What Is an Ecosystem?
Ecosystem: Living organisms + their non-living environment
An ecosystem is all the living organisms in a particular area, together with the non-living components of the environment that they interact with, such as soil, air, water, temperature, and light.
Imagine a woodland on a spring morning. There are oak trees, bluebells, mosses, fungi, earthworms, beetles, rabbits, foxes, owls, and thousands of species of bacteria in the soil. There is sunlight filtering through the canopy, rainfall soaking into the ground, minerals dissolving in the soil water, and carbon dioxide in the air. All of this, taken together (every living thing and every non-living factor) makes up the woodland ecosystem.
Ecosystems come in every shape and size. A vast tropical rainforest is an ecosystem. So is a small garden pond. So is a single rotting log on a forest floor. What defines an ecosystem is not its size, but the fact that the living and non-living components within it interact with and influence each other constantly.
Scientists divide the components of an ecosystem into two categories:
- Biotic components represents all the living organisms within the ecosystem. This includes every plant, animal, fungus, bacterium, and other microorganism present. Biotic components interact with each other through feeding, competing, pollinating, and decomposing.
- Abiotic components represents all the non-living factors. These include sunlight intensity, temperature, rainfall, humidity, soil type, soil pH, wind speed, and the availability of minerals and nutrients. Abiotic factors shape which organisms can survive in an ecosystem and how well they thrive.
Biotic and abiotic components do not exist independently of each other. Plants shape the soil through their roots and fallen leaves. Animals change the landscape through grazing and burrowing. Rainfall determines which plants grow, and which plants grow determines which animals can live there. Everything is connected.
What Is Biodiversity and Why Does It Matter?
Biodiversity: The variety of life within an ecosystem
Biodiversity is the variety of different species living within an ecosystem. An ecosystem is described as biodiverse if it contains many different species. The greater the number of different species, the higher the biodiversity.
A lush tropical rainforest might contain tens of thousands of species of plants, insects, birds, mammals, and microorganisms all living within a few square kilometres. That ecosystem has extremely high biodiversity. A field of a single crop of wheat, sprayed with pesticides to kill insects and weeds, might contain only a handful of species. That ecosystem has very low biodiversity.
Biodiversity matters enormously, and not just for aesthetic or ethical reasons. High biodiversity makes an ecosystem more stable and more resilient. When many different species are present and fulfilling different roles, the ecosystem can absorb changes and disturbances without collapsing. If one species declines, another can often fill its role. If a disease kills one plant species, many others remain to produce food and oxygen, provide shelter, and hold the soil in place.
Low biodiversity, on the other hand, makes an ecosystem fragile. If a single-crop farm is hit by a fungal disease, the entire crop can be wiped out because there is no variety to resist it. This is why the potato famine in Ireland in the 1840s was so catastrophic — the entire potato crop planted was a single variety, and when a blight struck, there were no other varieties with natural resistance to survive.
Why Biodiversity Is Under Threat
Human activities including deforestation, intensive farming, pollution, climate change, and the introduction of non-native species are reducing biodiversity across the planet at an alarming rate. Scientists estimate we are losing species at a rate far faster than at any point in the past millions of years. Once a species goes extinct, it is gone forever and the consequences for the ecosystems that depended on it can be profound and unpredictable.
Interdependence (How Organisms Depend on Each Other)
Interdependence: Every species depends on other species
Interdependence is the way in which all organisms in an ecosystem depend upon each other for their survival. A change in the population of one species will affect other species throughout the ecosystem.
No organism exists in isolation. Every species within an ecosystem has relationships with other species, whether those relationships are immediately obvious or hidden beneath the surface. These relationships are so numerous and so intertwined that ecosystems are often described as a web of life rather than a simple chain.
Within a community, each species depends on other species in a variety of ways:
- Food: Almost every organism depends on other organisms for energy. Herbivores eat plants. Carnivores eat herbivores. Omnivores eat both. Decomposers break down dead organisms and return nutrients to the soil, which plants then absorb to grow. Remove any one link in this feeding web and the effects ripple outward in every direction.
- Shelter: Many animals depend on specific plants or other species for shelter, nesting sites, and protection from the weather and predators. Squirrels nest in tree hollows. Birds build nests in hedgerows and dense shrubs. Many insects shelter under bark or within leaf litter. Remove the shelter species and the animals that depend on it face exposure and decline.
- Pollination: The majority of flowering plants depend on animals, primarily insects such as bees, butterflies, and hoverflies, to transfer pollen between flowers and enable reproduction. Without pollinators, these plants cannot produce seeds or fruit and their populations collapse. Since many animals depend on those plants for food, the consequences extend far beyond the plants themselves. This is why the global decline of bee populations is treated as a serious ecological crisis.
