Consequences of food chain disruption
A classic example of anthropogenic intervention in an ecosystem is the mass extermination of sparrows, which led to the death of 10–30 million people. The logic of the answer is based on two interrelated points:
- the decline in the sparrow population led to a massive proliferation of agricultural pests (locusts, caterpillars) because their natural regulator disappeared;
- the growth in the pest population caused a sharp drop in crop yields, leading to mass starvation and the death of people.
A similar pattern occurs when predatory fish are exterminated in a water body. First, the population of herbivorous commercial fish increases, but then intraspecific competition for resources intensifies among them, diseases and parasites spread rapidly, ultimately leading to a sharp population decline.
Natural ecosystem vs. agrocenosis
Differences between a natural ecosystem and a human-created agrocenosis must be presented in pairs:
- in a natural ecosystem, food chains are long, whereas in an agrocenosis they are short due to low species diversity;
- in a natural ecosystem, there is high species diversity, whereas an agrocenosis is dominated by a single crop species;
- in a natural ecosystem, nutrient cycling is closed, whereas in an agrocenosis it is open because part of the biomass (the harvest) is removed by humans;
- in a natural ecosystem, self-regulation is carried out by natural selection, whereas in an agrocenosis it is maintained by humans.
Soil depletion and crop rotation
When a single crop is grown in a field for a long time, the soil becomes depleted because plants selectively absorb certain elements. Additionally, the population of specialized parasites and pests accumulating in the soil increases.
Soil fertility can be restored without the use of fertilizers by sowing the field with legumes. Nodules form on their roots, which harbor nitrogen-fixing symbiotic bacteria. These bacteria convert atmospheric nitrogen into plant-accessible compounds, enriching the soil and restoring its fertility.
Population limitation and rare species
Although any organism is potentially capable of unlimited reproduction, population size in nature is regulated. The existence of rare and endangered species is explained by the action of limiting factors:
- biotic and abiotic environmental factors (predators, parasites, climate, food availability);
- human economic activity — poaching, habitat destruction, pollution;
- environmental changes to which the species fails to adapt in time.
Rule of the ecological pyramid
The 10% rule (Lindeman's efficiency): when energy passes from one trophic level to the next, about 10% of the energy and biomass of the previous link is transferred.
Problem: calculate the mass of phytoplankton required to support a 150-tonne whale. Food chain: phytoplankton → zooplankton → whale. There are two trophic links above the whale, so the mass must be multiplied by 10 twice:
- zooplankton: 150 t × 10 = 1,500 t;
- phytoplankton: 1,500 t × 10 = 15,000 tonnes (15 million kg).
Classification of natural resources
Exhaustible non-renewable resources include:
- fossil fuels — oil, natural gas, coal;
- metallic mineral ores;
- non-metallic mineral raw materials;
- subsurface earth resources in general.
Some of these resources can be renewed through geological cycles, but the duration of such cycles spans hundreds of millions of years, which is incomparable to the timeframe of human consumption.
Mechanism of ecological speciation
Ecological speciation occurs without geographic isolation, within the original range, and includes the following stages:
- Populations of the same species find themselves in different environmental conditions within a shared, undivided range, occupying distinct ecological niches.
- Individuals with mutations beneficial to specific conditions are preserved through natural selection and spread within the population.
- The gene pool of the populations gradually changes, and differences in morphology and physiology accumulate.
- Individuals from different populations stop interbreeding, resulting in reproductive isolation.
- A new species is formed.
Key points
- The destruction of a single link in a food chain triggers cascading consequences for the entire ecosystem.
- An agrocenosis differs from a natural ecosystem by having short food chains, low diversity, and an open nutrient cycle.
- Legumes restore soil fertility through symbiosis with nitrogen-fixing bacteria.
- Species populations in nature are limited by biotic, abiotic, and anthropogenic factors.
- The 10% rule allows for calculating the biomass of organisms at different trophic levels.
- Fossil fuels belong to exhaustible non-renewable resources.
- Ecological speciation proceeds without geographic isolation through niche divergence and reproductive isolation.