Biospheres, biomes and ecosystems
The definition of an ecosystem is any area of nature that includes living organisms and non-living substances interacting to produce an exchange of materials between the living and non-living parts. A problem with the definition is that of a boundary, as the definition can be applied on any scale. For example, a pond with its interaction between the water, micro- organisms, invertebrates, fish and amphibians could be defined as an ecosystem. But if water birds that occasionally frequent the pond and feed on fish are included in this ecosystem, then the boundary of the ecosystem expands to include the living area of the water birds. Ecosystems can best be studied by observing the flows of energy matter through an ecosystem. The definition of the biosphere is the part of the earth in which life is permanently possible and which contains all living organisms. This includes the oceans, the surface of continents and the atmosphere. Animal and plant species are classified into communities that are adapted to a particular environment. The major biomes are, the aquatic biome, the desert biome, the forest biome, the grassland biome and the tundra biome.
The Aquatic Biome
Water covers ¾ of the Earth’s surface. A large number of animals and plants are adapted to living in water and it is thought that life began in water billions of years ago. The aquatic biome can be divided into salt water and fresh water.
Fresh water is essential for life on Earth. Fresh water has low salt concentrations. Fresh water environments include ponds, lakes, streams, rivers and wetlands. Ponds and lakes are still water environment, unlike rivers and steams where water flows in a specific direction. These still water bodies can vary from a few metres in size in the case of ponds to thousands of kilometres in the case of lakes. Ponds and lakes may only contain water during the rainy season. In this case they are called seasonal. If they always contain water then they are called permanent. Ponds and lakes may have a small number of different plant and animal species as the ponds and lakes are often separated from each other by long distances and the plants and animals from one pond or lake cannot mix with the plants and animals of another pond lake.
In South Africa, there is only one natural lake. This is Lake Sibaya in Kwazulu Natal.
Ponds and lakes can be classified into zones according to the vertical layers of water.
Uppermost zone of the littoral zone, is the warmest and most productive of the zones as it receives the most sunlight, It is also a zone of high oxygen content. This zone is dominated by algae, macrophytes, snails, clams, insects, crustaceans, fishes and amphibians. Animals in this zone provide food for turtles, snakes & water birds.
The middle zone is the limnetic zone. This zone is usually well lighted. This zone is usually dominated by plankton.
The last layer is the deepest layer and there is usually little light, less oxygen and the temperature is lower in this zone. This zone is dominated by animals who feed as predators or on waste sinking to the bottom.
These three clear layers do not occur in shallow lakes and ponds, as wind causes the layers to mix together.
Streams and rivers start at headwaters. A headwater can be a spring, a region with melting snow or a lake. Water at this region is usually cleaner, clearer and colder. There are a greater number of different species in the middle regions of streams and rivers, which are wider and dominated macrophytes (water plants) and algae (microscopic water plants). The mouths of rivers are usually have murky water with a lower number of different species and lower productivity. These regions are dominated by carp and catfish who can tolerate lower oxygen levels.
Wetlands are areas of standing water that support aquatic plants. These include marshes, swamps and bogs. Plants called hydrophytes grow in wetlands and are adapted to the moist and humid conditions. These plants include pond-lilies, cattails, spruce Cyprus and Gum. These areas have a high number of different species, including amphibians, reptiles and birds.
Salt water or marine regions covers ¾ of the Earth’s surface. The oceans provide a great service as marine plants and algae produce much of the Earth’s oxygen, while absorbing large amounts of harmful carbon dioxide. The evaporation of the seawater also provides rainwater for the land in a process called the hydrological cycle.
A simple representation of the hydrological cycle.
Oceans, like lakes are divided into separate zones. The deepest part of the ocean is called the abyssal zone. There is comparatively little known about the animals that live in this zone. The animals here are adapted to very cold water and the high pressure of the water. Because of the lack of sunlight in this zone, nutrients enter this zone from the shallower waters above. There is one exception however. This is the presence of hydrothermal vents in some parts of the ocean floor. This will be dealt with in more detail, later in this chapter.
The area of the ocean that is above the abyssal zone is called the benthic zone. Here less light penetrates and the water gets colder as the water gets deeper. This area is more productive then the abyssal zone since it is nutrient rich. Here you will find seaweed, fungi, sponges, sea anemones, worms, sea stars, fishes and many other animals.
The pelagic zone is that area of ocean that is usually referred to as open ocean. Light can penetrate this zone, and microphytoplankton and seaweed at the surface are the primary producers of this zone. Many fish, as well as whales and dolphins inhabit this zone.
The intertidal zone is the zone where the ocean meets the land. Depending on the tide, this area is either covered or uncovered by water. Rocky parts of the intertidal zone are stratified, with less diverse and more hardy species inhabiting the highest areas of the intertidal zone where only the fullest tides reach. In areas of the zone that are submerged during high tide, there are more types of animals and plants such as algae, snails, crabs, sea stars and fish. The part of the rocky shore that is only exposed at the lowest tides many plants and animals are also found. There are not as much difference between zones on sandy shores. Here the sand is always moving and animals and plants struggle to establish themselves. Here animals such as clams, crustaceans, worms and shore birds are found.
Coral reefs are found in warm shallow waters. They are found around islands and as barriers around continents. The organisms that make the coral reefs are called corals. Corals are tiny communities consisting of both algae and tiny animal cells, The corals obtain nutrients from the sun via photosynthesis as well as filtering plankton from the surrounding water. Coral reefs provide a refuge for a huge number of different animals and are therefore very important to conserve.
