Chapter 3 — Ecosystems

Definition and scope: An ecosystem is a functional unit made up of living organisms and the physical environment with which they interact. It is not only a list of species but the processes that link them: feeding, reproduction, decay, energy flow and nutrient cycling. Ecosystems vary in size and complexity. A small rock pool, a classroom terrarium, a farm, a forest or an ocean are all ecosystems, each with its own specific set of organisms and environmental conditions.

Biotic components: Biotic components are all living elements: plants, animals, fungi and microorganisms. They interact through feeding relationships and other connections. Producers create organic matter using sunlight; consumers depend on other organisms for food; decomposers break down dead matter. The balance among these groups shapes population sizes and community structure.

Abiotic components: Abiotic components are the physical and chemical parts: sunlight, temperature, water, soil, minerals, pH, and air. These factors determine which organisms can survive and how productive an ecosystem can be. For example, sunlight and water availability limit plant growth, while soil nutrients determine the kinds of plants that can thrive.

Function and processes: Ecosystems capture energy (mainly from the sun), transform it into biological forms, and recycle matter. Photosynthesis converts light into chemical energy; consumers transfer that energy by eating; respiration releases energy and carbon dioxide; decomposers return nutrients to the soil. These flows of energy and matter keep ecosystems functioning. Disturbances such as storms, fires, or human activities can alter these processes and lead to changes in species composition. Understanding ecosystems means paying attention to both the parts and how they connect, so that we can predict how changes — natural or human-caused — will ripple through communities.

Habitat — the address: Habitat describes the physical place where an organism lives. It supplies the essentials: food, water, shelter and space for reproduction. Habitats can be very specific: the bark of an old mango tree, the shaded underside of a rock, the muddy edge of a pond or the canopy of a banyan. A habitat includes microhabitats — small zones with slightly different conditions — that different organisms prefer. Temperature, moisture, light and soil type within a habitat influence the species found there.

Niche — the job: A niche describes how an organism fits into the ecosystem: what it eats, when it is active (day or night), where it nests, how it obtains resources and how it interacts with other species. It is a multidimensional concept combining behaviour, diet, trophic role and environmental tolerances. Two species can share a habitat but must occupy different niches to coexist long-term. If niches overlap too closely, competition may force one to change, move or decline.

Examples to contrast: Consider a tree. Multiple organisms may live on the same tree but in different niches: birds may nest in branches and eat fruits, insects may feed on sap or leaves, lichens may grow on shaded bark surfaces. Their niches differ in food source, place of activity and time of activity. Understanding niches explains why species have certain adaptations: a bird with a stout beak cracks seeds, a long-tongued insect visits deep flowers, and nocturnal animals avoid daytime predators.

Adaptations and niche partitioning: Niche partitioning is how similar species reduce competition by using different resources or acting at different times. This can be seen in two species of birds feeding on the same tree but taking insects from different heights. Behavioural, physiological and morphological adaptations help organisms occupy their niches. Learning about habitat and niche helps predict where to find organisms and how they might respond to changes such as habitat loss or introduction of a competitor.

Interactions shape communities: Species in an ecosystem do not live in isolation. Their interactions — positive, negative or neutral — influence survival, reproduction and distribution. Four major types are competition, predation, parasitism and mutualism. Each affects population size and can lead to adaptations in the species involved.

Competition: Competition occurs when organisms require the same limited resource. It can be intraspecific (within the same species) or interspecific (between species). For example, two plants in a small pot compete for light, water and nutrients. Competition can be direct (fighting or allelopathy where a plant releases chemicals) or indirect (both using the same resource). Outcomes include competitive exclusion (one species replaces another), coexistence through resource partitioning, or reduced growth and reproduction for both.

Predation: Predation involves one organism (the predator) killing and eating another (the prey). Predators control prey populations and can maintain balance. Prey species evolve defences such as camouflage, speed, herd behaviour or toxic chemicals. Predators evolve hunting skills: sharp teeth, claws, camouflage or social hunting. Predator-prey dynamics often produce cyclical changes in numbers, with increases in prey followed by increases in predators and then declines.

