Element Cycles

In addition to energy, organisms require a supply of materials to live. These chemical elements are available in the biosphere but must be constantly recycled in order to maintain their availability. This section provides an account of some of these cycles.

Q2.30
What are the 'biogeochemical' cycles? Biogeochemical cycles (bio meaning 'life', geo for 'earth' and chemical for the changing of matter from one form to another) are natural processes that recycle elements in various chemical forms from the environment, to organisms, and then back to the environment. Water, carbon, oxygen, nitrogen, phosphorous, and other elements pass through these cycles, connecting the living and non-living components of the Earth. The element cycles are one of the basic life support systems of the biosphere.

Q2.31
What is the water cycle? The water cycle (or hydrological cycle) is the circulation of the waters of the Earth between land, freshwater lakes and rivers, the salt seas and oceans, and the atmosphere. This cycle collects, purifies, and distributes the Earth's fixed supply of water. This cycle comprises several main steps:

• Through evaporation, water on the land and in the oceans is converted by solar energy into water vapour. Winds then move masses of water vapour around the Earth.

• Through condensation, water vapour is turned into water droplets, which form clouds or fog.

• Through precipitation, water returns to the Earth in the form of dew, rain, hail, or snow.

• Through transpiration, water is absorbed by the roots of plants, passed through their stems and other structures, and released from their leaves as water vapour.

• Through run-off, water moves from the land to the sea, or else from the land into the ground where it is stored (from which it eventually returns to the surface or to lakes, streams, and oceans).

Q2.32
What are two important ways that human beings affect the water cycle?

• We can deplete groundwater supplies, causing water shortages and subsidence (sinking of land when groundwater is withdrawn).

• We clear vegetation from land areas. When vegetation is removed, water flows over the ground more quickly, allowing less time for it to sink into the soil. This results in groundwater depletion. This increased surface run-off also results in accelerated soil erosion.

Q2.33
What is the carbon cycle? Carbon is an essential component of all living things. It exists mostly as carbon dioxide in the atmosphere, oceans, and in fossil fuels stored beneath the Earth's surface. The major steps of the carbon cycle are the following:

• Carbon dioxide in the atmosphere is absorbed by plants and converted into sugar, by the process of photosynthesis.

• Animals eat plants, breaking down the sugars and releasing carbon into the atmosphere, oceans or soil.

• Other organisms break down dead plant and animal matter, returning carbon to the non-living environment.

• Carbon is also exchanged between the oceans and the atmosphere. This occurs in both directions at the interface between the air and water. When the level of carbon dioxide in the oceans (atmosphere) exceeds a certain level, it will flow from the higher concentration in the oceans (atmosphere) to the lower concentration in the atmosphere (oceans), maintaining equilibrium.

Q2.34
What is the oxygen cycle? Oxygen composes nearly 21% of the atmosphere. It combines chemically with a multitude of other elements to form important substances such as water, carbon dioxide, plant nutrients and organic substances. The steps of the oxygen cycle are the following:

• Plants release oxygen into the atmosphere as a by-product of photosynthesis.

• Animals take in oxygen and, by the process of respiration, use it to break down sugars obtained from food.

• Carbon dioxide is released by animals and used by plants in photosynthesis.

• Oxygen is also cycled between the oceans and the atmosphere, maintaining oxygen equilibrium.

Q2.35
What are the two major processes connecting the oxygen and carbon cycles, and how do these processes fit together? The two processes are photosynthesis and respiration.

Photosynthesis is the process by which most autotrophic organisms organisms that
manufacture their own food nourish themselves. Green plants, for example, are photosynthetic. Energy from the sun is used to convert carbon dioxide and water into sugars. Sugar is used to form tissues, allowing the organism to grow and reproduce. It is also used as a source of energy. (Heterotrophic organisms organisms that cannot produce their own food, also use sugars, but cannot perform photosynthesis. Instead, they obtain sugars by eating other organisms.)

Respiration is a process carried out in the cells of both plants and animals. Sugars,
whether obtained through photosynthesis or consumption, contain stored (potential) energy which is released when they are burned. This burning takes place in a slow, controlled way within cells, fuelling basic life processes. This conversion of sugars into energy is called respiration; it uses oxygen and produces carbon dioxide.

