Frog Swimming in Pond
It’s a frog’s life. Ponds provide homes for a diverse range of species. Photo: © Laurie Mac Bride

What is an Ecosystem? How Does it Work?

What’s an ecosystem? How does it work? And how are ecosystems affected by climate change? These are some of the questions we’ll be examining in this article.

No Creature Exists in Isolation

The mighty blue whale is the largest animal ever known to have lived on earth. But without microscopic phytoplankton, not even a blue whale could survive. In fact, without marine algae like phytoplankton, or terrestrial plants like trees and other vegetation, no human being could survive either. 1

This illustrates a very important point that applies throughout the biosphere: no creature exists in isolation. All species within an ecosystem are interdependent. They need each other – sometimes, in ways that aren’t at all obvious – if they are to succeed in their continuous struggle to survive and reproduce. For example, a lion may be lord of the African savanna, but he can’t make the oxygen he needs to breathe. For that, he needs plants. A leopard is a deadly predator, but without trees to protect his kills, he’d starve.

What is Ecology?

Every second, millions of complex interactions take place between individual organisms, among species, and between the biotic (living) and abiotic (no-living) components of the environment. Ecology is the branch of biology that deals with these interactions – the relations of organisms to one another and to their physical surroundings.

What is an Ecosystem?

The word ‘ecosystem’ describes a ‘system’ – a system of interactions between the living (biotic) and non-living (abiotic) components of an area. Typical interactions include: how animals react to each other in the food web (who eats who), and how they relate to their local environment – its climate, landscape and water supply.

Environments that we usually speak of as having their own ecosystem include: ponds, meadows, forest canopies, beaches, peat bogs, woodland, desert sands, coral reefs, sea ice, cave systems, ocean floors, and hundreds more.

Each of the four layers of the rainforest - emergent, canopy, understory and forest floor - have their own ecosystem.
The Amazon rainforest has several ecosystems. The Emergent Layer is the top layer of the rainforest. Trees that break through the Canopy Layer and into the Emergent Layer can reach 200 feet. Many animals that live in the Emergent Layer never touch the ground. The Canopy Layer is the next layer down, and this is where much of the action in the rainforest happens, including photosynthesis as well as plant respiration and transpiration. It filters about 80 percent of the sunlight. Very little sunlight reaches the bottom two layers. Diagram: © Md. Islam

The size of an ecosystem can be vast or tiny. A rock pool left by the ocean as the tide goes out, is a tiny ecosystem, although it is almost certainly inhabited by tens of thousands of microscopic phytoplankton, plus seaweed, crabs, sea anemones and other creatures. And all these living components will respond to each other, as well as to the temperature and salinity of the water, the composition of the rocks and sand, and the sunlight and wind.

In a similar way, a forest is not just an expanse of trees, but rather a complex interlocking system of soil, air, and water, of insects, bacteria, viruses, fungi, grasses, herbs, birds, mammals and foliage.

In South America, the Amazon rainforest includes four layers of foliage – emergent, canopy, understory and forest floor. Each layer has its own ecological system, with its own food chain, as well as relationships with its three sister layers. The same applies in the Congo Rainforest in Africa.

The soil of the tropical rainforest floor contains less nutrients than the soil of a temperate deciduous forest. Roughly 48 percent of nutrients are found in tropical forests, versus 69 percent in temperate forests (the balance in both cases is found in vegetation). Even so, the soil’s own ecosystem supports a wide variety of living organisms that quickly absorb the nutrients in dead or decaying plant or animal matter.

Multiple Ecosystems Form Biomes

The entire surface of the planet is a patchwork quilt of connected ecosystems. Mostly, but not always, they form larger ecozones or “biomes” – large areas of land or sea that form little biological worlds with specific climate characteristics together with a specific set of plants and animals. Examples of biomes include: polar and tundra biomes, temperate and tropical grasslands, rainforests, alpine, wetland and aquatic biomes.

The Sahara Desert biome, for example, embraces a variety of ecosystems. They include arid expanses of treeless, windblown sand dunes where little survives the scorching temperatures; areas of rocky outcrops and caves with their own particular microclimate, flora and fauna; shaded oases with freshwater rivers or streams, watering a variety of olive, lemon, apricot and fig trees, as well as date palms and other crops. The Sahara also hosts several coastal ecosystems, from the cool and breezy Northwest coast of Morocco, to the warmer Mediterranean ecosystems of coastal Libya.