Parasitism and mutualism: Parasitism benefits the parasite and harms the host but usually does not kill it quickly; parasites can change host behaviour or health. Examples include tapeworms or external lice. Mutualism benefits both species: pollinators get nectar while flowers get pollinated; lichens are a mutualism of fungi and algae. Commensalism is another type where one benefits and the other is unaffected, such as epiphytes on tree branches. The balance of these interactions determines community structure, resource flow and the adaptability of ecosystems to change.

Linear chains and complex webs: A food chain is the simplest way to show feeding relationships: a linear pathway from producers through consumers to decomposers. In nature, however, species feed on many others and are eaten by several predators. These multiple linkages form a food web, which is a realistic map of who eats whom in an ecosystem. Food webs reveal the multiple pathways for energy and matter and show how interconnected systems are.

Trophic levels and roles: Trophic levels classify organisms by how many energy-transfer steps separate them from producers. Level 1 are producers (plants and algae), level 2 are primary consumers (herbivores), level 3 are secondary consumers (carnivores that eat herbivores) and so on. Decomposers work across levels by breaking down dead material into nutrients. An organism may occupy more than one trophic level: a crow eats seeds (as a primary consumer) and also eats insects (as a secondary consumer).

Energy and matter flow: Food chains and webs show energy moving from sunlight to plants and then through consumers. Arrows point from food to eater (e.g., grass → rabbit means the rabbit eats grass). Food webs also help trace how pollutants or toxins move through an ecosystem via biomagnification — higher concentrations in predators at the top of the web. A disturbance removing one species can affect others; for instance, removing a top predator may allow prey populations to rise and overconsume plants, changing the whole web.

Using food webs in study and conservation: Drawing local food webs trains observation skills: note what species eat and who eats them. Food webs help identify key species and possible consequences of human activities. They are valuable for conservation planning because protecting one species may benefit many others linked in the web.

How energy enters and moves: Energy enters ecosystems mainly as sunlight. Producers (green plants and algae) convert light energy into chemical energy by photosynthesis, storing it in sugars. Consumers obtain this energy by eating plants or other animals. At each transfer, much energy is used for life processes (movement, growth, heat production) and lost as heat to the surroundings. Because of these losses, less energy is available at higher trophic levels.

The 10% rule and limits on food chains: A practical rule is that around 10% of the energy at one trophic level becomes available to the next—though this is a simplification. Some ecosystems may transfer slightly more or less energy. Still, the main idea holds: energy decreases markedly at each step, which limits the length of food chains and the number of top predators an ecosystem can support. This explains why trophic pyramids typically narrow toward the top.

Types of ecological pyramids: There are three common pyramids. Pyramids of numbers show how many organisms exist at each trophic level. These can be upright or inverted depending on the organisms (e.g., many parasites on one host). Pyramids of biomass show the total dry mass of living tissue at each level and usually narrow upward. Pyramids of energy display the energy flow per unit area per time (for example, kJ m-2 yr-1) and are always upright because energy transfer is unidirectional and decreases at higher levels.

Applications and human choices: Understanding energy flow helps explain ecological efficiency and human diet choices. Eating lower trophic levels (plants, grains) uses ecosystem energy more efficiently than eating top predators. This knowledge informs sustainable food production and emphasises why conserving primary productivity (plants and phytoplankton) is essential for supporting ecosystems and human societies.

The continuous movement of water: The water cycle describes how water circulates between the atmosphere, land and oceans. Driven by solar energy and gravity, this continuous movement keeps water available for ecosystems and human use. The cycle has several linked processes that move water in different forms (liquid, vapour, ice) and between reservoirs.

Key processes explained: Evaporation is when liquid water on oceans, lakes or soil becomes water vapour and rises into the atmosphere. Transpiration is the release of water vapour by plants from their leaves; together with evaporation it is called evapotranspiration. Condensation happens when water vapour cools to form tiny droplets, creating clouds or fog. Precipitation returns water to the surface as rain, drizzle, sleet or snow. Infiltration is the movement of surface water into soil, recharging groundwater. Runoff is water moving over the surface into streams and rivers, eventually reaching lakes or the sea. Some water is stored temporarily as snow or ice in glaciers or as groundwater in aquifers.