Through photosynthesis, carbon dioxide is converted into sugar and oxygen, whereas through respiration, oxygen and sugar are converted back into carbon dioxide.

Q2.36
How do humans affect the cycling of oxygen and carbon?

• We breathe, taking oxygen out of the air and adding carbon dioxide.

• We burn fossil fuels, thereby decreasing oxygen levels and adding carbon dioxide to the atmosphere.

• We cut down forests without sufficient replanting. This leaves less vegetation to release oxygen and to absorb carbon dioxide.

• We pollute our oceans, destroying photoplankton (microscopic aquatic plants). This has the same effect less oxygen, more carbon dioxide.

Q2.37
What is the nitrogen cycle? Nitrogen is essential for life; living things need it to make proteins and DNA. Nitrogen gas constitutes about 78% of the atmosphere, but it is useless to most living things in this form. There are three main steps in the cycle that converts nitrogen gas into proteins and DNA and back into nitrogen gas again:

• Certain kinds of organisms (mainly a type of bacteria called cyanobacteria) are able to transform nitrogen gas into water soluble compounds that can be taken up by the roots of plants; this process is called nitrogen fixation.

• Plants then convert these compounds into protein and DNA. Animals get the nitrogen they need by eating plants or other animals.

• When plants and animals die, a series of processes, again mediated by bacteria, converts the nitrogen compounds in the dead matter back into nitrogen gas, completing the cycle.

Q2.38
What are three ways that human beings intervene in the nitrogen cycle?

• We burn fossil fuels. This releases nitrogen compounds into the atmosphere; these compounds can in turn be converted to nitric acid, a component of acid rain.

• We fertilize our crops and lawns. The nitrogen contained in fertilizer is important for agricultural productivity, yet much of it seeps into groundwater or runs off into lakes and rivers. An excessive supply of this nutrient causes rapid growth of algae, beginning a chain of effects that disrupts life in aquatic ecosystems.

• Through certain kinds of harvesting practices, we deplete nitrogen compounds from the soil.

Q2.39
What is the phosphorus cycle? Phosphorus is an essential nutrient of both plants and animals; it is a part of DNA, energy storage molecules, certain fats, and of bones and teeth. It originates in certain types of rock and is usually found in very small amounts. Thus, phosphorus is the limiting factor for plant growth in many soils and aquatic ecosystems. The major steps of the cycle are as follows:

• Phosphorus is released by the slow breakdown or weathering of rocks which contain it.

• Plants take up phosphorus from the soil or water. Animals get the phosphorus they need by eating plants or other animals.

• Plant and animal remains decompose, releasing phosphorus into the soil and water.

Q2.40
What are the two main ways we intervene in the phosphorus cycle?

• We mine large quantities of rock containing phosphorus compounds to produce commercial inorganic fertilizers and detergents.

• We add excess phosphorus to aquatic ecosystems in run-off of animal wastes from livestock feedlots, run-off of commercial fertilizers from cropland, and discharge of both untreated and treated municipal sewage. As with nitrogen compounds, an excessive supply of this plant nutrient causes explosive growth of algae and other aquatic plants that disrupts life in aquatic ecosystems.

Q2.41
How do these cycles interact with each other? All biogeochemical cycles interact with the different levels of the physical planet. Through photosynthesis and respiration the carbon and oxygen cycles are intimately connected. The nitrogen cycle serves to regenerate dead organic material and release carbon and oxygen for further cycling. The phosphorus cycle provides nutrient value for the biological component of the other cycles. The hydrological cycle, finally, acts as a conduit by which all of these elements and cycles are transported through the system. The interaction of all of these elements and cycles make up a system that we call the biosphere.

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Bird guano: more than meets the eye
Phosphorus is an important constituent of life, present in the biosphere only in rock, deep oceans, and in dust in the atmosphere. On small islands off the coasts of South America and Africa, however, accumulations of dried bird excrement guano have been a source of commercial phosphorus for many years. The existence of these stocks of phosphorus near the planet's surface is dependent upon a series of processes involving the particular geography, climate, and species found on these islands.