Saharan Horned Vipers play a key part in the desert ecosystem
Saharan Horned Viper (Cerastes cerastes). A poisonous snake which is native to the deserts of northern Africa. This viper is easily recognisable by its horns. Their diet consists mainly of lizards, but also small rodents and birds. Horned Vipers play a key role in their ecosystem by controlling the rodent populations. Photo: © H. Krisp CC BY 3.0

Where Does the Energy in an Ecosystem Come From?

Carbon is the main element in organic compounds (the stuff that makes up living organisms), so it’s essential to all life forms. It enters ecosystems in the form of carbon dioxide (CO2) during plant photosynthesis. Plants absorb it from the air and combine it with water (from their roots) and sunlight to form complex carbohydrates, which are then stored in plant tissues. This is the main pathway in the fast carbon cycle leading from the atmosphere to the ground.

If the plant is eaten, the carbohydrate is transferred to other organisms through the food chain. If it remains uneaten when the plant dies, it is broken down and its nutrients recycled by decomposers. 2

In land-based ecosystems, around 90 percent of the photosynthesized carbohydrates (known as ‘net primary production’) end up being broken down by decomposers. The remainder is either consumed by animals while still alive and enters the plant-based food chain, or it is consumed after it has died, and enters the detritus-based trophic system. 3

In freshwater or ocean ecosystems, the proportion of plant biomass that gets consumed by herbivores is much higher.

How Does Carbon Move Through the Food Chain of an Ecosystem?

The organisms who manufacture food by themselves via photosynthesis are known as primary producers and are classified as autotrophs (self-nourishers). The only organisms who do this are plants – land or marine types.

These primary producers are preyed on by other creatures known as consumers, who are classified as heterotrophs (organisms who get nourishment from others). As we have seen, uneaten remains of plants and animals are broken down and recycled by decomposers, who are classified as saprotrophs (organisms who get nourishment from rotting material).

Each step in the food chain (nowadays, more commonly called the food web) is referred to as a trophic level (feeding level). The first level (primary producers) consists solely of plants, who get most of their food energy from sunlight as they photosynthesize. They are preyed upon by herbivores like rabbits or zooplankton, who make up the second level (primary consumers). In turn, herbivores are devoured by carnivores (secondary consumers) on level three, who are eaten by larger carnivores (tertiary consumers) on level four. At the top of the food chain are apex predators (like lions, crocodiles and killer whales), who are not preyed on by anyt other animal.

Of course, in real life, the food web is much more complex and interlinked than this explanation suggests. Organisms may eat other organisms from more than one trophic level and may prey on organisms from their own level.

The marine food web is a case in point. Zooplankton (primary consumers) eat phytoplankton (primary producers) but they also prey on other zooplankton. Blue whales (tertiary consumers on level four) eat huge amounts of tiny krill, whereas other whales like sperm whales eat mostly giant squid. On a microscopic level, ocean ecosystems are extremely violent environments characterized by continuous vicious attacks by predators, as well as viruses exploding their victim hosts. For details, see: Marine Microbes Drive the Aquatic Food Web.

Loss of Energy as it Moves Through the Food Chain

As we move up the food chain, we find that the number of organisms decreases. This is because there is less energy available to the next level from the previous one. This is the ’10 percent rule’, which says that each trophic level only gets 10 percent of the energy of the level immediately below it.

The reason for this, is that each living organism expends quite a bit of the energy it receives doing the work necessary for living – such as breathing, digesting, moving, growing, fleeing predators, keeping warm, reproducing, and so on.

The expended energy is returned to the environment as heat, via respiration, which also returns CO2 to the atmosphere. This loss of energy explains why there are far more creatures at the bottom of most food webs than at the top.

Ecological Pyramid
An ecological pyramid (also called trophic pyramid). Photo: © Thompsma/CC BY-SA

Biomass and the Ecological Pyramid

Biomass is a measure of the amount of organic material in a particular ecosystem. The term “phytomass” refers to plant material, while “zoomass” describes animal matter. Biomass is typically measured in terms of the dry weight of organisms found at a particular trophic level in a food chain.

Given the loss of energy at each level, it’s clear that biomass is going to decrease with each trophic level. Thus, the top level will have a biomass which is far, far smaller than the base level. This is well demonstrated by the ecological pyramid.

There are varying estimates of global biomass, some of which are woefully outdated. According to a recent research paper published in PNAS, the total biomass on Earth adds up to about 550 billion tonnes of carbon, of which around 82 percent (450 billion tonnes) are plants, predominantly land plants (embryophytes).

The second largest source of biomass is bacteria (70 billion tonnes), which constitutes about 12.5 percent of the total biomass. The remaining 5.5 percent consists of a mixture of life forms, including (in descending order) fungi, archaea, protists, animals, and viruses. 4

Everything in an Ecosystem is Interconnected

The food web illustrates the cardinal fact of life on Earth: every living thing depends upon other living things for the necessities of life, including energy, nutrients, oxygen, fresh water, habitat, shelter, reproduction and good health.