Importance for ecosystems: The water cycle influences soil moisture, which determines plant growth and therefore primary productivity. It controls the availability of freshwater for animals and humans. Wetlands, rivers and ponds are habitats maintained by regular water inputs. Groundwater discharge can support springs and maintain river flow during dry periods, supporting aquatic life.

Human impacts and management: Human activity can alter the water cycle. Deforestation reduces transpiration and may change local rainfall patterns; urbanisation increases runoff and reduces infiltration, leading to floods and less groundwater recharge. Over-extraction of groundwater lowers water tables, sometimes causing wells to dry. Pollution of water bodies harms organisms and disrupts the cycle’s role in purifying water. Sustainable water management—protecting forests, creating recharge zones, reducing pollution and using water wisely—helps maintain a healthy hydrological cycle and resilient ecosystems.

Carbon moves through living and non-living parts: Carbon is a central element in living molecules such as carbohydrates, proteins and fats. The carbon cycle describes how carbon atoms move among the atmosphere, living organisms, soil, oceans and rocks. This movement occurs through biological processes (photosynthesis, respiration, decomposition), chemical processes and human activities such as burning fossil fuels.

Biological pathways: Photosynthesis by plants and algae removes carbon dioxide (CO2) from the air and converts it into organic compounds (sugars). Animals and other heterotrophs eat these plants, transferring carbon through food chains. Organisms respire, returning CO2 to the atmosphere. When plants and animals die, decomposers break down organic matter, releasing carbon to the soil and releasing CO2 through microbial respiration. Some carbon becomes part of soil organic matter, helping soil fertility.

Geological and oceanic storage: Over long time scales, some organic matter becomes buried and, under pressure and heat, forms fossil fuels (coal, oil, natural gas). Oceans absorb CO2 from the atmosphere and store carbon as dissolved bicarbonate or as organic matter in marine organisms. Marine sediments can store carbon for long periods. These sinks and sources help regulate atmospheric CO2 levels and thus influence climate.

Human influence and consequences: Burning fossil fuels and deforestation release stored carbon rapidly as CO2, increasing greenhouse gas concentrations and contributing to global warming. This affects ecosystems by shifting species distributions, altering timing of life-cycle events (phenology), and changing precipitation patterns. Protecting forests, restoring degraded lands, and reducing fossil fuel use help maintain the natural balance of the carbon cycle and reduce climate change impacts on ecosystems.

Why nitrogen cycles matter: Nitrogen is essential for proteins, enzymes and DNA. Most atmospheric nitrogen (N2) is unavailable to plants. The nitrogen cycle converts atmospheric nitrogen into forms that plants and animals can use, and then returns nitrogen back to the atmosphere, closing the loop. This cycle depends heavily on different kinds of bacteria and on certain physical processes.

Major stages and organisms involved: Nitrogen fixation converts N2 gas into ammonia (NH3) or ammonium (NH4+) that plants can use. Biological fixation is carried out by free-living bacteria in soil and by symbiotic bacteria (Rhizobium) in root nodules of legumes. Lightning can also fix nitrogen by converting N2 to nitrates. Nitrification is a two-step process in which ammonia is oxidised first to nitrite (NO2-) by bacteria such as Nitrosomonas, and then to nitrate (NO3-) by Nitrobacter; plants mainly absorb nitrates. Assimilation is the incorporation of nitrate into plant proteins and other organic molecules; consumers obtain nitrogen by eating plants. Ammonification (or mineralisation) is the conversion of organic nitrogen from dead organisms and waste into ammonia by decomposers. Denitrification, performed by bacteria like Pseudomonas and Paracoccus under low-oxygen conditions, converts nitrate back into gaseous N2, releasing it to the atmosphere.

