Ecosystem

An ecosystem is a community of living organisms and the non-living environment they live in. They interact together as a single unit. A pond, a forest, a desert: each is an ecosystem. Even the whole biosphere can be seen as one giant ecosystem. What holds it together is the one-way flow of energy and the cycling of nutrients. These ideas are the heart of ecology and a frequent exam theme.

Every ecosystem has two kinds of components:

• Abiotic components: the non-living factors: sunlight, temperature, water, air, soil and minerals. They set the physical stage for life.
• Biotic components: the living organisms, grouped by how they get their food:
  • Producers (autotrophs): green plants and algae that make their own food by photosynthesis. They are the base of every ecosystem.
  • Consumers (heterotrophs): animals that eat other organisms: herbivores (primary consumers), carnivores (secondary and tertiary consumers).
  • Decomposers: bacteria and fungi that break down dead matter and return nutrients to the soil.
  • Detritivores: animals that feed on dead organic matter (detritus) and break it into smaller pieces, speeding the work of decomposers. Earthworms, millipedes and woodlice are typical detritivores. Do not confuse them with predators such as jellyfish and seahorses, which hunt live prey and are consumers, not detritivores.

In the open ocean the producers are the phytoplankton, the tiny floating plant-like organisms. Naming them by type is a common exam point:

• Diatoms: single-celled algae with glassy shells. They photosynthesise, so they are producers.
• Cyanobacteria: photosynthetic bacteria, also called blue-green algae. They too are producers.
• Copepods: tiny crustaceans that graze on the plankton. They are zooplankton, so they are consumers, not producers.
• Foraminifera: single-celled organisms with chalky shells. They feed on other organisms, so they are heterotrophic consumers, not producers.

So among ocean plankton, only diatoms and cyanobacteria are primary producers. Copepods and foraminifera are consumers.

Two terms describe where and how an organism lives, and exams often test the difference:

• Habitat: the physical place where an organism lives, such as a pond or a forest floor.
• Ecological niche: the full role of a species, both the physical space it occupies and the functional role it plays, including what it eats, when it is active and how it interacts with others. Two species can share a habitat but occupy different niches.
• Ecotone: the transition zone where two ecosystems meet, such as the edge between a forest and a grassland.
• Home range: the area over which a single animal normally moves to find food and mates.

Plants of dry places, called xerophytes, change the shape of their leaves to lose less water. Desert plants show three classic leaf modifications that inhibit water loss:

• Waxy leaves: a thick waxy coating seals the surface and slows evaporation.
• Tiny or absent leaves: a smaller leaf area means less surface from which water can escape.
• Thorns instead of leaves: in plants such as cacti the leaves are reduced to spines, cutting water loss to a minimum while the green stem takes over photosynthesis.

Energy and matter pass from one organism to another along a food chain, for example: grass → grasshopper → frog → snake → hawk. Each step in a food chain is a trophic level (feeding level).

Food chains work in water as well as on land. A common marine chain runs diatoms → crustaceans → herrings. Here the diatoms are the producers, the small crustaceans (such as copepods) are the primary consumers that graze on them, and the herrings are the larger fish that eat the crustaceans. The order always follows the flow of energy, from producer upward.

In nature, food chains are not isolated. They cross and link to form a food web, a network of many interconnected chains. A food web is more realistic and more stable than a single chain. If one species declines, others can fill the gap.

Energy thins out as it moves up a food chain, but some substances do the opposite. Persistent poisons build up to higher and higher concentrations at each level. This is called biomagnification (or bioaccumulation). Pollutants that resist breakdown, such as chlorinated-hydrocarbon pesticides, are stored in body fat and are not excreted. A predator eats many contaminated prey, so it takes in all their stored poison. With each step up the chain the dose grows. The apex predator, for example a hawk, ends up with the highest concentration of all, even though it never touched the pesticide directly. So energy decreases up the chain while these pollutants increase, the two trends running in opposite directions.

The ultimate source of energy for almost all ecosystems is the Sun. Producers capture solar energy in photosynthesis, and it then passes up the trophic levels as one organism eats another.

The key rule is the ten per cent law: only about 10 per cent of the energy at one trophic level is passed on to the next. The rest is lost as heat in life processes. Because so much is lost at each step, energy flow is one-way and cannot be recycled. Food chains rarely have more than four or five levels. Beyond that, there is simply not enough energy left to support more.

