Chapter 15 Life On The Earth

Previous chapters have introduced the fundamental components of Earth's environment: the lithosphere (land), the atmosphere (air), and the hydrosphere (water). This chapter focuses on the biosphere, which encompasses all living organisms on Earth and their intricate interactions with these other environmental realms.

The biosphere includes all forms of life, from the smallest microorganisms to complex plants and animals, wherever they are found on Earth. Life exists in diverse environments, from the frozen poles to the hot equator, from the deepest ocean trenches to high altitudes in the atmosphere, and within groundwater beneath the surface. Organisms reside within or move between the lithosphere, hydrosphere, and atmosphere.

The biosphere and its living components are crucial parts of the environment. Living organisms interact significantly with non-living (abiotic) elements like land, water, soil, and are influenced by atmospheric conditions such as temperature, rainfall, moisture, and sunlight. These interactions are fundamental to the growth, development, and long-term evolution of life on Earth.

Ecology

The increasing awareness of environmental and ecological challenges leads us to the study of Ecology. Derived from the Greek words 'oikos' (meaning 'house' or 'habitat') and 'logy' (meaning 'science' or 'study of'), ecology literally means the study of Earth as a 'household' where diverse life forms live together interdependently. German zoologist Ernst Haeckel first used the term 'oekologie' in 1869.

Ecology is defined as the scientific study of the relationships and interactions between living organisms (biotic components) and their physical environment (abiotic components), as well as the interactions among the organisms themselves.

Within a specific environment, a particular group of organisms interacts with abiotic factors (temperature, light, water, soil, chemicals). These interactions result in defined patterns of energy flow and material cycling, creating functional units known as ecological systems.

A habitat, in an ecological context, refers to the complete set of physical and chemical conditions that characterize the environment where an organism or a population lives. An ecological system or ecosystem is a community of living organisms (biotic components) interacting with each other and with the non-living components (abiotic factors) of their environment. All parts of an ecosystem are interconnected and influence one another.

Different types of ecosystems exist across the globe, each with a unique range of environmental conditions. The organisms within these ecosystems have developed specific adaptations through evolution that enable them to survive and thrive in those conditions.

Types Of Ecosystems

Ecosystems are broadly classified into two major types:

• Terrestrial Ecosystems: Land-based ecosystems. These are further divided into major categories called biomes. A biome is a large geographical area characterized by a specific climate that supports a distinct community of plants and animals. The distribution and characteristics of terrestrial biomes are primarily determined by climate, particularly temperature and precipitation, along with humidity and soil conditions.
• Aquatic Ecosystems: Water-based ecosystems. These are subdivided into:
  • Marine Ecosystems: Include oceans, seas, estuaries (areas where freshwater rivers meet the sea), and coral reefs.
  • Freshwater Ecosystems: Include inland water bodies like lakes, ponds, rivers, streams, marshes, and bogs.

Structure And Functions Of Ecosystems

The structure of an ecosystem involves identifying and describing the different plant and animal species present, as well as the non-living environmental factors. Structurally, all ecosystems consist of two main components:

• Abiotic Factors: Non-living components of the environment. These include physical factors like rainfall, temperature, sunlight intensity, atmospheric humidity, and soil characteristics, as well as inorganic chemical substances essential for life, such as carbon dioxide ($CO_2$), water ($H_2O$), nitrogen ($N_2$), calcium ($Ca$), phosphorus ($P$), potassium ($K$), etc.
• Biotic Factors: Living components of the ecosystem. These are grouped by their functional roles:
  • Producers: Organisms, primarily green plants and some bacteria, that can create their own food (organic matter) using energy from sunlight or chemical reactions. Photosynthesis is the main process by which producers convert inorganic substances ($CO_2$ and water) into organic compounds (sugars).
  • Consumers: Organisms that obtain energy by feeding on other organisms. Consumers are categorized by what they eat:
    • Primary Consumers (Herbivores): Feed directly on producers (plants). Examples include deer, goats, mice, insects that eat plants.
    • Secondary Consumers (Carnivores or Omnivores): Feed on primary consumers. Examples of carnivores are snakes, tigers, lions that eat herbivores. Omnivores eat both plants and animals.
    • Tertiary Consumers (Higher-level Carnivores): Feed on secondary consumers. Some carnivores are eaten by other carnivores (e.g., a snake eaten by a hawk), making the hawk a tertiary consumer.
  • Decomposers: Organisms that break down dead organic matter (dead plants, animals, waste products) from all trophic levels. Scavengers like vultures and crows eat dead organisms, and then microorganisms like bacteria and fungi further decompose the remaining material, releasing nutrients back into the environment for producers to use.

