Ecosystems and Food Webs

In the previous lesson, we looked at Earth as an interconnected planetary system. The atmosphere, water, land, ice, and living world constantly interact.

Now we can look more closely at the living part of that system.

A forest contains trees, insects, birds, mammals, fungi, bacteria, soil, water, air, and sunlight. A coral reef contains corals, fish, algae, microorganisms, seawater, minerals, and sunlight. Even a small pond contains an enormous number of living and nonliving components.

Together, these form ecosystems.

An ecosystem is not simply a place where organisms live.

Understanding those relationships helps explain why changes involving one species—or one part of the environment—can sometimes affect an entire ecosystem.

What Is an Ecosystem?

An ecosystem consists of a community of living organisms interacting with one another and with the nonliving environment around them.

The living components include plants, animals, fungi, bacteria, and other microorganisms.

The nonliving components include water, air, soil, minerals, sunlight, temperature, and other physical conditions.

Ecosystems exist at many scales.

A forest is an ecosystem. So is a lake. So is a coral reef.

But ecosystems can also be much smaller. A fallen log can support fungi, insects, microorganisms, mosses, and other organisms interacting within a particular physical environment.

At the other extreme, ecosystems can extend across enormous regions, such as grasslands, deserts, tropical forests, or the open ocean.

The boundaries are not always precise. Organisms, water, nutrients, and energy move across them.

Once again, the categories we use are tools for understanding nature—not walls that nature itself necessarily follows.

Organisms Need More Than Food

Every organism requires certain conditions and resources to survive.

Plants need sunlight, water, carbon dioxide, nutrients, and suitable temperatures. Animals need food, water, and appropriate habitat. Fungi and microorganisms require their own combinations of nutrients, moisture, temperature, and other conditions.

The particular role an organism plays within an ecosystem is often described as its ecological niche.

A niche includes much more than where an organism lives. It includes what it eats, what eats it, where and when it is active, how it reproduces, what resources it uses, and how it interacts with other organisms.

A bee, for example, needs sources of nectar and pollen, suitable places to nest, and environmental conditions in which it can survive.

But the bee is also part of the ecosystem around it.

As it moves among flowers, it transfers pollen. That helps many flowering plants reproduce. Those plants may provide fruits, seeds, shelter, and other resources for still other organisms.

The bee's life therefore cannot be understood entirely by studying the bee alone.

It exists within a web of relationships.

Energy Moves Through Ecosystems

In Lesson 1, we saw that most life ultimately depends on energy from the Sun.

Plants, algae, and some microorganisms capture solar energy through photosynthesis and store it as chemical energy.

These organisms are known as producers because they produce energy-rich organic material from simpler substances.

Other organisms obtain energy by consuming plants or other organisms. These are consumers.

Herbivores eat plants or algae. Carnivores eat other animals. Omnivores eat both plant and animal material.

But the process does not end when an organism dies.

Fungi, bacteria, and other decomposers break down dead organisms and waste. In doing so, they obtain energy and return nutrients to soil, water, and other parts of the environment.

This creates one of the fundamental patterns of ecosystems:

That distinction is important.

Energy enters most ecosystems primarily as sunlight and eventually leaves as heat.

Matter—including water, carbon, nitrogen, phosphorus, and other nutrients—is used and reused.

From Food Chains to Food Webs

A simple way to represent feeding relationships is with a food chain.

For example:

This shows one possible path through which energy can move.

But real ecosystems are rarely that simple.

A grasshopper may eat several kinds of plants. A frog may eat grasshoppers, flies, beetles, and other organisms. A snake may eat frogs, rodents, birds, or other animals. A hawk may feed on snakes, rodents, rabbits, or birds.

Each species can therefore be connected to many others.

When these connections are mapped together, they form a food web.

A food web provides a much more realistic picture of an ecosystem than a single food chain.

Species Depend on One Another in Different Ways

Food is only one kind of ecological relationship.

Organisms interact in many other ways. They compete for resources. They provide shelter. They pollinate plants. They disperse seeds. They alter habitats. They break down waste and dead material.

Some species form close relationships in which both benefit.

Flowers and pollinators provide a familiar example. An insect may obtain nectar or pollen from a flower while transporting pollen that helps the plant reproduce.

