PLATE TECTONIC THEORY

                                

Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen "plates" that move around across the earth's surface relative to each other, like slabs of ice on a lake.

The drawing above is a cross section of the earth showing the components that lie within plate tectonic theory. The cross section should really be curved to correspond to the earth's curvature, but it has been straightened out here. 

Note the continental craton (stable continent) in the middle of the drawing. Note the line under the craton; that is the lower boundary of the plate. Everything above that line is the plate. All similar lines in the cross section mark the bottom of the plates. Technically, everything above that line is lithosphere, the rigid, brittle shell of the earth. Everything below is asthenosphere, the hot, plastic interior of the earth.

Within the asthenosphere are convection cells, slowly turning over hot, plastic rock. The convection cells bring heat from the earth's interior out to the surface, but slowly. Movement is about 10 centimeters a year. When the convection cells reach the base of the lithosphere they release heat to the surface at the divergent plate boundary to escape to space. The cooled plastic rock then turns sideways and moves parallel to the earth's surface before descending back into the earth at subduction zones to become reheated. It is this turning over of the convection cells the drives the plate movements. 

THE NITROGEN CYCLE

The nitrogen cycle represents one of the most important nutrient cycles found in terrestrial ecosystems (Figure 9s-1). Nitrogen is used by living organisms to produce a number of complex organic molecules like amino acids, proteins, and nucleic acids. The store of nitrogen found in the atmosphere, where it exists as a gas (mainly N2), plays an important role for life. This store is about one million times larger than the total nitrogen contained in living organisms. Other major stores of nitrogen include organic matter in soil and the oceans. 

Despite its abundance in the atmosphere, nitrogen is often the most limiting nutrient for plant growth. This problem occurs because most plants can only take up nitrogen in two solid forms: ammonium ion (NH4+ ) and the ion nitrate (NO3- ). Most plants obtain the nitrogen they need as inorganic nitrate from the soil solution. Ammonium is used less by plants for uptake because in large concentrations it is extremely toxic. Animals receive the required nitrogen they need for metabolism, growth, and reproduction by the consumption of living or dead organic matter containing molecules composed partially of nitrogen.

Figure 9s-1: Nitrogen cycle.

In most ecosystems nitrogen is primarily stored in living and dead organic matter. This organic nitrogen is converted into inorganic forms when it re-enters the biogeochemical cycle via decomposition. Decomposers, found in the upper soil layer, chemically modify the nitrogen found in organic matter from ammonia (NH3 ) to ammonium salts (NH4+ ). This process is known as mineralization and it is carried out by a variety of bacteria, actinomycetes, and fungi.

THE CARBON CYCLE

All life is based on the element carbon. Carbon is the major chemical constituent of most organic matter, from fossil fuels to the complex molecules (DNA and RNA) that control genetic reproduction in organisms. Yet by weight, carbon is not one of the most abundant elements within the Earth's crust. In fact, the lithosphere is only 0.032% carbon by weight. In comparison, oxygen and silicon respectively make up 45.2% and 29.4% of the Earth's surface rocks.

Carbon is stored on our planet in the following major sinks (Figure 9r-1 and Table 9r-1): (1) as organic molecules in living and dead organisms found in the biosphere; (2) as the gas carbon dioxide in the atmosphere; (3) as organic matter in soils; (4) in the lithosphere as fossil fuels and sedimentary rock deposits such as limestone, dolomite and chalk; and (5) in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms.

Figure 9r-1: Carbon cycle.

Table 9r-1: Estimated major stores of carbon on the Earth.

Sink	Amount in Billions of Metric Tons
Atmosphere	578 (as of 1700) - 766 (as of 1999)
Soil Organic Matter	1500 to 1600
Ocean	38,000 to 40,000
Marine Sediments and Sedimentary Rocks	66,000,000 to 100,000,000
Terrestrial Plants	540 to 610
Fossil Fuel Deposits	4000

Ecosystems gain most of their carbon dioxide from the atmosphere. A number of autotrophic organisms have specialized mechanisms that allow for absorption of this gas into their cells. With the addition of water and energy from solar radiation, these organisms use photosynthesis to chemically convert the carbon dioxide to carbon-based sugar molecules. These molecules can then be chemically modified by these organisms through the metabolic addition of other elements to produce more complex compounds like proteins, cellulose, and amino acids. Some of the organic matter produced in plants is passed down to heterotrophic animals through consumption.