Human impacts and management: Use of synthetic nitrogen fertilisers increases reactive nitrogen in soils and water. Runoff transports nitrates to water bodies, causing algal blooms and eutrophication. Excess nitrogen in the atmosphere from fossil fuel combustion contributes to air pollution and acid rain. Sustainable practices include using organic fertilisers, crop rotation with legumes to fix nitrogen naturally, precise fertiliser application to reduce runoff, and protecting wetlands that can denitrify excess nutrients before they reach rivers.

Producers — foundations of food webs: Producers, or autotrophs, make organic compounds from inorganic sources. Green plants and photosynthetic algae use sunlight, water and carbon dioxide to form sugars and other molecules. Some bacteria are chemosynthetic producers, using chemical energy from inorganic reactions. Producers set the energy and biomass base for an ecosystem; their productivity limits how many consumers the system can support.

Consumers — energy users: Consumers, or heterotrophs, obtain carbon and energy by eating producers or other consumers. Primary consumers are herbivores feeding directly on plants (e.g., insects, cows). Secondary consumers feed on herbivores (e.g., frogs, small carnivorous birds). Tertiary consumers are higher predators that may feed on other carnivores. Omnivores eat both plants and animals. Consumers have adaptations for their diets: specialized teeth, digestive systems, hunting behaviours, or body shapes that suit their feeding style.

Decomposers and detritivores — recyclers of ecosystems: Decomposers (fungi and bacteria) chemically break down dead organisms and waste, returning nutrients such as nitrogen, phosphorus and minerals to the soil. Detritivores (earthworms, woodlice, certain insects) mechanically fragment organic matter, increasing surface area for decomposers. This recycling maintains soil fertility and allows nutrients to be reused by producers, closing material cycles. Without decomposers, nutrients would remain locked in dead tissue and ecosystems would run out of usable matter.

Interactions and balance: The balance between these groups determines ecosystem health. If decomposers are absent or slowed (e.g., in cold or dry conditions), nutrient release is reduced and plant growth can decline. Overharvesting of producers or important consumers disrupts food webs. Recognising the roles of each group emphasises why conserving habitats and maintaining diversity are essential for continued ecosystem functioning and for human benefits such as food production and clean soil and water.

What is stability? Stability in an ecosystem means its ability to remain functioning and to recover after disturbances. A stable ecosystem maintains its basic structure — such as species composition and processes like nutrient cycling — despite shocks like storms, droughts or human impacts. Stability depends on species diversity, redundancy of roles (several species doing similar jobs) and the complexity of links in the food web.

Why complexity can help: In a complex web with many links, if one food source declines, consumers may switch to other prey. This flexibility can buffer changes. On the other hand, some links are crucial: if a species that performs a unique role is lost, the system may change rapidly. Thus, both diversity and the presence of key functional species influence resilience.

Keystone species defined: A keystone species has a disproportionately large effect on its community relative to its abundance. Its activities maintain community structure and biodiversity. Predators often act as keystone species by controlling herbivore populations and preventing overgrazing. Mutualists such as pollinators or seed dispersers can also be keystones because many plants depend on them for reproduction. Removing a keystone can cause cascading changes, altering populations at many trophic levels and sometimes collapsing the ecosystem’s function.

Indicators and conservation: Signs of instability include sudden population crashes, loss of habitat specialists, algal blooms after nutrient input, or invasive species dominating a habitat. Conservation strategies often focus on protecting keystone species, maintaining habitat complexity and promoting biodiversity. Monitoring simple indicators — species counts, water quality, and presence of key predators or pollinators — helps detect problems early and guides restoration efforts.

Succession as orderly change: Succession is the gradual and predictable change in species composition and ecosystem structure over time following a disturbance or the creation of new substrate. It describes how communities develop from simple pioneer stages to more complex and stable communities, often termed climax communities under a given climate.