The functions of an ecosystem involve the processes of energy flow and nutrient cycling, which connect the biotic and abiotic components. The most fundamental functional link is the flow of energy through feeding relationships, organized into food chains and food webs.

A food chain represents a linear sequence of organisms where energy is transferred from one trophic level (feeding level) to the next when one organism eats another. It illustrates the flow of energy and nutrients. For example, a grazing food chain starts with producers (plants), followed by primary consumers (herbivores eating plants), then secondary consumers (carnivores eating herbivores), and potentially tertiary or higher-level consumers. Energy is transferred at each step, but a significant amount is lost at each transfer (typically about 90%) through metabolic processes like respiration, excretion, or decomposition (Figure 15.1 shows a simplified food chain concept).

In reality, feeding relationships are rarely simple linear chains. Organisms often eat more than one type of food, and are eaten by more than one type of predator. These interconnected feeding relationships form a complex network called a food web. A food web shows the multiple pathways of energy flow through an ecosystem, where food chains are interlinked.

There are generally two types of food chains:

• Grazing food chain: Starts with living primary producers (plants) and involves herbivores and carnivores.
• Detritus food chain: Starts with dead organic matter (detritus) and involves decomposers (bacteria, fungi) and detritivores (organisms that eat detritus), breaking down the organic waste from the grazing food chain.

Types Of Biomes

As discussed, terrestrial ecosystems on a large geographical scale are classified as biomes. A biome is essentially a major ecological community of plants and animals extending over a large natural area, primarily determined by climate. Biomes are characterized by specific types of vegetation and associated animal life that have adapted to the prevailing environmental conditions.

While there are variations in classification, common major terrestrial biomes include forests, grasslands, deserts, and tundra. Aquatic biomes include marine and freshwater types. Altitudinal zonation in mountains can create conditions similar to different latitudinal biomes over short vertical distances.

Table 15.1 provides characteristics of some major world biomes:

Biogeochemical Cycles

Life on Earth relies on the continuous flow of energy and the cycling of matter. While the Sun provides the basic energy source, essential chemical elements circulate between the living (biotic) and non-living (abiotic) components of the biosphere. These cyclic movements are known as biogeochemical cycles.

"Bio" refers to living organisms, "geo" refers to Earth's rocks, soil, air, and water, and "chemical" refers to the elements and compounds involved. These cycles describe how chemical elements move through organisms and the environment, maintaining the relatively stable composition of the atmosphere and hydrosphere over long geological timescales.

Biogeochemical cycles are largely powered by solar energy. The cycle typically involves organisms absorbing elements from the environment, the transfer of these elements through food chains, and their return to the air, water, or soil through excretion and decomposition.

There are two main types of biogeochemical cycles based on where the primary reservoir of the element is located:

• Gaseous Cycles: The main reservoir for the element is the atmosphere or the oceans. Examples include the carbon, oxygen, and nitrogen cycles.
• Sedimentary Cycles: The main reservoir for the element is the Earth's crust (soil, rocks, minerals). Examples include the phosphorus, sulfur, and calcium cycles.

The Water Cycle

Water is essential for all life and is constantly exchanged between the atmosphere, oceans, land, and living organisms. The movement of water in its various states (solid, liquid, gas) is described by the water or hydrological cycle. This cycle, driven by solar energy, is a fundamental biogeochemical cycle that links all Earth's spheres (as covered in detail in Chapter 13).

