Structure and key components of a biogeocoenosis as an ecological system

In the previous chapter, basic concepts and definitions regarding the ecosystem and the closely related concept of biogeocenosis were provided.

In accordance with the formulations above, a biogeocenosis includes two main components: the totality of abiotic factors in a specific territory, i.e., the ecotope, and the totality of living organisms — the biocenosis. In turn, the ecotope consists of a combination of climatic (climatotope) and soil-ground (edaphotope) factors, while the biocenosis includes communities of animals (zoocenosis), plants (phytocenosis), and microorganisms (microbiocenosis).

One of the most important properties of a biogeocenosis is the interconnection and interdependence of all its components, which are indicated by arrows in Fig. 2.6. Climate determines the state and regime of soil factors and creates a habitat for living organisms. Soil, in turn, influences the climate (its reflectivity — albedo, and therefore temperature and air humidity, depend on the color of the soil surface), and also affects animals, plants, and microorganisms. All living organisms are closely linked, serving for each other as a source of food, a habitat, or mortality factors.

The arrows in the figure represent channels of information transfer between the various components of the biogeocenosis. Through their activities, humans can directly or indirectly interrupt these channels and flows of matter and energy, or distort them. Anthropogenic activity has always been directed at biogeocenoses (ecosystems), without which there is no life on Earth.

Ecological systems are distinguished by their spatial and species structure. The spatial structure is determined by the fact that the subsoil, soil, water basins, and atmosphere have a tiered structure, which, in turn, influences the distribution of living organisms in space. As a result of long-term evolution, in accordance with abiotic and biotic conditions, different species of living organisms have distributed themselves in biogeocenoses in such a way that they not only do not interfere with each other, but, on the contrary, contribute to the most complete and efficient utilization of all material and energy resources of a given biotope. It has been established that complex multi-tiered communities are more productive than simple single-tiered ones.

Thus, tiering is the phenomenon of the vertical stratification of biocenoses into structural parts of varying heights. It is most clearly expressed in plant communities (phytocenoses). Thanks to tiering, different plants and their nutrient-absorbing organs are located at different heights (or depths) and therefore easily coexist within the community.

In a forest, up to six tiers are often distinguished:

• I — primary canopy trees (spruce, pine, oak, birch, aspen);
• II — secondary canopy trees (rowan, bird cherry);
• III — understory of tall shrubs (rose hip);
• IV — understory of medium shrubs and large herbs (marsh tea, bog blueberry, fireweed);
• V — low dwarf shrubs and small herbs (cranberry);
• VI — mosses, ground lichens, etc.

Meadow communities can be subdivided in a similar way. Underground parts of plants are also arranged in tiers. Tree roots, as a rule, penetrate to a greater depth than those of shrubs; roots of small herbaceous plants are located closer to the surface, and moss roots are directly on the surface. At the same time, there are significantly more roots in the surface layers of the soil than in the deep ones.

Plants of each tier determine a specific microclimate and create a particular environment (ecological niche) for the habitation of strictly specific animals. As a result, groupings of plants and animals arise that are closely linked to one another. For example, in the soil layer of the forest, filled with plant roots, bacteria, fungi, insects, mites, and worms live. In the forest floor, among decomposing plant residues, mosses, and lichens, insects, mites, spiders, and a multitude of microorganisms also live. Higher tiers — the grass cover and understory — are occupied by herbivorous insects, birds, mammals, and other animals. Furthermore, even birds, which move freely in space, usually adhere to a certain tier. This is especially evident during the nesting period.

Each specific ecosystem has a species structure, i.e., it is characterized by its species composition. In an oak forest, this is oak; in a pine forest, it is pine; in a feather grass steppe, it is feather grass, etc. In a forest consisting of many dozens of plant species, only one or two species provide 90% of the timber. These species are called dominant. They occupy a commanding position in the biocenosis. Usually, terrestrial biocenoses are named after the dominant species: larch forest, feather grass steppe, etc. In addition, there may be so-called edificators in an ecosystem, which not only dominate but play the role of community builders, determining the regime of temperature, humidity, light, and the specifics of soil-ground conditions.

The same applies to water systems. Water bodies are divided into two large groups: standing water bodies, or lentic environments (calm), which include lakes, ponds, and swamps, and flowing water bodies, or lotic environments (washing), which include rivers and streams.

The specifics of water bodies are determined by many factors, primarily the thermodynamic characteristics of water, including a higher specific heat capacity than that of air, a large latent heat of fusion, the highest heat of vaporization among known substances, and the greatest density at a strictly defined temperature (+4°C). The abiotic factors of the aquatic environment discussed above determine the distribution of living organisms inhabiting water bodies, some of which live at depth, others at the surface, and still others in the water column.

In any lentic water body, four main life zones can be distinguished.

Fields and adjacent water bodies form a single system where processes on land directly determine the state of aquatic ecosystems. In any water body, four main zones are distinguished: shallow coastal water (littoral) with rooted plants, frogs, and snails; the surface illuminated layer (pelagic) with phytoplankton and zooplankton; the deep-water part (profundal); and the bottom (benthic). At the bottom, numerous decomposers, mollusks, and insect larvae process organic residues coming from the surface. In rivers and streams, the ecosystem is divided into shallow riffles and deep pools.

In flowing water bodies, the speed and turbulence of the flow determine how washed-off substances move, settle, and decompose, as well as how effectively self-purification occurs. Plants, animals, and microorganisms in these zones evolved together and developed a close relationship. These components cannot exist separately; therefore, any changes on land instantly affect the inhabitants of the water.

Nutrient runoff from arable land and eutrophication processes

The most important link between land and water is the runoff of organic matter and mineral elements, primarily nitrates and phosphates. Entering rivers and lakes from fields, they serve as food for aquatic plants and microorganisms. However, during erosion, soil material also enters water bodies along with the runoff. Settling at the bottom of lakes and slow-flowing rivers, bottom sediments gradually change the composition of flora and fauna, over time turning the aquatic ecosystem into land.

Natural factors and human economic activity can sharply accelerate the movement of soil and fertilizer from fields into water. Ploughing areas, clearing land, or flooding increase runoff volumes, overloading water bodies with organic matter. This leads to eutrophication — a long-term change in the physical, chemical, and biological properties of the aquatic environment.

The transport of substances also occurs in the opposite direction — from water to land. Fish and mollusks become food for terrestrial animals, poultry, and humans. Along with excrement, nutrients return from water systems to land, completing the natural cycle of nitrogen and phosphorus.