Primary succession: Primary succession begins on surfaces that previously lacked soil and life, such as bare rock after lava flows, glacial retreat, or newly formed sand dunes. Pioneer organisms like lichens and certain mosses colonise first. They weather rock and add organic matter as they grow and die, gradually forming a thin soil layer. This allows small herbaceous plants to establish, followed by grasses, shrubs and eventually trees as soil depth and fertility increase. Primary succession is slow because soil formation is the rate-limiting step.

Secondary succession: Secondary succession occurs where a community has been disturbed but soil remains intact — for example, after fires, floods, or abandoned agricultural land. Because seeds, roots and soil microorganisms often persist, secondary succession proceeds faster than primary succession. Early stages commonly involve fast-growing, sun-loving species (weeds and grasses) that prepare the site for later species by improving soil and altering light conditions.

Processes and outcomes: Succession involves changes in resource availability, light penetration, soil depth and nutrient levels. Early colonisers are typically r-selected species that reproduce quickly; later stages favour K-selected species that compete well in crowded environments. Succession is not always linear: disturbances or environmental changes can reset stages or create alternate stable states. Understanding succession helps in restoration ecology, predicting recovery after disturbance and managing landscapes for biodiversity.

Humans as agents of rapid change: Human activities affect ecosystems on local to global scales. While natural changes occur slowly, human impacts such as land clearing, pollution, overharvesting and introduction of non-native species can be rapid and large. These actions often reduce biodiversity, destabilise food webs and impair ecosystem services like clean water, pollination and soil fertility that people rely on.

Habitat loss and fragmentation: Conversion of forests, grasslands and wetlands to agriculture, industry or housing destroys habitats. Fragmentation divides continuous habitats into smaller patches, isolating populations and reducing genetic exchange. Small, isolated populations are more vulnerable to disease, inbreeding and local extinction. Fragmentation also increases edge effects — changes in light, temperature and predation at habitat borders — which can alter species composition.

Pollution, eutrophication and climate change: Chemical pollutants, plastic waste and untreated sewage contaminate air, soil and water, harming plants and animals. Excess fertilisers run off into lakes and rivers, causing eutrophication: algal blooms followed by oxygen depletion and fish kills. Burning fossil fuels releases greenhouse gases and pollutants, driving global climate change and acid rain. Changes in temperature and rainfall shift habitat suitability, forcing species to migrate or face extinction.

Overexploitation and invasive species: Overfishing and excessive hunting reduce populations below sustainable levels. Introducing invasive species — intentionally or accidentally — can displace native species through competition, predation or disease. Invasive plants can alter fire regimes and nutrient cycling, further damaging ecosystems. Effective management involves regulation of harvesting, controlling invasive species, pollution reduction, habitat restoration and community-based conservation to balance human needs with ecosystem health.

Goals and principles: Conservation aims to protect species, habitats and ecological processes so that ecosystems remain healthy and continue providing services for people and nature. Sustainability means using resources at a rate that does not deplete them for future generations. Both ideas require combining science, policy and community action to manage landscapes responsibly.

Practical conservation measures: Creating protected areas such as national parks, wildlife sanctuaries and community reserves preserves habitat and reduces human pressures. Habitat restoration — replanting native species, removing invasive plants, and restoring water flows — helps degraded areas recover. Buffer zones around protected areas can reduce harmful edge effects. Legal tools like anti-poaching laws and trade controls protect threatened species.

Sustainable resource use: Sustainable agriculture includes crop rotation, intercropping, agroforestry and use of organic fertilisers to maintain soil fertility and reduce pollution. Water conservation through rainwater harvesting, groundwater recharge and efficient irrigation reduces stress on freshwater ecosystems. Sustainable fishing uses quotas, protected breeding areas and gear that reduces bycatch. Energy choices, such as using renewables, decrease greenhouse gas emissions and limit climate impacts on ecosystems.

Community and individual actions: Conservation succeeds when local people are involved and benefit. Community forestry, citizen science monitoring and education programmes build stewardship. Individuals can help by reducing waste and single-use plastics, planting native trees, saving water, choosing sustainable food, and supporting conservation projects. Small actions add up: widespread responsible behaviour can maintain ecosystem functions and biodiversity for the long term.