SOIL ORGANIC MATTER DECOMPOSITION AND NUTRIENT CYCLING

In most terrestrial ecosystems the bulk of nutrient cycling occurs in the topmost layers of soil. The main sources of the nutrient inputs to these soil layers comes from weathering, rainfall, fertilizers, atmospheric fallout, and organisms. Organism add nutrient matter via excreted wastes, shed tissues, and from the decomposition of their tissues when they die. Under most conditions, plants are the greatest single source of nutrients to soils. 

Plants not only supply nutrients released by organic decomposition of shed tissues and dead body parts, but also substances carried in from the plant leaves when water flows over them (foliar leaching). Losses or outputs of nutrients within ecosystems are by leaching, erosion, gaseous loss (like denitrification), and plant root uptake for growth purposes. Within the soil, nutrients are found attached to the surface of soil particles by chemical bonds, stored within the chemical structure of dead organic matter, or in chemical compounds.

Organic matter decomposition is the main process that recycles nutrients back into the soil. Decomposition of organic matter begins with large soil organisms like earthworms, arthropods (ants, beetles, and termites), and gastropods (slugs and snails). These organisms breakdown the organic matter into smaller pieces which can be decomposed by smaller organisms like fungi and heterotrophic bacteria (Figure 9q-1).

Figure 9q-1: Fungi play an important role in the decomposition process converting organic matter back into basic inorganic chemicals.

CITATION

Pidwirny, M. (2006). Fundamentals of Physical Geography, 2nd Edition. 11/1/2012. http://www.physicalgeography.net/fundamentals/9g.html 

BIOCHEMICAL CYCLING

The patterns of cycling nutrients in the biosphere involves both biotic and abiotic chemical reactions. Understanding the biogeochemical cycle of any biologically important element requires the knowledge of chemical processes that operate in the biosphere, lithosphere, atmosphere, and hydrosphere.

The biogeochemical cycles of all elements used by life have both an organic and an inorganic phase. For most of these nutrients, how efficiently these elements cycle from the organic component back to the inorganic reserviors determines how much is available to organisms over the short term. This cycling involves the decomposition of organic matter back into inorganic nutrients. The major reservoirs for all metabolically important elements are found either in the atmosphere, lithosphere (mainly rock, soil and other weathered sediments) or hydrosphere. Flow from these reservoirs to the organic phase is generally slower than the cycling of nutrients through organic matter decomposition.

TROPHIC PYRAMIDS AND FOOD WEBS

So far we have described food chains as morphological systems of energy flow. The energy flow within food chains can also be described in more quantitative terms. Several different quantitative models are commonly seen in the academic literature. One of these models, called a pyramid of biomass, quantifies all of the living biomass found in each of the trophic levels. Biomass can be defined as the weight of living matter (usually measured in dry weight per unit area). Figure 9o-1 describes the pyramid of biomass for an aquatic community living in a shallow experimental pond.

Figure 9o-1: Pyramid of biomass for a pond. (Source: Data from Whittaker, R.H. 1961. Experiments with radiophosphorus tracer in aquarium microcosms. Ecological Monographs 31:157-188).

ORGANIC DECOMPOSITION AND THE DETRITUS FOOD CHAIN

The detritus food chain differs from the grazing food chain in several significant ways. First, the organisms making it up are in general smaller in size. Second, the functional roles of the different organisms do not fall as neatly into categories like the grazing food chain's trophic levels. Finally,detritivores live in environments (soil, sea bed, etc.) that are rich in scattered food particles. As a result, decomposers are less mobile than herbivores or carnivores.

The organisms of the detritus food chain include members of many different species of animals and plants, such as algae, bacteria, slime molds, fungi, protozoa, insects, mites, crustaceans, centipedes, mollusks, worms, sea cucumbers, and even some vertebrates (Figure 9n-1). These organisms consume organic wastes, shed tissues, and the dead bodies of both plants and animals.

Figure 9n-1: Earthworms are one of the most important soil decomposers. These organisms consume vast amounts of organic matter and mineral soil. As the organic matter passes through their digestive system, it is subjected to digestive enzymes and the grinding action of mineral soil particles. The amount of material consumed per day is often equal to their body weight.

