Streamsare usually well-oxygenated at their headwaters. There are usually no plankton - drifting organisms that are often microscopic - in the turbulent headwaters of a stream. Green algae, diatoms and water mosses may be attached to stones or other objects in the water. These organisms may completely cover the bottom of the stream. These organisms are eaten by insect larvae, which are themselves eaten by small fish. Insects continually fall into the stream, and rain washes detritus (dead organic matter) into it as well. Whatever is not eaten immediately washes further downward, so there is very little food for detritivores (also known as saprovores or saprophages), organisms that eat detritus, in the headwaters. As the water moves on, it begins to move more slowly. The bed of the stream becomes larger, and the total volume of water increases. Some of the sediment from upstream is deposited. Dead organic matter accumulates, so there is now food for detritivores to consume. As the stream grows wider and starts to become a river, less water is shaded by the trees belong the bank. This means that direct sunlight can reach most of the surface of the water. As the levelof light increases, the rateof photosynthesis increases. There are some plankton, but many plankton organisms are swept downstream. Plants with roots grow in the sediments at the bottom. During floods, these plants may be washed away. Snails, mussels, crayfish and insect larvae live at the bottom. These may be eaten by perchandtrout. Leeches may feed on these fish. As a river comes close to the sea, it usually slows down even further. It drops large amounts of sediment. Banks along the lower reaches of the river may grow higher than the land behind it, forming natural levees. Waters in this part of a river are usually muddy. Because of this, the amount of sunlight that can penetrate the water is reduced, and organisms that depend on photosynthesis cannot live at the bottom. However, many plankton - some of which perform photosynthesis - live in the waters near the surface. Floating plants and emergent plants - plants that extend from below the water into the air - grow in the swampy lands along a river. During floods, the fruits and seeds of these plants are swept into the river. Large predators at the lower end of a river eat zooplankton - animal forms of plankton. Large fish, large crustaceans and large mollusks live at this end of the river. Many birds and mammals come here to obtain food. Crocodiles can be found at the lower ends of tropical rivers.
The nitrogen cycle is the movement of nitrogen from the Earth's soil to the atmosphere and back again. Most of the Earth's atmosphere - about 78% - is made up of nitrogen (N2). Most living things, including human beings, cannot use it in this form. When we breathe, we inhale nitrogen and then exhale it. We don't use the nitrogen that we take in from the air. However, nitrogen is necessary for life. All organisms contain nitrogen compounds. Nitrogen is an essential component of proteins and nucleic acids (DNA and RNA). Many microorganisms that live in soil decompose proteins, breakingit down into simpler materials. During this process, ammonia (NH3) is produced as a waste product. When ammonia, which is a gas, is dissolved in soil water, it reacts with hydrogen ions to form ammonium ions (NH4+). Sometimes, the roots of plants will absorb these ammonium ions and convert them back into proteins. However, plants often do not absorb ammonium ions directly. Instead, bacteria in soil convert ammonium ions to nitrates, which are the main source of nitrogen for most plants. Soil contains two groups of bacteria known as nitrifying bacteria. One group converts ammonium ions to nitrite ions (N02-). The second group then changes these nitrite ions into nitrate ions (NO3-). Nitrifying bacteria only break down ammonium ions when there is oxygen in the soil water.
Oxygen dissolves into soil water from the air spaces that normally occur in soil. However, if all the spaces become filled with water, leaving no room for air, then the soil water has no source of oxygen, and nitrifying bacteria cannot do this. In anaerobic conditions (when there is no oxygen available) a group of bacteria known as denitrifying bacteria change remaining nitrates to nitrogen gas, which gradually escapes into the atmosphere. Lightning changes smalls amount of gaseous nitrogen to nitrogen compounds. When these are dissolved in rainwater, they form weak nitrous and nitric acids. In soil, these combine with other elements to form nitrates and nitrites. Some bacteria, known as nitrogen-fixing bacteria, change nitrogen from the atmosphere into nitrogen compounds. They play an important role in returning gaseous nitrogen to the soil.