• Phosphorus settles in the ocean as a result of erosion on land.

• Upwelling currents near the islands carry phosphorus from deep water to the surface.

• The phosphorus is stored by algae, which grow abundantly near the surface, and is passed on through the food chain to planktonic animals, fish, and finally birds.

• The islands are a favoured nesting area for many sea birds because they provide protection from predators. Large colonies of birds have nested on these islands for thousands of years, resulting in the accumulation of bird excrement rich in phosphorus.

Climate

Climate processes are an essential aspect of our planet's life support system. This section explains some of the basic facts and concepts related to climate, and their importance for life.

Q2.42
What is climate? Every place on Earth experiences a distinctive and fairly regular pattern of weather from year to year. There is variation, but generally the amount of sunshine and rain, the range of temperatures, and other aspects of the weather are similar from one year to the next. It is this 'average weather' that we call climate.

Q2.43
Why do we have different sorts of weather and different climates? Weather is caused primarily by the exchange of energy between the sun, the Earth's surface, atmosphere, and oceans. The sun supplies the energy; the Earth's shape and motion determine how much sunlight and heat are received and in what places. The atmosphere and oceans redistribute this energy, balancing the pools of heat and cold between the equator and the poles. Other factors which influence weather and climate include surface water, the shape of the land, and the nature of ground cover.

Q2.44
Why is climate important? Climate affects all living things on the planet. It regulates the life cycles of plants and animals, affects their growth and vitality, and is a principal factor in determining how they distribute themselves around the globe. Almost all complex life forms are adapted to live in a specific, relatively narrow climatic area.

Q2.45
What is the greenhouse effect? The 'greenhouse effect' is the popular term for one of the functions of the atmosphere plays in the global ecosystem. The Earth receives energy from the sun in the form of short-wave radiation. It absorbs this energy and re-emits it in the form of long-wave infrared radiation. Some infrared radiation escapes into space immediately; most, however, is trapped by water vapour and various other gases, warming the lower atmosphere. As a result of this process, the surface temperature of the Earth is higher than it would be if all of the radiation was allowed to escape immediately. The gases which prevent infrared radiation from escaping are called 'greenhouse gases'.

Q2.46
What role does the greenhouse effect play in the global ecosystem? By retaining heat within the lower atmosphere, the greenhouse effect makes it possible for life to thrive. The present average temperature on Earth is about 15oC. Without the natural greenhouse effect, the average temperature of the Earth would be 33 degrees colder than it is now: minus 18oC.

Q2.47
How does the water cycle affect climate? The water cycle plays a major but complex role in the Earth's climate. In all of its forms vapour, clouds, liquid, snow, and ice water is an important part of climate processes. Evaporation, for example, accounts for 50% of all surface cooling. Water vapour in the atmosphere, on the other hand, is a powerful greenhouse gas. Clouds reflect incoming solar radiation, reducing the sun's input of energy into the environment. Yet they also trap part of the energy the Earth emits. Due to the number and complexity of the processes at work, the overall effect of the water cycle on climate is not yet known.

Q2.48
What role do oceans play in climate processes? Since they cover so much of the Earth's surface, it is not surprising that oceans influence climate greatly. Since water loses and gains heat more slowly than air, oceans provide a moderating effect on the climate of coastal areas. For the biosphere as a whole, however, oceans play two major roles. First, they distribute heat around the globe. In the subtropics, ocean currents transport warm water toward the poles (as does the atmosphere). This balances the extra solar radiation that the equatorial regions receive. Second, oceans provide long-term memory for the climate. Whereas water vapour stays in the atmosphere an average of 10 days (before moving on to another part of the biosphere), the average 'residence time' of water in the oceans is 3,000 years. Along with land glaciers and ice caps, these reservoirs of slowly moving water introduce considerable delay into the response of the climate system to the forces of change.

Ecosystems

Although we speak of individual organisms as being either alive or dead, sustained life is, in fact, a property of systems involving many individuals and species and their non-living environment ecosystems. It is for this reason that people often speak of the 'web of life'. Despite our particularities, humans are part of this web. This section explains the ecosystem concept and examines some of the key biological processes that go on within ecosystems.