The Carbon Cycle

Carbon is a fundamental building block of all organic matter and is found in all living organisms and organic compounds. The carbon cycle primarily involves the circulation of carbon dioxide ($CO_2$), the main form of carbon in the atmosphere and dissolved in oceans (Figure 15.2 conceptually illustrates the carbon cycle pathways).

Diagram showing the main reservoirs of carbon (atmosphere, ocean, land plants/soil, rocks) and the processes (photosynthesis, respiration, decomposition, combustion, volcanic activity) that move carbon between them.

The cycle begins with photosynthesis by green plants (producers), which absorb $CO_2$ from the atmosphere (or dissolved in water) and convert it into organic compounds, primarily glucose (carbohydrates), using sunlight as energy. Oxygen is released as a byproduct. Some of these carbohydrates are used by the plant itself for energy through respiration, releasing $CO_2$ back into the atmosphere. The remaining organic matter becomes part of the plant's tissues.

When herbivores (primary consumers) eat plants, they obtain these carbon compounds. Animals also use some of this organic matter for energy through respiration, releasing $CO_2$ into the atmosphere. Carbon is also transferred up the food chain when carnivores eat herbivores.

When plants and animals die, or when organisms excrete waste, their organic matter is broken down by decomposers (bacteria and fungi). Decomposition oxidizes the organic carbon, returning $CO_2$ to the atmosphere or dissolved in water. Over long geological periods, some organic matter is not fully decomposed and can form fossil fuels (coal, oil, gas), which represent large carbon reservoirs. The burning of fossil fuels (combustion) rapidly releases large amounts of $CO_2$ back into the atmosphere, significantly impacting the carbon cycle balance.

The Oxygen Cycle

Oxygen is essential for respiration in most organisms and is a key component of the atmosphere (about 21%). The oxygen cycle is complex, involving the movement of oxygen through the atmosphere, biosphere, and lithosphere. Oxygen is produced primarily through photosynthesis as a byproduct when water molecules are split by sunlight (photolysis of water). This oxygen is released into the atmosphere.

Organisms (plants and animals) use atmospheric oxygen for respiration, which involves oxidizing organic compounds (like carbohydrates) to release energy, producing $CO_2$ and water. Oxygen also combines with many elements and minerals in the Earth's crust through oxidation processes, forming various oxides (e.g., iron oxide). Oxygen also exists in other compounds like nitrates (combined with nitrogen).

The Nitrogen Cycle

Nitrogen is the most abundant gas in the atmosphere (about 78%) and is a crucial component of essential organic molecules like amino acids (building blocks of proteins), nucleic acids (DNA and RNA), vitamins, and pigments. Although abundant in the atmosphere, most organisms cannot directly use atmospheric nitrogen ($N_2$) in its gaseous form. The nitrogen cycle involves processes that convert atmospheric nitrogen into usable forms and its circulation through ecosystems (Figure 15.3 conceptually depicts the nitrogen cycle).

Diagram showing the various steps of the Nitrogen Cycle, including nitrogen fixation (converting atmospheric N2 to usable forms), assimilation by organisms, and decomposition/denitrification returning nitrogen to the atmosphere or soil.

The key step is nitrogen fixation, converting inert atmospheric $N_2$ into reactive nitrogen compounds (like ammonia, $NH_3$). This is primarily done by:

• Biological Fixation: Carried out by certain species of free-living bacteria in soil and water, and symbiotic bacteria living in the root nodules of leguminous plants (like beans and peas). This accounts for about 90% of nitrogen fixation. Blue-green algae (cyanobacteria) also contribute, especially in aquatic environments.
• Atmospheric Fixation: Small amounts of nitrogen are fixed by lightning and cosmic radiation, which provide enough energy to break nitrogen molecules apart and combine with oxygen to form nitrogen oxides, which dissolve in rain and reach the Earth as nitrates.
• Industrial Fixation: Human processes, particularly the Haber-Bosch process for making fertilizers, convert large amounts of atmospheric nitrogen into ammonia.