A pond is a relativelysmall body of water. Ponds are usually smaller than lakes.
Ponds contain standing water. However, most ponds are fed by springs or streams and most have an outlet, so a small current passes through the water and the water gradually changes. Huge numbers of plankton - drifting organisms that are mostly microscopic in size - float through ponds. Some of these organisms are plants, some are animals, some are bacteria and some are archaea. Bacteria and archaea are the two types of prokaryotes - one-celled organisms that do not have nuclei. Prokaryotes were the first living things to have existed on Earth. Most of the photosynthesis in ponds is performed by plankton known as phytoplankton. Diatoms are usually the most common form of plankton in ponds. Dinoflagellates, a type of plankton that has a flagellum - a tail-like structure that assists with locomotion - are also common in some ponds. Zooplankton, animal forms of plankton, can include rotifers and tinycrustaceans. Enough light penetrates through to the bottom of most ponds so that photosynthesis can take place at the bottom. At the bottom of a pond, Chlorophyta, a type of green algae, may wave filaments back and forth in the current while diatoms turn the mud a golden-green color. Sometimes green algae can become so abundant in a pond that the whole surface of the pond turns green. Large emergent plants - plants that piercethe surface of the water and grow partially in the air - such as irises, may grow around the edge of a pond, but there usually aren't any rooted plants growing in the center of a pond. Plants may grow within the water, some floating on the surface and others remaining submerged.
Fish can be found mostly around the edge of a pond, where they can hide among plants. Large predatory fish, such as pike, may swim through a pond in search of prey. Fish may eat insects and other invertebrates, or other fish. Numerous insects can often be found above and around ponds. Frogs eat these insects. Herons, kingfishersandotters come to ponds to eat pond fish. As dead creatures sinkto the bottom of a pond, large amounts of organic matter accumulate at the bottom.
Soil is vital to the many organisms that live in it and to all animals that eat plant food. A soil starts to form when bacteria and small plants, such as mosses, begin to grow in decomposed, weathered rock. Humus, a dark organic material, is addedwhen plants and animals die and rot. Then plant roots, as well as burrowing animals, mix the contents of the new soil, keeping it porous and spongelike. This allows water, air and minerals to circulate. Plants stabilize the soil by their root systems. It takes about 50 years for one centimeter of soil to form. Most soils contain three distinct layers, or horizons. The A horizon (topsoil) consists of decomposed rock and humus. Horizon B (subsoil) is red or brown in color and is made up of clay and iron with little organic material. The C horizon consists of partly decomposed rock. It grades down to solid, unaltered rock. Soil types and colors depend partly on the type of rock from which they develop and partly on the climate of the area in which they occur. Different types of soils include chernozems (black earths), chestnut-brown soils, podsoils, prairie soils and red, tropical laterites.
Most microorganisms that live in soil are detritivores, also known as saprovores or saprophages. Detritivores are organisms that eat detritus - decomposing organic matter. Many detritivores only consume certain substances in a dead organism, rather than the whole body. As they decay, organic substances in soil pass through a food chain, in which the most complex pieces of detritus are reduced to simpler and simpler substances. For example, some detritivores consume cellulose, pectin and lignin- substances found in plant cell walls - from dead leaves that fall on top of soil. They then leave simpler organic substances as waste, and other detritivores will consume this waste. These detritivores will leave even simpler waste products, such as sugars, which then will be used by another group of detritivores. Food continues to pass downward through this food chain until eventually only inorganic materials - minerals, water and carbon dioxide - are left. Many intermediate organic substances that are formed during this process affect the environment of the soil. Many soil organisms create acids as waste products. These tend to accumulate in woodlandsoils, making woodland soil intolerable for many bacteria. However, fungican tolerate acidic soil and therefore thrive in this type of soil. Some microorganisms in soil produce antibiotics, which prevent competing organisms from growing. Some of these, such as Tetracycline and Streptomycin, have been used to fight bacterial infections in human beings.
