NCERT Solutions for Class 12 Biology Chapter 14: Ecosystem

Answer:

1. Plants are called as producers because they fix carbon dioxide.
2. In an ecosystem dominated by trees, the pyramid (of biomass) is upright type.
3. In aquatic ecosystems, the limiting factor for productivity is light.
4. Common detritivores in our ecosystem are earthworms.
5. The major reservoir of carbon on earth is ocean.

Answer: (d) Decomposers

Decomposers have the largest population in a food chain because they break down dead organic matter from all trophic levels. They are essential for nutrient cycling and are present throughout the ecosystem, resulting in a larger population compared to other trophic levels.

Answer: (b) Zooplankton

The first trophic level consists of producers (phytoplankton in a lake). The second trophic level comprises primary consumers, which in a lake ecosystem are zooplankton that feed on phytoplankton.

Answer: (a) The rate of formation of new organic matter by consumers

Secondary productivity refers to the biomass generated by consumers (heterotrophs) through consumption and assimilation of organic matter produced by primary producers. It measures the growth and reproduction of consumers in an ecosystem.

Answer: An ecosystem consists of two main components:

1. Biotic Components:

• Producers (Autotrophs): Organisms that produce their own food using photosynthesis or chemosynthesis. Examples include green plants, algae, and certain bacteria.
• Consumers (Heterotrophs): Organisms that depend on other organisms for food. They are categorized as:
  • Primary consumers (herbivores) - feed on producers
  • Secondary consumers (carnivores) - feed on primary consumers
  • Tertiary consumers (top carnivores) - feed on secondary consumers
• Decomposers: Organisms that break down complex organic matter into simpler inorganic substances. Examples include bacteria and fungi.

2. Abiotic Components:

• Inorganic substances: Carbon, nitrogen, oxygen, water, etc.
• Organic compounds: Proteins, carbohydrates, lipids, etc.
• Climatic factors: Temperature, light, wind, humidity, etc.

Answer: Ecological succession is the process of change in the species structure of an ecological community over time. It is a directional, predictable process where sequential changes occur in the composition and structure of communities.

Answer: Energy flow in an ecosystem follows these key principles:

1. Unidirectional Flow: Energy flows in one direction - from the sun to producers and then through consumers. It never flows back from higher trophic levels to lower ones.
2. Decreasing Energy at Higher Trophic Levels: As energy moves from one trophic level to the next, a significant portion (about 90%) is lost as heat or used for metabolic processes, with only about 10% being transferred.
3. 10% Law: Proposed by Lindeman, this law states that only about 10% of energy from one trophic level is transferred to the next. This is why food chains rarely have more than 4-5 trophic levels.
4. Solar Energy Entry: The sun is the primary source of energy for most ecosystems. Producers capture solar energy through photosynthesis.
5. Energy Loss: Energy is lost at each trophic level through respiration, excretion, and as heat during metabolism.
6. Singularity of Energy Entry: Unlike nutrients which cycle through the ecosystem, energy enters only once (mainly through photosynthesis) and exits as heat.

This unidirectional and decreasing pattern of energy transfer explains why higher trophic levels contain fewer organisms and why top predators are generally fewer in number compared to primary consumers.

Answer: Ecological pyramids are graphical representations of the trophic structure of an ecosystem. They show the quantitative relationships between different trophic levels. They are of three types:

1. Pyramid of Number: Represents the number of individuals at each trophic level. In most ecosystems, it is upright with maximum numbers at the producer level and decreasing numbers at higher trophic levels. However, it can be inverted (e.g., in a tree ecosystem where a single tree supports thousands of herbivores).
2. Pyramid of Biomass: Represents the total biomass (living matter) at each trophic level. It is generally upright in terrestrial ecosystems but inverted in aquatic ecosystems where the biomass of producers (phytoplankton) is less than that of consumers.
3. Pyramid of Energy: Represents the amount of energy at each trophic level. It is always upright because energy decreases at each trophic level according to the 10% law, with maximum energy at the producer level and minimum at the highest trophic level.

These pyramids help visualize the trophic structure, energy efficiency, and biomass distribution in ecosystems, providing insights into ecological relationships and limitations of food chain length.

Answer: Nutrient cycling, also called biogeochemical cycling, is the movement of chemical elements between living organisms and the non-living environment. Key aspects include:

1. Carbon Cycle: Carbon moves between the atmosphere (as CO2), oceans, organisms, and fossil fuels. Processes like photosynthesis, respiration, decomposition, and combustion drive this cycle. Human activities like burning fossil fuels have significantly increased atmospheric CO2.
2. Nitrogen Cycle: Nitrogen, essential for proteins and nucleic acids, moves through processes of nitrogen fixation (converting atmospheric N2 to usable forms), nitrification, assimilation by plants, ammonification, and denitrification. Nitrogen-fixing bacteria play a crucial role in this cycle.
3. Phosphorus Cycle: Unlike carbon and nitrogen, phosphorus has no gaseous phase. It cycles through rocks, soil, water, and organisms. Weathering of rocks releases phosphate ions, which are absorbed by plants and passed through food chains. Decomposition returns phosphorus to soil, where it may be reused or washed into water bodies and eventually sedimented.
4. Importance: These cycles ensure the availability of essential elements to living organisms. They maintain ecosystem stability and support life. Human activities can disrupt these cycles, causing environmental problems like eutrophication and climate change.