The structure of an ecosystem can be shown as an ecological pyramid, with producers at the base and top carnivores at the apex. There are three kinds:

• Pyramid of numbers: the number of organisms at each level.
• Pyramid of biomass: the total mass of living matter at each level.
• Pyramid of energy: the energy at each level. This is always upright, because energy always decreases up the chain.

Pyramids of number and biomass can sometimes be inverted (for example, one large tree supporting many insects), but the pyramid of energy never is.

Unlike energy, nutrients are not lost. They are used again and again. The movement of a nutrient through the living and non-living parts of an ecosystem is a biogeochemical cycle.

In the carbon cycle, carbon passes from the air into plants by photosynthesis. It then moves through animals that eat the plants. Several processes return carbon dioxide to the air: respiration by living things, the decay of dead matter, the burning of fuels and volcanic action, which vents carbon dioxide from deep in the Earth. Photosynthesis is the one major process that does the reverse, removing carbon dioxide from the air. In the phosphorus cycle, phosphorus moves from rocks and soil into plants and animals. Decomposers then return it to the soil. These cycles keep the limited stock of nutrients in constant circulation. That is why decomposers are so vital.

Primary productivity is the rate at which producers fix solar energy into organic matter. The amount left after the producers use some for their own respiration is the net primary productivity (NPP). Productivity is not the same everywhere. Ecosystems can be ranked by how much organic matter they produce per unit area. In decreasing order of productivity, a useful sequence is mangroves, then grasslands, then lakes, then the open ocean. Mangroves and other tropical wetlands are among the most productive systems on Earth, while the vast open ocean produces the least per unit area.

The sea is still a giant producer overall. Marine phytoplankton, the tiny floating algae, together with photosynthetic bacteria, make about half of the world's oxygen. This means the oceans produce more oxygen than rainforests, not less. Note also that dissolved oxygen in water is far more dilute than the oxygen in air: well-oxygenated surface water holds much less oxygen than the atmosphere, not more.

Two ideas about ocean productivity are worth fixing in memory:

• Upwelling zones: places where deep water rises to the surface. They raise productivity by bringing nutrients up to the sunlit surface, where phytoplankton can use them. They do not bring decomposers or bottom-dwelling animals up.
• Red tides: estuaries sometimes show sudden blooms of pigmented dinoflagellates, a kind of single-celled alga. The dense reddish cells discolour the water, so the bloom is called a red tide.

Ecosystem services are the benefits that people get from nature for free. The Millennium Ecosystem Assessment, a major global study of the state of ecosystems, sorted these benefits into four main groups:

• Provisioning services: the goods we take out, such as food, fresh water, timber and fibre.
• Regulating services: the control of natural processes, such as climate regulation, flood control, water purification and the regulation of disease.
• Supporting services: the basic processes that make all the other services possible, such as nutrient cycling, soil formation and pollination of crops.
• Cultural services: the non-material benefits, such as recreation, beauty and spiritual or educational value.

A vivid way to picture one service: if rainforests are the lungs of the Earth, wetlands act as its kidneys. Just as kidneys filter toxins from blood, wetland plants absorb heavy metals and excess nutrients from the water passing through, cleaning it naturally.

Pollination is a supporting service worth a closer look. Many animals carry pollen between flowers and let plants reproduce. The pollinators include more than just insects:

• Bees: the best-known pollinators.
• Bats: they pollinate many night-blooming flowers.
• Birds: sunbirds and hummingbirds pollinate many flowering plants.

Ecological succession is the gradual change in the community of an area over time, as one set of species replaces another until a stable climax community is reached. Primary succession starts on bare ground with no soil, such as a fresh rock surface. Secondary succession starts where a community has been cleared but soil remains, such as an abandoned field.

The first species to colonise a bare habitat are the pioneer species. On bare rock the classic pioneers are lichens. A lichen is not one organism but a symbiotic partnership of a fungus and an alga (sometimes a cyanobacterium). The fungus gives structure and holds water, while the alga makes food by photosynthesis. Lichens slowly break down the rock and build the first thin soil, preparing the ground for mosses and later plants.

Succession does not always march on to forest. Local conditions can hold a community at an earlier stage:

• Grasslands: trees do not replace the grasses because periodic fires and limited water kill tree seedlings before they can establish. The grasses survive and stay dominant.
• Tropical rainforest: if cleared, it regrows far more slowly than a tropical deciduous forest. The reason is that rainforest soil is poor in nutrients: almost all the nutrients are locked in the living biomass, and once the trees are removed the thin topsoil is quickly washed away.