Other examples are found throughout nature. Many plants have relationships with mycorrhizal fungi that live on or around their roots. The fungi help the plants obtain water and nutrients from the soil, while receiving sugars produced by the plants through photosynthesis.

On coral reefs, corals and microscopic algae live in a close partnership. The algae use sunlight to produce food, providing much of the energy the coral needs, while the coral provides the algae with shelter and access to nutrients.

These relationships illustrate an important point: organisms do not merely compete with or consume one another. Cooperation and mutual dependence are also fundamental parts of ecosystems.

In other relationships, one organism benefits while another is harmed. These are known as parasitic relationships.

A tick, for example, attaches itself to a mammal and feeds on its blood, gaining nutrients while potentially harming its host. A tapeworm can live inside the digestive system of an animal, absorbing nutrients that would otherwise be available to the host. Some plants are parasites as well: mistletoe attaches to trees and shrubs and draws water and nutrients from them.

In each case, the parasite benefits from the relationship while the host bears a cost.

Still other organisms may compete for the same food, water, nesting sites, sunlight, or territory.

An ecosystem is therefore shaped by many overlapping relationships—not merely by who eats whom.

Habitat Provides the Setting

Every species needs habitat: the physical environment that provides the conditions and resources necessary for survival and reproduction.

Habitat may include food, water, shelter, nesting areas, breeding grounds, appropriate temperatures, and access to other members of the same species.

Different species can use the same landscape in very different ways.

A mature tree might provide nesting cavities for birds, bark for insects, branches for mammals, shade for plants, food for herbivores, and eventually dead wood for fungi and decomposers.

A wetland can provide breeding habitat for amphibians, feeding grounds for birds, nursery areas for fish, and conditions needed by specialized plants and microorganisms.

When habitat is changed, the consequences therefore depend not only on how much area is lost, but also on what ecological functions that habitat provided.

Biodiversity Adds Complexity

The variety of life within an area is called biodiversity.

Biodiversity includes more than simply the number of species.

It can refer to variation within species, the variety of species within ecosystems, and the variety of ecosystems themselves.

Why does that matter?

Different organisms perform different ecological roles.

Plants capture solar energy. Pollinators help plants reproduce. Predators influence prey populations. Decomposers recycle nutrients. Soil organisms affect soil structure and fertility.

Many species perform overlapping functions, which can sometimes give an ecosystem alternative ways of continuing to function when conditions change.

But species are not interchangeable.

Some perform roles that few others can easily replace.

This is one reason biodiversity will receive its own lesson later in this Learning Path.

For now, the important point is that the variety of life contributes to the structure and functioning of ecosystems.

What Happens When a Species Changes?

Because species are connected, a large change in one population can affect others.

Suppose a predator declines sharply.

Some of its prey may increase.

If those prey animals consume vegetation, greater numbers of them may place additional pressure on plants.

Changes in vegetation can then affect insects, birds, soil, streams, and other organisms.

The result can be a trophic cascade—a series of ecological effects that moves through different levels of a food web.

But these effects are not automatic or identical in every ecosystem.

A prey species may have several predators. Predators may switch to different prey. Other species may compensate for some of the change. Weather, disease, habitat conditions, and human activity may also influence the outcome.

Ecological relationships are real, but they are often complex.

That is why removing or restoring a species does not always produce a simple, predictable result.

Some Species Have an Outsized Influence

Not every species affects an ecosystem to the same degree.

Some have effects that are especially large relative to their abundance. These are often called keystone species.

Sea otters provide a well-known example.

Otters eat sea urchins. Sea urchins eat kelp.

Where otters keep urchin populations in check, kelp forests can flourish. These underwater forests provide food and habitat for a wide variety of fish, invertebrates, marine mammals, and other organisms. They also absorb carbon dioxide, produce oxygen, and can help reduce the force of waves along coastlines.

Where otters disappear, urchin populations can sometimes increase dramatically and consume large amounts of kelp. As kelp forests decline, the many species that depend on them can lose food, shelter, and habitat. The effect therefore extends well beyond otters and sea urchins. By influencing urchin populations, sea otters can indirectly help maintain an entire kelp-forest ecosystem.