Q2.49
What is required to sustain life? All forms of life require energy and materials for their existence. The biosphere receives energy from the sun and, in very small amounts, from the Earth's core. The biosphere also contains the various chemical elements that are necessary for life. In the process of living, organisms use and give off energy while recycling materials.

Q2.50
What is an ecosystem? An ecosystem consists of a community of living organisms and their local physical environment. The living and non-living elements of an ecosystem are connected through flows of energy and the cycling of chemical elements. No part of the biosphere that is smaller than an ecosystem can sustain life.

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Imagine placing a single green plant in a glass jar with air, water, soil, and light from the sun. The plant could make its own food through photosynthesis (from water, carbon dioxide, and light). But eventually the plant would die as it used up all of the nutrients in the soil. It would die because it couldn't recycle the materials it needs to live; no green plant can decompose its own products into the inorganic compounds necessary for plant life. In fact, no single organism, population, or species is able to produce all of its own food and recycle all of its metabolic products. Life requires the interaction of several species in an environment that includes air and/or water to transport materials and energy. This is one of the fundamental principles of ecology; sustained life is a property of ecosystems, not organisms or species.

Q2.51
Is there a difference between a population and a community? Yes. A population consists of interbreeding members of one species living in a specific area, more or less isolated from other members of their species. A community consists of a collection of populations of different species interacting within a specific area, functioning more or less as a unit with certain identifiable characteristics.

Q2.52
What is an example of an ecosystem? A forest is a common ecosystem. Forests consist of air, soil, water, nutrients, and particular species of animals, birds, insects, micro-organisms, trees, and other plant life. If some of the trees are cut down, each of the other elements will be affected. Animals and birds may lose their habitats, soil may erode, nutrients may be displaced, and the flow of waterways may change.

Q2.53
What characteristics does an ecosystem have? Ecosystems can be looked at in terms of: (1) their structure, (2) the processes that go on within them, or (3) the way they change over time. Every ecosystem is made up of a set of physical components (soil, minerals, water, etc.) and a set of populations of different species. Secondly, energy flows through ecosystems, and chemical elements cycle within them. Finally, ecosystems go through different patterns of change over time.

Q2.54
Why is the ecosystem concept important? The concept of an ecosystem is important because it conveys one of the key insights that we have gained from the science of ecology everything is related to everything else. Everyday perception tells us that we live in a world composed of distinct units: trees, rocks, animals, buildings, and so on. Yet all of these seemingly unconnected fragments are in fact part of one system; they are interrelated, and this interrelation is essential for life. Since no piece exists independently of another, none can be modified without affecting the others. It is this idea that is behind the term 'ecosystem.'

Q2.55
Are there clear boundaries between different ecosystems?
There are no absolute boundaries between ecosystems. Some ecosystem borders are, however, quite well defined. For example, there is a relatively clear transition from a rocky ocean coast to the forest along its edge, or from a pond to the wood that surrounds it. Other borders are much more vague, as is the case with the gradual transition from deciduous forests in the south to coniferous forests farther north.

Q2.56
Are all ecosystems the same size? No. Everything from a local wetland to the whole of the western prairie can be described as an ecosystem. The great vegetation regions of the world are probably the most familiar large-scale ecosystems (arctic tundra and tropical rainforests, for example). One can even consider the biosphere itself as one big ecosystem, although we often adopt a much more local perspective.

Q2.57
What is a food web? One of the ways organisms interact is by eating each other. A food 'web' is a model of who eats what or, in more scientific terms, a model of the flows of energy and chemical elements through organisms in an ecosystem. A food chain is one particular sequence of organisms in this web. All organisms are part of at least one food chain.

Q2.58
What are the two main types of food chain? The first type of food chain begins with green plants and extends to herbivores, carnivores, and omnivores. The second type begins with a base of dead organic matter the remains of dead plants and animals and proceeds to a variety of other organisms including scavengers, insects, and micro-organisms.

Q2.59
How are these two types of food chains linked? Dead organisms from the first type of food chain provide the organic matter that forms the basis of the second. Decomposers from this second type of chain break up organic matter into the inorganic compounds that are vital nutrients for the first.