Energy and Oxygen

All living organisms depend upon plants for food to eat and for oxygen to breathe. (Okay, they may not eat plants themselves but their prey, or their prey’s prey probably does.)


All living organisms rely upon decomposers and detritivores to clean up the environment and recycle valuable nutrients. For example, read about the essential contribution of the Indian Vulture in our article: 10 Endangered Birds of Prey.

The Nitrogen Cycle: How Does It Work?
The Phosphorus Cycle: How Does It Work?

The ‘whale pump’ is another interesting example of species interdependence. Whales release their feces near the surface of the ocean, where it is broken down by decomposers like bacteria, for the benefit of tiny microbes like zooplankton, who are the favorite diet of krill. Now guess which creatures relies most on krill as a food source? Yup, whales.

Fresh Water

A wide range of plants and bacteria are involved in water purification in land based aquatic ecosystems. While in the ocean, oysters help to purify and detox polluted seawater. Other marine purifiers include kelp, eelgrass, mangroves, and other vegetation.

Mangrove trees seen from above and below the water line. Mangrove ecosystems are famous for storing "blue carbon"
A red mangrove tree and its roots above and below the water surface. Mangrove forests contain an amazingly rich biodiversity of wildlife, including birds, fish, mammals, reptiles and amphibians. They also store ‘blue carbon.’ Photo: © Caroline Rogers, USGS.


When plants and trees are destroyed in disasters like the recent Arctic fires, so is the habitat for billions of creatures. Forests are a vital habitat for an enormous number of plant and animal species, as are trees and shrubs in a number of biomes around the world. This is why deforestation is responsible for so much loss of biodiversity, notably in the Amazon Rainforest, and in Southeast Asia. Aquatic ecosystems, like Brazil’s Pantanal, are another precious habitat for aquatic plants, reptiles, birds, mammals and birds. These wetlands are also vital staging points for migrating birds.

Pileated Woodpecker USA
Pileated woodpecker feeding her young offspring. Otsego Marsh, Wisconsin. This woodpecker is native to the deciduous forests in eastern North America, the Great Lakes and the boreal forests of Canada. It is the second-largest woodpecker in America, after the endangered and possibly extinct ivory-billed woodpecker. Once a woodpecker’s brood is raised, they abandon the holes they pecked into trees and never return. These holes become good homes for songbirds and a wide variety of other animals. This is why ecologically, woodpeckers are important to the wellbeing of many other species. Photo: © Mark Martin


Half of all birds are cavity nesters. Birds like owls, woodpeckers, parrots, chickadees, tree swallows, house wrens and nuthatches, to name but a few, all live in cavity nests inside trees which they rely on for their safety and that of their chicks. Many other birds nest in trees, including rooks, buzzards and sparrow hawks. Other arboreal species include koalas, sloths, geckos, and opossums, as well as numerous monkeys and a vast number of insects. Plants also house a large number of organisms from the insect kingdom. Even a large predator like a leopard depends upon trees to protect its kills from the attentions of other animals.

10 Birds Threatened by Climate Change
What is Biodiversity?

Back in 1982, a researcher discovered 1,200 species of beetles living inside one particular species of tree in the tropical rainforest. Of these, he estimated 163 were unique to the tree in question. 5


Many plants rely on animals and birds to disperse their seeds in the forest, thus creating new growth. 6 In tropical rainforests, for instance, plants rely on animals such as gorillas, orangutans, lemurs, wild pigs, tapirs, and civets, as well as numerous birds, to scatter their seeds and maintain their growth as a species. 7 In total, an estimated 70-94 percent of tree species rely on vertebrates for the dispersal of their seeds. 8 For more about pollination, see: Why are Pollinators So Important to the Planet?

Stag Beetle
The stag beetle, seen here in Richmond Park’s designated National Nature Reserve, UK. The beetle lives up to 7 years. The stag beetle is a gentle giant of the insect world. Despite their scary appearance, they tend only to use their jaws for wrestling. Numbers are greatly reduced in the UK, and so a EU wide protection program was put in place to help them repopulate. Photo: © Royalparks UK

Crop plants, too, need the assistance of animals in order to pollinate, because pollen is a vital link in the reproductive cycle. Of the 1,400 crop plants grown around the world – that is, those that provide all our food and plant-based industrial products, nearly 80 percent require pollination by animals. 9 These animal pollinators include: ants, bats, bees, beetles, birds, butterflies, flies, hummingbirds, moths and rodents.