- Seed dispersal : Many plants depend on animals to carry their seeds away from the parent plant. Birds eat berries and deposit the seeds in their droppings elsewhere. Squirrels bury acorns and nuts and sometimes forget them, allowing them to germinate. Mammals carry burr-covered seeds on their fur. Without these animal partnerships, the plants would be unable to colonise new areas and their populations would become crowded and decline.
Food Chains and Feeding Relationships
The most common form of interdependence in any ecosystem is the feeding relationship. We represent these relationships using food chains. A food chain shows the flow of energy from one organism to the next as each one is eaten by the next.
In this food chain, grass is the producer as it makes its own food through photosynthesis using sunlight. The rabbit is the primary consumer as it eats the producer. The fox is the secondary consumer as it eats the primary consumer. The arrows show the direction of energy flow: from the grass, to the rabbit, to the fox.
Now consider what happens when one part of this chain is disrupted. This is where interdependence becomes dramatically clear.
Stable Community: Balance between all species populations over time
A stable community is one in which the size of the populations of all species remain relatively constant over time. The different populations are living in a healthy, self-regulating balance within their environment.
In the grass-rabbit-fox food chain, a stable community would look like this: a consistent, healthy population of grass sustaining a moderate population of rabbits, which in turn sustains a smaller population of foxes. When fox numbers rise slightly, more rabbits are eaten and rabbit numbers fall. With fewer rabbits to eat, some foxes starve and fox numbers fall. With fewer rabbits grazing, the grass recovers and rabbit numbers rise again. The system naturally self-corrects, oscillating around a balanced point.
This self-regulating balance is sometimes called a dynamic equilibrium. It is not perfectly static (populations fluctuate up and down) but over time, no single population grows without limit or collapses entirely. Each population acts as a natural check on the others.
Stable communities are resilient. They can absorb moderate disturbances (a harsh winter, a drought, a temporary disease) and return to balance. But they have limits. Sufficiently large or prolonged disturbances (the complete removal of a keystone species, catastrophic habitat loss, severe pollution) can push an ecosystem past the point from which it can recover.
Competition: The Struggle for Resources
Competition occurs when organisms compete for the same limited resources in an ecosystem. Resources are limited, meaning there is not enough for every organism to have all it needs. Organisms that compete most effectively are more likely to survive and reproduce.
Resources in nature are never unlimited. There is only so much sunlight, so much water, so many nutrients in the soil, so much food to eat, and so much territory to occupy. Because resources are limited and organisms need them to survive and reproduce, competition is inevitable. It is one of the fundamental forces shaping life on Earth.
Competition takes two distinct forms, depending on whether the competing organisms belong to the same species or different species:
Intraspecific competition is often the most intense, because members of the same species need exactly the same resources i.e. the same food, the same water, the same nesting sites, the same mates. When resources are scarce, individuals of the same species are each other's closest rivals.
Interspecific competition between different species can be equally fierce when two species occupy the same ecological niche that is, when they fill the same role in the ecosystem and depend on the same resources. A classic example is the grey squirrel, introduced to Britain from North America, which outcompetes the native red squirrel for food and habitat. The grey squirrel is larger, more adaptable, and can digest a wider range of foods, including unripe acorns that red squirrels cannot eat. As a result, red squirrel populations have collapsed across most of Britain wherever grey squirrels have been introduced.
The Competitive Exclusion Principle
If two species in the same ecosystem compete for exactly the same resources in exactly the same way, one will eventually outcompete the other completely. The losing species will either go extinct in that area or be forced to adapt (shifting its diet, its habitat use, or its behaviour) to reduce the overlap. This is called the competitive exclusion principle and it is one of the key drivers of the evolution of different species into distinct ecological niches.
How Plants Compete
Plants cannot run, fight, or call out for help. Their competition is slower and quieter than that of animals, but no less intense or consequential. Plants compete primarily through growth i.e. growing faster, taller, wider, and deeper than their neighbours to secure the resources they need.
- Light: Sunlight is the energy source for photosynthesis, and without enough of it, a plant cannot produce glucose for growth, reproduction, or any other life process. Plants in dense woodland or crowded fields must grow tall quickly to reach the light before their neighbours shade them out. Trees have evolved enormous heights precisely as a competitive strategy to place their leaves above the canopy of rivals. Forest floor plants like bluebells have adapted by flowering in early spring before the trees above them come into leaf and block the light.
- Space: A plant that is heavily shaded by its neighbours will struggle to photosynthesise effectively. Plants compete for space both above ground (spreading their leaves to maximise light capture) and below ground (spreading their root systems to maximise water and mineral absorption). Many plants produce chemicals in their roots that inhibit the germination and growth of competitors nearby — a phenomenon called allelopathy. Bracken fern, for example, produces chemicals that suppress the growth of almost all other plant species beneath it.