Q2.60
Do ecosystems change over time? Yes. This is referred to as 'succession'; ecosystems change over time according to certain recognizable, repeatable patterns. The composition of species of trees in a forest, for example, changes in a relatively predictable way over time. It was once thought that the final stage of succession was a steady-state, 'climax community' that remained constant unless disturbed (by humans, for example). However, it is now known that even 'mature' ecosystems change over time.

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The living organisms of the world can be divided into two categories producers and consumers depending upon the way they nurture themselves. Producers (or 'autotrophs') produce their own food from inorganic compounds and a source of energy. The most common and most familiar autotrophs are green plants, which manufacture their food with energy from the sun and compounds from soil. Consumers (or 'heterotrophs') nourish themselves by feeding on other organisms or dead organic matter. Consumers are either:

• herbivores, who eat plants,
  
• carnivores, who eat animals,
  
• omnivores, who consume both plant and animal matter, or
  
• detritivores, who feed on dead plant and animal matter.
  

One kind of detritivore, the decomposer, breaks down dead organic matter into inorganic compounds that can be used again as food by plants. Familiar examples of decomposers are bacteria and fungi.

Evolution and Extinction

Over the course of the Earth's history, millions of different species have evolved, flourished, and then become extinct. Our species was born during the period of the greatest diversity of species the planet has ever witnessed. This section explores the concepts of evolution and extinction, as well as their history and likely future.

Q2.61
What is evolution? Evolution is the process by which life developed on Earth; the species that exist today are the product of evolution. It is a process that continues today.

Q2.62
Is evolution a theory or a fact? It is both. Evidence that species have changed over the course of the Earth's history is provided by the fossil record, by patterns of physiological and biological similarity in organisms, and by laboratory experiments demonstrating the ability of living things to mutate over the course of generations. In this sense evolution is a fact. Theories of evolution try to explain why and how it took place. The fact of evolution is not disputed by scientists, although there are still vigorous debates in evolutionary theory.

Q2.63
What is natural selection? Natural selection is the basic mechanism of evolution. All living organisms are adapted to a specific set of environmental conditions. Within every species, however, genetic variation leads to a degree of variation in physical characteristics between different individuals. Some variations allow those who possess them to function more effectively in their particular environment, giving them a greater chance of living long enough and being healthy enough to produce offspring. This is natural selection; genetic variations that improve the adaptation of an organism to its environment have a better chance of being passed on than variations that hinder adaptation, simply because better adapted organisms are more likely to reproduce.

Q2.64
What is an example of natural selection? Some giraffes have taller necks than others. Having a longer neck opens up higher levels of foliage for feeding, and, during a drought, this may make the difference between life and death. Having a greater change of survival, long-necked giraffes are more likely to reproduce. Offspring of long-necked giraffes are also likely to have long necks, since characteristics resulting from genetic makeup are passed on from parent to offspring (unlike traits acquired in the course of one's life such as large muscles from hard work). If over a long period of time it remains advantageous to have a long neck, then the long-necked variety of giraffe will become dominant.

Q2.65
What is speciation? Speciation is the process by which new species are born. A new species is born when a population of a species evolves sufficiently that interbreeding with other populations of what used to be the same species becomes first unusual then impossible. One way this often happens is through geographic isolation. When a population becomes physically isolated from other populations of the same species, adaptive variation particular to it and its environment may in time lead to the development of two different species.

Q2.66
What is extinction? Extinction is the opposite of speciation; it is the process by which species die out. It is a natural process, which has occurred throughout the history of the Earth at almost the same rate as species have originated. The incredible variety of species that inhabit the Earth today represents a tiny fraction of the species that have ever existed.

Q2.67
Has the rate of extinction been constant? No. There have been a number of mass extinctions in the Earth's history. These were relatively short periods of time wherein a great number of species died. The most serious of these, 250 million years ago, killed off more than three quarters of the species in the oceans and had a significant effect on terrestrial life as well. The most recent mass extinction occurred 65 million years ago, killing off the last of the dinosaurs. Our own species was born during the period of the greatest diversity of species in the Earth's history.