Estuaries are areas where freshwater streams or rivers merge with the ocean. Estuaries are known to be very productive and provide a relatively safe nursery for the young of many marine organisms. Plants such as algae, seaweeds, marsh grasses, and mangrove trees are found in estuaries. Many different types of animals are found in estuaries including worms, oysters, crabs, fish and waterfowl.
The desert biome
Deserts occur in areas of the planet where the annual rainfall is less than 50 cm. Animals and plants that live in the desert are highly adapted for the desert environment. Deserts cover approximately 1/5 of the Earth’s surface.
Hot and dry deserts
These deserts are usually very cold at night and hot in the day. Daylight temperatures remain hot throughout the year and get particularly hot during summer. In addition there is very little rainfall during winter. Rainfall occurs in short bursts during the year. In fact, there are some desert areas that receive no rainfall. The soil in these deserts is coarse, shallow, rocky or gravely with very little ground water. Plants that grow in deserts are particularly adapted to the conditions. These include ground hugging shrubs and short trees. The leaves of these plants tend to be short, thick and covered in a thick cuticle. The spines of cacti are actually modified leaves. Many desert plants only transpire at night so as to minimise water loss.
Desert animals include small mammals, certain insects, arachnids, reptiles and birds. Most of the desert animals stay inactive during the day and hunt or forage during the night.
Semiarid deserts have summer daytime temperatures that seldom exceed 38° C. Night time temperatures are usually cool. The winter period in semiarid deserts is usually associated with a dry period, with even less than usual rainfall. The cool evenings allow both plants and animals to reduce water loss. Dew that accumulates on the land surface at night provides an important source of water for both plants and animals. The soil can range from sandy and fine-textured to loose rock fragments, gravel or sand.
Plants in these semiarid desert areas often have many spines with the dual purpose of shading from the sun and for protection from predators. Many plants may also have a taste or smell that is unpleasant to predators.
Animals in semiarid deserts include small mammals such as various rodents, insects such as grasshoppers and ants, reptiles such as lizards and snakes and various birds. Many of these animals are adapted to feeding at night or staying in shady places such as burrows during the day.
These deserts occur in moderately cool to warm areas such as the Nearctic and Neotropical realm. A good example is the Atacama of Chile. The cool winters of coastal deserts are followed by moderately long, warm summers. The average summer temperature ranges from 13-24° C; winter temperatures are 5° C or below. The maximum annual temperature is about 35° C and the minimum is about -4° C. In Chile, the temperature ranges from -2 to 5° C in July and 21-25° C in January.
The average rainfall measures 8-13 cm in many areas. The maximum annual precipitation over a long period of years has been 37 cm with a minimum of 5 cm.
The soil is fine-textured with a moderate salt content. It is fairly porous with good drainage. Some plants have extensive root systems close to the surface where they can take advantage of any rain showers. All of the plants with thick and fleshy leaves or stems can take in large quantities of water when it is available and store it for future use. In some plants, the surfaces are corrugated with longitudinal ridges and grooves. When water is available, the stem swells so that the grooves are shallow and the ridges far apart. As the water is used, the stem shrinks so that the grooves are deep and ridges close together. The plants living in this type of desert include the salt bush, buckwheat bush, black bush, rice grass, little leaf horsebrush, black sage, and chrysothamnus.
Some animals have specialized adaptations for dealing with the desert heat and lack of water. Some toads seal themselves in burrows with gelatinous secretions and remain inactive for eight or nine months until a heavy rain occurs. Amphibians that pass through larval stages have accelerated life cycles, which improves their chances of reaching maturity before the waters evaporate. Some insects lay eggs that remain dormant until the environmental conditions are suitable for hatching. The fairy shrimps also lay dormant eggs. Other animals include: insects, mammals (coyote and badger), amphibians (toads), birds (great horned owl, golden eagle and the bald eagle), and reptiles (lizards and snakes).
Cold Desert These deserts are characterized by cold winters with snowfall and high overall rainfall throughout the winter and occasionally over the summer. They occur in the Antarctic, Greenland and the Nearctic realm. They have short, moist, and moderately warm summers with fairly long, cold winters. The mean winter temperature is between -2 to 4° C and the mean summer temperature is between 21-26° C. The winters receive quite a bit of snow. The mean annual precipitation ranges from 15-26 cm. Annual precipitation has reached a maximum of 46 cm and a minimum of 9 cm. The heaviest rainfall of the spring is usually in April or May. In some areas, rainfall can be heavy in autumn. The soil is heavy, silty, and salty. It contains alluvial fans where soil is relatively porous and drainage is good so that most of the salt has been leached out.
The plants are widely scattered. In areas of shad-scale, about 10 percent of the ground is covered, but in some areas of sagebush it approaches 85 percent. Plant heights vary between 15 cm and 122 cm. The main plants are deciduous, most having spiny leaves. Widely distributed animals are jack rabbits, kangaroo rats, kangaroo mice, pocket mice, grasshopper mice, and antelope ground squirrels. In areas like Utah, population density of these animals can range from 14-41 individuals per hectare. All except the jack rabbits are burrowers. The burrowing habit also applies to carnivores like the badger, kit fox, and coyote. Several lizards do some burrowing and moving of soil. Deer are found only in the winter.