Actinobacteria, or actinomycetes, are a group of bacteria that are very common in soil. Actinobacteria are the chief agents of decomposition in the relatively dry and alkaline soil of grasslands. In 1900s Martinus Beijerinck, a Dutch botanist and microbiologist, was the first to point out how essential actinomycetes are to soil health. In the 1920s, soil microbiologists discovered that when soil samples are grown in a laboratory, between 30 and 40 percent of the colonies that develop are actinomycetes. Some actinomycetes decompose cellulose, which is one of the most abundant materials in the plant remains, while others act on the substances that result from this decomposition. In 1940, Selman Waksman, an American microbiologist and biochemist discovered that some actinobacteria produce actinomycin, which is an antibiotic. His work led to the discovery of other natural antibiotics, such as streptomycin. He received the 1952 Nobel Prize in Medicine for his work.
Fungi live in many different soils, but they are especially important as agents of decay in woodland soils. They seem to tolerate the acid conditions of woodland soils better than bacteria. If you turn over a few inches of woodland soil, you will probably expose an irregular network of thin grey or white filaments. These filaments are branching fungal cells known as hyphae. Soil fungi decompose cellulose, as well as pectins - substances found in plant cell walls - and chitins, which are found in insect exoskeletons.
Although we often think of algae as water organisms, many species of algae live in soil. Algae may form surface crusts in desert soils and so help reduce soil erosion. In rice paddy soils, algae increase crop yields by adding nitrogen and oxygen to the soil.
Common soil animals include earthworms, nematodes (roundworms), small insects, millipedes, centipedesand mites.
Tree bark protects the tissues of a tree from weather extremes, from disease and from attacks by animals. Bark consists of two layers. The inner layer consists of living tissue. It is known as the phloem. The outer layer is made up of dead tissue. The phloem transforms sugars from the leaves to other parts of the tree. The outer layer is waterproof, which prevents the living tissues from drying out. In parts of the world where there are seasonal forest fires, some trees have very thick bark, which acts as insulation against heat. The cork oak of the Mediterranean region is an example of one of these trees. Trees with thin barks often have much thicker barks near the base of the trunk. This helps protect the tree against large herbivores.
How Tree Bark is Formed
Underneath tree bark, there is a layer of wet, green tissue known as bark cambium or cork cambium. The bark cambium creates corky cells. The bark cambium, together with these cells, is known as the periderm. When a tree is young, the periderm first appears in the outer tissues of a shoot. Waxes and other materials in the periderm cause the color of the shoot to change, usually from green to grey.
In most trees, a succession of periderms arise one after another, each coming from deeper and deeper layers of the stem. Periderms contain lenticels, small pores that allow gases to be exchanged with the outer atmosphere. As each periderm forms, the tissue layers outside it die because they can no longer receivewater or nutrients. New layers of bark are always being created in the stem. This causes the bark to increase in size. In some trees, old layers of bark easily peel or break off. With other trees, the older layers remain attached so that the bark becomes very thick. These trees often have cracks in their bark because the trunk grows faster than the outside layers of bark can expand.
Many nutrients are stored in a tree's wood and bark. When a tree dies and then decays, nutrients are returned to the soil, to be recycled and then used by the next generation of trees. It takes about 20 years for a large log to decay completely. Insect activity, as well as the growth of fungi, can speed up the decay of dead wood. As a tree decays, various organisms began to colonize it. The first to arriveare those that invade the tree as it is dying. Next come organisms that live on wood that has recently died. Other creatures, which specialize in different stages of decay, come later, with the last group specializing in wood that has become crumbly. The species that colonize wood in its earlier stages of decay tend to be more specialized than those that colonize it in the later stages.