PRODUCTION BY CONSUMERS AND THE GRAZING FOOD CHAIN

Production by Consumers

Biological communities also include organisms that consume biomass for their nutrition. Such organisms include herbivores, carnivores, and detritivores. These organisms obtain their energy through respiration, a process that releases energy from organic molecules like glucose. The equation for respiration is described below:

C6H12O6 + 6O2 >>> 6CO2 + 6H2O + released energy

The amount of energy actually used by these animal populations is significantly less than the amount consumed. In all animals, digestion is an imperfect process and only a portion of the energy ingested is actually assimilated and then used for body maintenance, growth and reproduction. The remaining portion leaves the organism's body undigested.

We sometimes call the energy assimilated by consumer organisms the gross secondary productivity. Gross secondary productivity can be determined directly, unlike gross primary productivity, by measuring the amount consumed minus the material defecated.

PRIMARY PRODUCTIVITY OF PLANTS

The bodies of living organisms within a unit area make up a standing crop of biomass. More specifically, biomass can be defined as the mass of organisms per unit area and is usually expressed in units of energy (e.g., joules m-2) or dry organic matter (e.g., tons ha -1 or grams m -2). Most of the biomass in a community is composed of plants, which are the primary producers of biomass because of their ability to fix carbon through photosynthesis. This chemical reaction can be described by the following simple formula:

6CO2 + 6H2O + light energy >>> C6H12O6 + 6O2

The product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released into the atmosphere (Figure 9l-1). All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight (Figure 9l-1). This chemical reaction is catalyzed by chlorophyll acting together with other pigment, lipid, sugar, protein, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage, or it can be combined with other sugar molecules to form specialized carbohydrates, such as cellulose. Sugars can also be combined with other nutrients such as nitrogen, phosphorus, and sulfur, to build complex molecules such as proteins and nucleic acids.

Figure 9l-1: Inputs and outputs of the photosynthetic process.

The primary productivity of a community is the amount of biomass produced through photosynthesis per unit area and time by plants, the primary producers. Primary productivity is usually expressed in units of energy (e.g., joules m -2 day -1) or in units of dry organic matter (e.g., kg m -2 year -1). Globally, primary production amounts to 243 billion metric tons of dry plant biomass per year. 

CHARACTERISTICS OF THE EARTH'S TERRESTRIAL BIOMES

Many places on Earth share similar climatic conditions despite being found in geographically different areas. As a result of natural selection, comparable ecosystems have developed in these separated areas. Scientists call these major ecosystem types biomes. The geographical distribution (and productivity) of the various biomes is controlled primarily by the climatic variables precipitation and temperature. The map in Figure 9k-1describes the geographical locations of the eight major biomes of the world. Because of its scale, this map ignores the many community variations that are present within each biome category.

Figure 9k-1: Distribution of the Earth's eight major terrestrial biomes. Legend is below. (Adapted from: H.J. de Blij and P.O. Miller. 1996. Physical Geography of the Global Environment. John Wiley, New York. Pp. 290.)

Most of the classified biomes are identified by the dominant plants found in their communities. For example, grasslands are dominated by a variety of annual and perennial species of grass, while deserts are occupied by plant species that require very little water for survival or by plants that have specific adaptations to conserve or acquire water.

ECOSYSTEM

Ecosystem is defined as a dynamic entity composed of a biological community and its associated abiotic environment. Often the dynamic interactions that occur within an ecosystem are numerous and complex. Ecosystems are also always undergoing alterations to their biotic and abiotic components. Some of these alterations begin first with a change in the state of one component of the ecosystem which then cascades and sometimes amplifies into other components because of relationships.

In recent years, the impact of humans has caused a number of dramatic changes to a variety of ecosystems found on the Earth. Humans use and modify natural ecosystems through agriculture, forestry, recreation, urbanization, and industry. The most obvious impact of humans on ecosystems is the loss of biodiversity. The number of extinctions caused by human domination of ecosystems has been steadily increasing since the start of the Industrial Revolution. The frequency of species extinctions is correlated to the size of human population on the Earth which is directly related to resource consumption, land-use change, and environmental degradation. Other human impacts to ecosystems include species invasions to new habitats, changes to the abundance and dominance of species in communities, modification of biogeochemical cycles, modification of hydrologic cycling, pollution, and climatic change.

