between body shape and size to variations in average environmental temperatures.
The body size of a subspecies usually increases with the decreasing mean temperature of its habitat and, in warm-blooded species, the relative sizes of exposed body portions decrease as the mean temperature declines.
Further, in cold climates, body sizes tend be larger to provide body heat needed for survival and for protecting vital organs in the trunk of the body (Fig. 9.13).
Members of the same species living in colder climates eventually evolve shorter or smaller appendages (ears, noses, arms, legs, etc.) compared to their relatives in warmer climates.
In cold climates, small appendages are advantageous because they reduce the body areas that are subjected to temperature loss and frostbite.
In warm climates, long limbs, noses, and ears allow for heat dissipation in addition to that provided by panting or licking fur.
As a climatic control on vegetation, the wind is most significant in deserts, polar regions, coastal zones, and high lands.
Wind may directly injure vegetation and can also have an indirect effect by increasing evapotranspiration.
To prevent water loss in areas of severe wind stress, plants twist and grow close to the ground, minimizing their wind exposure (Fig. 9.14).
During severe winters, they are better off being buried by snow than being exposed to bitterly cold gales.
In some windy coastal regions, the shoreline may be devoid of trees or other tall plants.
In windy coastal and mountain regions where the trees do grow, they are often misshapen or swept bare of leaves and branches on their windward sides.
Soil and Topography
Soils supply much of the moisture and minerals for plant growth. Soil variations can strongly influence plant distribution and also can produce sharp boundaries between vegetation types.
This is partly a consequence of varying chemical requirements of different plant species and partly a reflection of factors such as soil texture.
Clay soils may retain too much moisture for certain plants, whereas sandy soils retain too little. Pines generally thrive in sandy soils, grasses in clays, cranberries in acid soils, and chili peppers in alkaline soils.
Topography, particularly in highlands, influences ecosystems by providing diverse microclimates within a relatively small area.
Plant communities vary from place to place in highland areas in response to the differing microclimatic conditions. Slope aspect has a direct effect on vegetation patterns in areas outside of the equatorial tropics.
North-facing slopes in the middle and high latitudes of the Northern Hemisphere have microclimates that are cooler and wetter than those on south-facing exposures (Fig 9.15).
Northern Hemisphere south facing slopes tend to be warmer and drier because they receive more direct sunlight. The steepness and shape of a hillslope also affect how long water is present before draining downslope.
Natural Catastrophes
Plant and animal distributions are also affected by a variety of natural processes frequently termed catastrophes. It should be noted, however, that this term is applied from a strictly human perspective.
What may be catastrophic to humans, such as a hurricane, wildfire, landslide, tsunami, or an avalanche are basically natural processes that can produce openings (gaps) in a region’s vegetative mosaic (Fig. 9.16).
The resulting succession, whether primary or secondary, produces a diverse set of patch habitats within the regional matrix of vegetation.
Natural catastrophes and the resulting patch dynamics they create among an area’s plant and animal residents are topics of much interest and research in modern landscape biogeography.
Biotic Factors
Although their influence on a particular species might tend to be overlooked, other plants and animals may also affect whether a given organism exists as part of an ecosystem.
Some interactions between organisms are beneficial to both species involved; this is called a symbiotic relationship. However, other relationships may be directly competitive and have an adverse effect on one or both species.
Because most ecosystems are suitable for a wide variety of plants and animals, there is always competition among species and also members of a given species to determine which organisms will survive.
The greatest competition occurs between species that occupy the same ecological niche. Among plants, there is great competition for light.
The dominant trees in the forest are those that grow tallest and partially shade the plants growing beneath them.
Other competition occurs underground, where the roots compete for soil water and plant nutrients.
Interactions between animals and plants as well as competition both within and among animal species also can significantly affect an ecosystem.
Many animals are helpful to plants through pollination or seed dispersal, and plants are the basic food supply for many animals. Grazing may also influence the species that make up a plant community.
During dry periods, herbivores may be forced to graze an area very closely and the taller plants are grazed out. Plants that are unpalatable, that have thorns, or that have the strongest root development are the ones that survive.
Grazing is a part of the natural selection process, yet serious overgrazing rarely results under natural conditions because wild animal populations increase or decrease with the available food supply. For most animals, predators are another control of population numbers.
Human Impact on Ecosystems
Throughout history, humans have modified ecosystems and their natural development. Except in regions too remote to be altered significantly by civilization, humans have eliminated or had a significant impact on much of Earth’s natural vegetation.
Farming, fire, domesticated animal grazing, deforestation, road building, urbanization, dam building and irrigation, impacts on water resources, mining, and the draining or infilling of wetlands are a few examples of how humans have modified plant communities.
Overgrazing by domesticated animals can seriously harm marginal environments in arid and semiarid climates.
Trampling and soil compaction by grazing herbivores may reduce the soil’s ability to absorb moisture, leading to increased surface runoff of precipitation.
In turn, the decreased absorption and increased runoff may lead to land degradation and gully erosion.
As humans alter ecosystems and natural environments, the changes can often produce negative effects on humans themselves. The desertification of large semiarid sections of East Africa has resulted periodically in widespread famine (Fig. 9.17).
Elsewhere, the continuing destruction of wet lands not only eliminates valuable plant and animal communities but also threatens the water supply quality and reliability for the people who drained the land.
ASSIGNMENT
Define the under-listed and look up how these weather elements are measured, the various instruments used to measure each one, and the units in which they are measured.
Air temperature
Air pressure
Humidity
Clouds
Precipitation
Visibility
Wind (wind speed & direction)