- Water: Water is essential as a solvent for chemical reactions, for photosynthesis, and for transporting minerals from the soil into the plant. In dry conditions, plants with deeper or more extensive root systems can access water that shallower-rooted competitors cannot reach. Cacti have evolved shallow but very wide-spreading root systems to capture rainfall quickly before it evaporates. Trees have deep taproots that reach groundwater sources unavailable to grasses.
- Minerals: Plants absorb minerals such as nitrates, phosphates, and potassium from the soil through their roots. These minerals are essential for producing proteins, DNA, chlorophyll, and many other biological molecules. Plants with more extensive root systems, or those that form beneficial relationships with soil fungi (called mycorrhizal networks), can access minerals that their competitors cannot. This is why invasive species like Japanese knotweed, which has an aggressive underground root system, are so devastating to native plant communities.
How Animals Compete
Animal competition is often more visible and dramatic than plant competition because animals can actively move, chase, fight, display, and vocalise. The resources animals compete for are food, mates, and territory and these three are deeply interconnected.
- Food: Animals must eat to survive, grow, and reproduce. When food is scarce, competition intensifies. This can take many forms: lions on the African savanna compete with hyenas, leopards, cheetahs, and wild dogs for the same prey. Seabirds on a cliff compete for the best fishing grounds. Urban foxes compete for the scraps in neighbourhood bins. Competition for food drives predators to become faster, stronger, or more cunning, and drives prey to become better at escaping. This evolutionary arms race between predator and prey shapes both species over generations.
- A mate: Reproduction is the biological imperative of every organism. An animal that cannot find a mate will not pass on its genes, and its genetic line will end. Competition for mates is often fierce and can involve elaborate displays of fitness — the peacock's spectacular tail, the red deer stag's antlers used in combat with rival males, the complex songs of birds. The winner gains access to a mate and passes on their genes. The loser does not. Over many generations, this competition drives the evolution of increasingly impressive traits.
- Territory: Many animals defend a territory: a defined area within which they live, feed, breed, and raise offspring. Territory provides exclusive or priority access to the food and shelter within it. Animals defend territories through a wide variety of behaviours: wolves mark their territory boundaries with scent. Robins sing loudly to warn rivals to stay away. Lions will fight to defend their territory from rival prides. A larger, better-quality territory generally leads to better food access, better breeding success, and better survival outcomes for offspring.
Survival of the Fittest and Evolution
Survival of the fittest is the idea that individuals within a species who are best adapted to compete for resources and survive in their environment are the most likely to reproduce and pass on their characteristics to offspring. Over generations, this drives evolution.
The phrase "survival of the fittest" was coined by the philosopher Herbert Spencer after reading Charles Darwin's work on natural selection. It does not mean "survival of the strongest" in a purely physical sense. "Fittest" in biology means best adapted to the environment i.e. best suited to find food, avoid predators, attract a mate, and raise offspring successfully. A fast cheetah is fit. But so is a bacterium that can digest an unusual chemical compound, or a plant that can tolerate dry soil, or a moth whose wing pattern makes it invisible on tree bark.
Individuals that cannot compete effectively are unlikely to reproduce, and may die without passing on their genes. Over time, if a species as a whole is unable to compete effectively in its ecosystem (because a stronger competitor has arrived, because the environment has changed, because its food source has disappeared) it may become extinct. Extinction is the permanent end of a species' evolutionary line.
Important: Organisms Do Not Evolve by Choice
A common misconception is that organisms decide to adapt or evolve in response to challenges. They do not. Evolution is not conscious or deliberate. Individual organisms do not change during their lifetime in response to competition. What changes, over many generations, is the gene pool of the population — as the genes of better-adapted individuals become more common and the genes of less-adapted individuals become rarer. Evolution happens to populations over generations, not to individuals during their lifetimes.
Revision Summary (Key Points to Remember)
- An ecosystem includes all the living organisms (biotic) and non-living components (abiotic) in a particular area, interacting together.
- Biodiversity is the variety of different species in an ecosystem. High biodiversity makes an ecosystem more stable and resilient to change.
- Interdependence means all organisms in an ecosystem depend upon each other for food, shelter, pollination, and seed dispersal.
- Food chains show the flow of energy through an ecosystem. Removing one species can have large, unpredictable effects on all others.
- A stable community is one where the populations of all species remain relatively constant over time through a natural self-regulating balance.
- Competition occurs when organisms compete for the same limited resources such as food, water, light, minerals, mates, and territory.
- Intraspecific competition is between individuals of the same species.
- Interspecific competition is between individuals of different species.
- Plants compete for light, space, water, and minerals. Animals compete for food, mates, and territory.
- Survival of the fittest means that individuals best adapted to compete are most likely to survive, reproduce, and pass on their genes.
- Over many generations, competition and natural selection drive evolution. Species that cannot compete effectively may become extinct