Once nitrogen is fixed into usable forms (ammonia or nitrates), green plants can absorb and assimilate it, incorporating it into their tissues. Animals obtain nitrogen by eating plants or other animals. When organisms die or excrete waste, organic nitrogen compounds are broken down by decomposers in the soil through ammonification, producing ammonia. Ammonia can then be converted into nitrites ($NO_2^-$) and then nitrates ($NO_3^-$) by specific types of bacteria (nitrification). Nitrates are readily absorbed by plants.

Finally, certain bacteria in oxygen-poor conditions (e.g., waterlogged soils) can convert nitrates back into gaseous nitrogen ($N_2$), which is released into the atmosphere. This process is called denitrification, completing the cycle.

Other Mineral Cycles

Besides carbon, oxygen, nitrogen, and hydrogen, numerous other mineral elements are essential nutrients for life (macro and micronutrients), such as phosphorus ($P$), sulfur ($S$), calcium ($Ca$), potassium ($K$), magnesium ($Mg$), etc. These elements participate in sedimentary biogeochemical cycles, as their main reservoir is typically the Earth's crust.

Minerals are released from rocks through weathering. Soluble mineral salts dissolve in soil water or are carried by runoff into lakes, streams, and eventually the oceans. Plants absorb dissolved minerals from the soil or water. Minerals are then transferred to animals when they consume plants or other animals. These elements become incorporated into the organisms' tissues.

When organisms die, decomposers break down the organic matter, returning the minerals to the soil and water in inorganic forms, making them available again for uptake by plants. Some minerals accumulate in sediments on the seafloor over geological time. These sediments can be uplifted through tectonic activity and exposed to weathering, allowing the minerals to re-enter the active cycle. Thus, these cycles involve movement between the lithosphere, hydrosphere, and biosphere.

Ecological Balance

Ecological balance refers to a state of dynamic equilibrium within an ecosystem. It is achieved when the diversity of living organisms and the populations of individual species remain relatively stable over time. While ecosystems are not static, changes usually occur gradually through natural processes like ecological succession.

This balance is maintained through the complex interactions of competition and cooperation among different species. Species compete for resources like food, water, light, and space. Simultaneously, species cooperate through mutualistic relationships, pollination, and, fundamentally, through predator-prey dynamics. In a balanced ecosystem, predator populations regulate prey populations, preventing overgrazing or resource depletion, and vice versa. For example, in grasslands, large herbivore populations are controlled by carnivores, maintaining the grassland ecosystem's health.

Any significant disturbance can upset this balance. Natural ecological succession involves the gradual replacement of one plant and animal community by another over time (e.g., a field slowly becoming a forest). In a healthy ecosystem, these changes are predictable and lead towards a stable climax community adapted to the environment.

However, ecological balance can be severely disturbed by:

• Introduction of New Species: Invasive species, if they outcompete native species or lack natural predators, can disrupt food webs and alter the entire ecosystem structure.
• Natural Hazards: Events like volcanic eruptions, large fires, severe storms, or prolonged droughts can cause rapid and extensive changes, eliminating populations or altering habitats.
• Human Activities: Human interference is currently the most significant cause of ecological imbalance. Activities such as deforestation, habitat destruction, pollution, overfishing, overhunting, and unsustainable resource extraction exert immense pressure on ecosystems. These actions disrupt food chains, reduce biodiversity, degrade habitats, and can trigger secondary successions where disturbed areas are colonized by different species, often less diverse and stable than the original community.

The increasing scale and intensity of human impact have led to widespread ecological imbalances globally. This loss of ecological stability has contributed to various environmental problems and natural calamities, including increased frequency and severity of floods, landslides, the spread of diseases, and erratic climatic events.

There is a strong interdependence between the plant and animal communities within their habitats. The diversity of life in an area serves as an indicator of the health and stability of the habitat. Understanding these complex relationships is crucial for developing effective strategies for protecting and conserving natural ecosystems and preventing further ecological imbalances.

Exercises

Content for Exercises is excluded as per your instructions.

Project Work

Content for Project Work is excluded as per your instructions.