PLANT SUCCESSION

Succession is a directional non-seasonal cumulative change in the types of plant species that occupy a given area through time. It involves the processes of colonization, establishment, and extinction which act on the participating plant species. Most successions contain a number of stages that can be recognized by the collection of species that dominate at that point in the succession. Succession begin when an area is made partially or completely devoid of vegetation because of a disturbance. Some common mechanisms of disturbance are fires, wind storms, volcanic eruptions, logging, climate change, severe flooding, disease, and pest infestation. Succession stops when species composition changes no longer occur with time, and this community is said to be a climax community.

The concept of a climax community assumes that the plants colonizing and establishing themselves in a given region can achieve stable equilibrium. The idea that succession ends in the development of a climax community has had a long history in the fields of biogeography and ecology. One of the earliest proponents of this idea was Frederic Clements who studied succession at the beginning of the 20th century. However, beginning in the 1920s scientists began refuting the notion of a climax state. By 1950, many scientists began viewing succession as a phenomenon that rarely attains equilibrium. The reason why equilibrium is not reached is related to the nature of disturbance. Disturbance acts on communities at a variety of spatial and temporal scales. Further, the effect of disturbance is not always 100 percent. Many disturbances remove only a part of the previous plant community. As a result of these new ideas, plant communities are now generally seen as being composed of numerous patches of various size at different stages of successional development.

SPECIES DIVERSITY AND BIODIVERSITY

Biologists are not completely sure how many different species live on the Earth. Estimates of how many species exist on the Earth range from low of 2 million to high of about 100 million. To date, about 2.1 million species have been classified, primarily in the habitats of the middle latitudes. Most of the unclassified species on this planet are invertebrates. This group of organisms includes insects, spiders, mollusks, sponges, flatworms, starfish, urchins, earthworms, and crustaceans. These species are often difficult to find and identify because of their small size and the fact that they live in habitats that are difficult to explore. In the tropical rain forest, the cataloging of species has been quite limited because of this later reason. Scientists estimate that this single biome may contain 50 to 90% of the Earth's biodiversity.

Many species have gone extinct over the Earth's geologic history. The primary reason for these extinctions is environmental change or biological competition. Since the beginning of the Industrial Revolution, a large number of biologically classified species have gone extinct due to the actions of humans. This includes 83 species of mammals, 113 species of birds, 23 species of amphibians and reptiles, 23 species of fish, about 100 species of invertebrates, and over 350 species of plants. Scientists can only estimate the number of unclassified species that have gone extinct. Using various methods of extrapolation, biologists estimate that in 1991 between 4000 to 50,000 unclassified species became extinct, mainly in the tropics, due to our activities. This rate of extinction is some 1,000 to 10,000 times greater than the natural rate of species extinction (2 - 10 species per year) prior to the appearance of human beings. The continued extinction of species on this planet by human activities is one of the greatest environmental problems facing humankind.

CONCEPT OF ECOLOGICAL NICHE

For a species to maintain its population, its individuals must survive and reproduce. Certain combinations of environmental conditions are necessary for individuals of each species to tolerate the physical environment, obtain energy and nutrients, and avoid predators. The total requirements of a species for all resources and physical conditions determine where it can live and how abundant it can be at any one place within its range. These requirements are termed abstractly the ecological niche.

G.E. Hutchinson (1958) suggested that the niche could be modeled as an imaginary space with many dimensions, in which each dimension or axis represents the range of some environmental condition or resource that is required by the species. Thus, the niche of a plant might include the range of temperatures that it can tolerate, the intensity of light required for photosynthesis, specific humidity regimes, and minimum quantities of essential soil nutrients for uptake.

A useful extension of the niche concept is the distinction between fundamental and realized niches (Figure 9g-1). The fundamental niche of a species includes the total range of environmental conditions that are suitable for existence without the influence of interspecific competition or predation from other species. The realized niche describes that part of the fundamental niche actually occupied by the species.

Figure 9g-1: The following diagram shows a hypothetical situation where a species distribution is controlled by just two environmental variables: temperature and moisture. The green and yellow areas describe the combinations of temperature and moisture that the species requires for survival and reproduction in its habitat. This resource space is known as the fundamental niche. The green area describes the actual combinations of these two variables that the species utilizes in its habitat. This subset of the fundamental niche is known as the realized niche.

