Sulfur and water cycle in the planet's global ecosystem

Sulfur is transformed into various compounds and circulates in the biosphere. From natural sources, it enters the atmosphere in the following forms:

• hydrogen sulfide (H2S) - a colorless, foul-smelling, toxic gas - from volcanic eruptions and the decomposition of organic matter in swamps and tidal lowlands;
• sulfur dioxide (SO2) - a colorless, choking gas – from volcanic eruptions;
• particles of sulfate salts (for example, ammonium sulfate) – from the atmosphere.

About one-third of all sulfur compounds and 99% of sulfur dioxide entering the atmosphere are of anthropogenic origin. Burning sulfur-containing coal and oil for electricity generation accounts for approximately two-thirds of all anthropogenic sulfur dioxide emissions into the atmosphere. The remaining third comes from such industrial processes as oil refining and the smelting of metals from sulfur-containing copper, lead, and zinc ores.

In the atmosphere, sulfur dioxide is oxidized by oxygen into gaseous sulfur trioxide, which, upon reacting with water vapor, forms minute droplets of sulfuric acid (H2SO4). By interacting with other atmospheric components, sulfur trioxide can form minute particles of sulfate salts. Sulfuric acid and sulfate salts contribute to the formation of acid precipitation, which disrupts the life activity of forest and aquatic ecosystems.

The hydrological cycle, during which the planet's water supply is accumulated, purified, and redistributed, consists of the following. Solar energy and gravity continuously move water between the oceans, the atmosphere, land, and living organisms. The most important processes in this cycle are evaporation, condensation, precipitation, and water runoff back to the sea to renew the cycle.

Under the influence of incoming solar energy, water evaporates from the surface of oceans, rivers, lakes, soil, and plants and enters the atmosphere. Winds and air masses transport water vapor to different regions of the Earth. A decrease in temperature in certain parts of the atmosphere leads to the condensation of water vapor, the formation of clouds and fog, and the occurrence of atmospheric precipitation.

A portion of fresh water returns to the Earth's surface as precipitation or freezes in glaciers. However, most of it fills depressions and basins and flows into the nearest lakes, streams, and rivers, which carry it back to the ocean, thereby completing the cycle. Such freshwater runoff from the land surface also causes soil erosion, which leads to the movement of various chemical substances within other biogeochemical cycles.

A significant part of the water returning to land infiltrates deep into the soil. There, groundwater accumulates in aquifers — underground reservoirs. Underground springs and watercourses eventually return water to the land surface and into rivers, lakes, and streams, from where it evaporates again or flows into the ocean. However, the circulation of groundwater occurs incomparably slower than the circulation of surface and atmospheric water.

Thus, all components of an ecological system, being in constant interaction with one another, form cycles of chemical elements. Metabolism of organisms among themselves and their exchange with the environment can be considered as processes of information and energy transfer. Consequently, in any ecosystem where food chains exist, there are specific channels for the transmission of this information: chemical, energetic, genetic, etc.

The balance of the biological cycle, and therefore the stability of ecological systems, from the point of view of cybernetics, is ensured by feedback mechanisms and processes. The principle of feedback is that a certain control component of a system receives information from the controlled components, using this information to make adjustments to the further management process.

To explain the principle of feedback in ecology, let us consider a hypothetical simple ecological system consisting of only two trophic levels: deer-wolf. That is, in this model ecosystem, the "producer" and "decomposer" parts are excluded, and the complex network of trophic channels existing in reality is represented by the link of matter and energy transfer from the 1st-order consumer (deer) to the 2nd-order consumer (wolf). Such a simplification will allow us to draw some fundamental conclusions.

In this system, wolves (predators) eat deer (prey). If the number of prey is constantly increasing, then the predator, which feeds only on this prey, also has the opportunity to increase its number (population size). This demonstrates positive feedback, which tends to push the system out of equilibrium. But since the wolf eats deer, it naturally reduces the deer population. This demonstrates negative feedback, which compensates for the deviation and returns the system to its initial state. If the number of wolves increases sharply for some reason, they will accordingly reduce the number of deer and will themselves be faced with the limitation of their own population size, as they will experience a shortage of food.

In a natural ecological system, equilibrium is constantly maintained, which excludes the irreversible destruction of certain links in trophic chains. The population of wolves and deer will always be kept at a certain level. No matter how strong an ecosystem may be, it is always balanced, stable, and only for this reason productive.

Any field or greenhouse is a living ecosystem, the stability of which is maintained by feedback loops between organisms. In natural conditions, the system regulates the population of species itself, but this balance is easily disrupted by external interference. Understanding these mechanisms helps an agronomist predict whether a planting will withstand climate stress and pest outbreaks, or whether the biocenosis will be completely destroyed.

The interaction of populations in the "deer-wolf" chain clearly shows how a system reacts to external stimuli. The role of interference in communication channels between species is played by various factors:

• climatic changes (for example, drought, which reduces crop productivity);
• infectious diseases that weaken the population;
• the appearance of new predators or competitors for food resources.

Such interference triggers natural selection. Weak individuals die or do not produce offspring, while the most resilient ones survive and pass on hereditary information to future generations.

The stability of a biocenosis directly depends on the diversity of connections within the system. The more complex an ecological system is and the more overlapping food chains it has, the higher its overall stability. This is why monocultures are the most vulnerable to external factors compared to multi-component systems.

Sustainability limit and critical loads on the ecosystem

Within certain limits, a system is capable of adapting to loads. Under moderate stress, the number of links in the chain decreases, but their thermodynamic ratio is maintained. A new level of stability in this case is ensured by negative feedback mechanisms.

However, the pressure of interference on an ecosystem cannot be limitless. At a critical level of stress, for example, during epidemics or the invasion of new competitors, defense mechanisms fail. Negative feedback can no longer compensate for deviations, and the system enters a stage of collapse.