The presence or absence of one predator can therefore influence the structure of an entire coastal ecosystem.

Other species transform habitat physically.

Beavers build dams, creating ponds and wetlands that can provide habitat for fish, birds, amphibians, insects, plants, and other organisms.

Large trees can create habitat used by hundreds of other species.

The importance of a species therefore cannot always be measured simply by how numerous it is.

Sometimes its ecological role matters more.

Decomposers Keep the System Moving

Large animals and plants tend to attract our attention.

But much of the work that keeps ecosystems functioning is performed by organisms we rarely notice.

Fungi, bacteria, insects, worms, and other decomposers break down dead material and waste.

Without decomposition, nutrients contained in dead organisms would remain locked away.

Decomposers help return those nutrients to soil and water, where they can again be used by plants and other organisms.

This process connects death to new growth.

A fallen tree may appear to be the end of something.

Ecologically, it can also become habitat, food, and raw material for another generation of life.

Ecosystems Change Naturally

Ecosystems are not fixed.

Storms occur. Fires burn. Rivers flood. Droughts come and go. Species migrate. Populations rise and fall. Diseases spread. Climate varies.

These changes can alter ecosystems, sometimes dramatically.

After a disturbance, an ecosystem may gradually change as species return, populations shift, and vegetation develops. This process is often called ecological succession.

Disturbance is therefore not necessarily the same thing as destruction.

Some ecosystems have developed with recurring disturbances.

Certain forests, for example, are shaped by periodic fire. Some plants depend on fire to create conditions favorable for germination or growth. Floods can replenish wetlands and floodplains with water and sediment.

The important questions include the type of disturbance, its intensity, its frequency, and whether organisms have time and opportunity to recover.

Resilience Has Limits

Ecosystems often have some capacity to absorb disturbance and continue functioning.

This capacity is known as resilience.

A grassland may recover after drought. A forest may regrow after fire. A fish population may recover after a period of decline.

But resilience is not unlimited.

Repeated disturbance, habitat fragmentation, pollution, invasive species, overharvesting, or rapid climate change can make recovery more difficult.

Multiple pressures can also occur at the same time.

A coral reef, for example, might face unusually warm water, ocean acidification, pollution, and overfishing simultaneously.

An ecosystem that could cope with one pressure may have much greater difficulty coping with several.

So when we ask whether an ecosystem can recover, we also need to ask:

Humans Are Part of Ecosystems Too

Humans depend on ecosystems for food, freshwater, materials, fertile soils, pollination, climate regulation, and many other benefits.

But we also change ecosystems.

Agriculture replaces some ecosystems with systems designed primarily to produce particular crops or livestock.

Cities replace vegetation and soil with buildings, roads, and other infrastructure.

Fishing changes marine populations. Dams alter rivers. Pollution changes air, water, and soil. Introduced species can create new ecological relationships.

Conservation and restoration can also change ecosystems—sometimes deliberately helping damaged habitats recover.

Humans are therefore not observers standing outside ecosystems.

The scale of our technology and economic activity means that our effects can extend far beyond the places where we live.

Why Ecosystems Matter

It is easy to value individual species because they are beautiful, useful, rare, or interesting.

But focusing only on individual species can obscure something larger.

A species exists within relationships.

A bee depends on flowering plants. Plants may depend on pollinators. Predators depend on prey. Plants depend on soil organisms. Aquatic organisms depend on water quality.

All depend on suitable physical conditions.

Protecting a species therefore often requires protecting the relationships and habitats that allow it to survive.

And protecting ecosystems is about more than preserving wildlife.

Healthy ecosystems help provide clean water, fertile soils, pollination, food, carbon storage, flood protection, erosion control, and many other functions upon which human societies depend.

Later in this Learning Path, we will examine these benefits in much greater detail.

For now, the essential idea is simple:

Understanding those relationships is fundamental to understanding the living planet.

Questions to Consider

1. Why is a food web a better representation of an ecosystem than a simple food chain?
2. Can you think of an organism that affects other species in ways besides eating or being eaten?
3. Why might protecting a species require protecting more than the species itself?
4. Why isn't every disturbance necessarily harmful to an ecosystem?
5. How does thinking of humans as participants in ecosystems change the way we think about environmental protection?