CITATION

Pidwirny, M. (2006). "Concept of Ecological Niche". Fundamentals of Physical Geography, 2nd Edition. 11/1/2012. http://www.physicalgeography.net/fundamentals/9g.html

BIOTIC INTERACTION AND THE DISTRIBUTION OF SPECIES

Interacting species have a tremendous influence on the size of each other's populations. The various mechanisms for these biotic influences are quite different from the way in which abiotic factors effect the size of populations. Biotic factors also regulate the size of populations more intensely. Finally, the influence of biotic interactions can occur at two different levels. Interspecific effects are direct interactions between species, and the intraspecific effects represent interactions of individuals within a single species.

Neutralism

Neutralism is the most common type of interspecific interaction. Neither population directly affects the other. What interactions occur are slight and indirect. The simple presence of the two species should not directly affect the population level of either. An example of neutralism would be the interaction between rainbow trout and dandelions living in a mountain valley.

Competition

When two or more organisms in the same community seek the same resource (e.g., food, water, nesting space, ground space), which is in limiting supply to the individuals seeking it, they compete with one another. If the competition is among members of the same species, it is calledintraspecific. Competition among individuals of different species it is referred to as interspecific competition. Individuals in populations experience both types of competition to a greater or lesser degree.

ABIOTIC FACTORS AND THE DISTRIBUTION OF SPECIES

GEOGRAPHIC RANGE

Each species on our planet occupies a unique geographic range where the members of its various populations live, feed, and reproduce. Some species have extensive geographic ranges that stretch over several continents. Species with such distributions are known as cosmopolitan species. Other species can have more restricted geographic ranges isolated to a small area on a single continent. This type of distribution is termed endemic.

Geographic ranges of organisms continually shift, expand, and contract with the passage of time. These changes are the result of two contrasting processes: establishment and extinction. The establishment of a species takes place when individuals colonize new areas and are able to maintain reproductively viable populations. New suitable habitats for establishment may open up because of abiotic and biotic environmental change. Species are always attempting to expand their spatial distribution as it betters their chance for long-term survival.

Extinction is a process that eliminates members of a species from all or part of its geographic range. Extinction occurs when large numbers of individuals from a species are killed by biotic interactions or abiotic environmental change. Limited extinctions occurring within small sub-regions of a species’ range are usually quite common.

DISPERSAL AND COLONIZATION

Many of the organisms that inhabit the Earth have the ability to move. This movement can be accomplished by either passive or active means.Active movement requires the organism to use some appendage to initiate walking, running, flying or swimming. In passive movement, the organism uses an external force to cause transit. Many plants use wind passively to disperse seeds over relatively long distances. Oyster larvae can travel hundreds of kilometers by using the power of sea currents.

THE HYDROLOGICAL CYCLE

The hydrologic cycle is a conceptual model that describes the storage and movement of water between the biosphere, atmosphere, lithosphere, and the hydrosphere (see Figure 8b-1). Water on this planet can be stored in any one of the following reservoirs: atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields, and groundwater.

Figure 8b-1: Hydrologic Cycle.

PHYSICAL PROPERTIES OF WATER

We live on a planet that is dominated by water. More than 70% of the Earth's surface is covered with this simple molecule. Scientists estimate that the hydrosphere contains about 1.36 billion cubic kilometers of this substance mostly in the form of a liquid (water) that occupies topographic depressions on the Earth. The second most common form of the water molecule on our planet is ice. If all our planet's ice melted, sea-level would rise by about 70 meters.

Water is also essential for life. Water is the major constituent of almost all life forms. Most animals and plants contain more than 60% water by volume. Without water life would probably never have developed on our planet.

Water has a very simple atomic structure. This structure consists of two hydrogen atoms bonded to one oxygen atom (Figure 8a-1). The nature of the atomic structure of water causes its molecules to have unique electrochemical properties. The hydrogen side of the water molecule has a slight positive charge (see Figure 8a-1). On the other side of the molecule a negative charge exists. This molecular polarity causes water to be a powerful solvent and is responsible for its strong surface tension (for more information on these two properties see the discussion below).

Figure 8a-1: The atomic structure of a water (or dihydrogen monoxide) molecule consists of two hydrogen (H) atoms joined to one oxygen (O) atom. The unique way in which the hydrogen atoms are attached to the oxygen atom causes one side of the molecule to have a negative charge and the area in the opposite direction to have a positive charge. The resulting polarity of charge causes molecules of water to be attracted to each other forming strong molecular bonds.