Four main habitats can be distinguished within the biosphere. This is an aquatic environment, a terrestrial-air environment, soil and an environment formed by the living organisms themselves.
Water serves as a habitat for many organisms. From water, they receive all the substances necessary for life: food, water, gases. Therefore, no matter how diverse aquatic organisms are, they must all be adapted to the main features of life in the aquatic environment. These features are determined by the physical and chemical properties of the water.
Aquatic organisms (inhabitants of the aquatic environment) live in both fresh and salt water and are divided into 3 groups according to their habitat:
- plankton – organisms that live on the surface of water bodies and passively move due to the movement of water;
- nekton – actively moving in the water column;
- benthos – organisms that live at the bottom of water bodies or burrow into silt.
In the water column, many small plants and animals constantly hover, leading life in a suspended state. The ability to soar is provided not only by the physical properties of water, which has a buoyant force, but also by special adaptations of the organisms themselves, for example, numerous outgrowths and appendages that significantly increase the surface of their body and, therefore, increase friction against the surrounding liquid.
The body density of animals such as jellyfish is very close to that of water.
In addition, their characteristic body shape, reminiscent of a parachute, helps them to stay in the water column.
Active swimmers (fish, dolphins, seals, etc.) have a spindle-shaped body, and the limbs are in the form of fins.
Their movement in the aquatic environment is facilitated, in addition, due to the special structure of the outer covers, which emit a special lubricant – mucus, which reduces friction against water.
Water has a very high heat capacity, that is, the ability to accumulate and retain heat. For this reason, water does not experience the sudden temperature fluctuations that often occur on land. Very deep waters can be very cold, but due to the constant temperature, animals were able to develop a number of adaptations that ensure life even in these conditions.
Animals can live in the vast ocean depths. Plants, on the other hand, survive only in the upper layer of water, where the radiant energy necessary for photosynthesis enters. This layer is called the photic zone.
Since the surface of the water reflects most of the light, even in the most transparent ocean waters, the thickness of the photic zone does not exceed 100 m. Animals of great depths feed on either living organisms or the remains of animals and plants that constantly descend from the upper layer.
Like terrestrial organisms, aquatic animals and plants breathe and require oxygen. The amount of oxygen dissolved in water decreases with increasing temperature. Moreover, oxygen dissolves worse in seawater than in fresh water. For this reason, the waters of the open sea of the tropical zone are poor in living organisms. Conversely, polar waters are rich in plankton – small crustaceans that feed on fish and large cetaceans.
The salt composition of water is very important for life. Ca2 + ions are of particular importance for organisms. Molluscs and crustaceans need calcium to build a shell or carapace. The concentration of salts in water can vary greatly. Water is considered fresh if one liter of it contains less than 0.5 g of dissolved salts. Sea water is characterized by constant salinity and contains an average of 35 g of salt per liter.
The ground-air environment was assimilated by living organisms later than the aquatic environment and is distinguished by a wide variety of living conditions. It is inhabited by more highly organized living organisms.
The most important factor in life for the inhabitants of this environment is the characteristics of the air around them. Air has a low density and cannot support organisms. For this reason, supporting elements are well developed in terrestrial plants and animals. The forms of movement are very diverse: running, jumping, crawling, flying, etc. Birds and some species of insects fly in the air. The wind carries plant seeds, spores, microorganisms.
The air is constantly in motion. Its temperature is not constant. Therefore, the inhabitants of the ground-air environment are well adapted to withstand sudden temperature fluctuations.
The most remarkable of them is the development of warm-bloodedness, which arose precisely in the ground-air environment.
The chemical composition of the air (78% nitrogen, 21% oxygen and 0.03% carbon dioxide) is of great importance for the life of plants and animals. Carbon dioxide, for example, is the most important raw material for photosynthesis. Air nitrogen is essential for the synthesis of proteins and nucleic acids.
The amount of water vapor in the air (relative humidity) determines the intensity of the processes of transpiration in plants and evaporation from the skin of some animals. Organisms living in low humidity conditions have numerous adaptations to prevent severe water loss. For example, desert plants have a powerful root system capable of sucking water into the plant from great depths. Cacti store water in tissues and use it sparingly. In many plants, the leaf blades are turned into thorns to reduce evaporation. Many desert animals hibernate during the hottest period, which can last for several months.
Soil is the top layer of land, transformed as a result of the vital activity of living things. It is an important and very complex component of the biosphere, closely related to its other parts. There are many inhabitants in the soil. There are numerous cavities between soil particles, which can be filled with water or air. Therefore, the soil is inhabited by both aquatic and air-breathing organisms. Soil is very important for plants.
One of the important factors affecting life in the environment is temperature. However, as it sinks into the soil, temperature fluctuations become less and less noticeable: daily temperature changes quickly fade, and as depth increases, seasonal temperature changes.
Even at shallow depths, complete darkness reigns in the soil. In addition, as the soil sinks into the soil, the oxygen content decreases and the carbon dioxide content increases. Therefore, only anaerobic bacteria can live at a considerable depth, while in the upper layers of the soil, in addition to bacteria, fungi, protozoa, roundworms, arthropods and even relatively large animals that make tunnels and build shelters, for example, moles, shrews, mole rats, are abundant.
Environment formed by living organisms
The bodies of many organisms serve as a living environment for other organisms. This applies not only to parasitism, but also to some other forms of relationships between organisms, which will be discussed in more detail in the following sections.
It is obvious that the conditions of life inside another organism are characterized by greater constancy in comparison with the conditions of the external environment. Therefore, parasites often have reduced organs and systems, without which free-living organisms cannot do. They do not have developed sense organs or organs of movement, but adaptations (often very sophisticated) appear for keeping the host in the body and for effective reproduction.