Good Health

It’s well-known that one of the key functions of apex predators is to regulate the populations and health of species below them, in order to maintain the overall balance and health of their ecosystem. Reef sharks for instance habitually patrol coral reefs in order to limit the numbers of ‘mid-species’ fish that eat the grazing fish that keep corals clean of algal overgrowth.

In the same way, lions (and other predators) help to limit the numbers of herbivores grazing the African savannas like the Masai Mara in Kenya. Without lions, the antelopes, gazelles and other herbivores would eat all the grass destroying the ecosystem for all the animals.

How Does Climate Change Affect Ecosystems?

Global warming damages ecosystems in numerous ways, some of which appear contradictory. Bear in mind that warming leads to warmer air, which can hold more water, leading to higher rainfall. The biggest single impact of global warming on our climate system is its destabilizing influence. It makes everything more extreme. Thus, heatwaves become hotter and more frequent; wet weather wetter; dry weather even dryer; and so on. Here’s how it affects our ecosystems:

(1) It destabilizes the water system. It melts glaciers and dries up water supplies, causing water shortages. At the same time, it increases the amount of intense rainfall, causing floods and run-off, as well as soil erosion and chemical pollution. This can lead to a cascade of problems in the ecosystems affected. See also: What is the Water Cycle?

(2) It increases salinity. The rapid melting of polar ice sheets already resulting in global sea level rise. This, combined with more frequent ocean storms is causing saltwater flooding of low-lying coastal ecosystems.

(3) It also reduces salinity(!) Polar ice melt increases the freshwater content of the oceans. When the salinity of surface ocean water decreases, it can interfere with the downwelling of deep-water ocean currents driven by thermohaline circulation which requires a high degree of salinity. This can cause severe repercussions for marine ecosystems around the world.

(4) It unbalances the traditional patterns of regional weather systems, such as the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM). This typically results in more extreme alternations, such as the 2020 floods in East Africa, the 2017-2019 Australian droughts and the 2019-2020 Australian Bushfires, to name three recent examples. This unbalancing effect has consequences for thousands of different ecosystems across several continents.

(5) The effects of global warming on oceans are well documented. Ocean warming and acidification is destroying coral reef ecosystems; marine heatwaves are also killing corals as well as mangrove forests. 10 Ocean deoxygenation is occurring in marine ecosystems in the upper 1,000 meter-layer, with serious breathing consequences for large ocean fish, like tuna and marlin. 11

(6) Recent Arctic wildfires in Siberia, and also in California and Australia, caused primarily by elevated temperatures resulting from climate change, have devastated very large numbers of ecosystems, along with the deaths of billions of animals. See also: Effects of Climate Change on Animals.

(7) In what could be a series of environmental disasters, Pacific Island states are currently bracing themselves for submergence, from predicted sea level rise. Whole islands could end up underwater.

In short, global warming poses an existential threat to thousands, perhaps millions of ecosystems. Indeed, if our climate crisis is not brought under control, most ecosystems are likely to undergo dramatic change, with severe consequences for plants and animals as they struggle to cope with rising temperatures, loss of habitat and intense competition from newly uprooted species. 12

10 Endangered Animals
List of Endangered Species


  1. Phytoplankton” Woods Hole Oceanographic Institute. []
  2. “Principles of Terrestrial Ecosystem Ecology” (Second ed.). Chapin, F. Stuart; Pamela A. Matson; Peter M. Vitousek (2011). New York: Springer. pp. 123–150. ISBN 978-1-4419-9503-2. []
  3. “Principles of Terrestrial Ecosystem Ecology” (Second ed.). Chapin, F. Stuart; Pamela A. Matson; Peter M. Vitousek (2011). New York: Springer. pp. 244–264. ISBN 978-1-4419-9503-2. []
  4. “The biomass distribution on Earth.” Yinon M. Bar-On, Rob Phillips, Ron Milo. PNAS June 19, 2018 115 (25) 6506-6511; first published May 21, 2018. []
  5. “Tropical Forests: Their Richness in Coleoptera and Other Arthropod Species”. The Coleopterists Bulletin. 36 (1): 74–75. Erwin, Terry L. (March 1982). The Coleopterists Society (ed.) ISSN 0010-065X []
  6. “Animal seed dispersal and the diversity of tropical forest trees.” Susan Harrison. PNAS October 3, 2017 114 (40) 10526-10527; September 25, 2017. []
  7. “Availability of large seed-dispersers for restoration of degraded tropical forest.” Lindsell, J. al; Tropical Conservation Science Vol.8 (1): 17-27. []
  8. “Fruits and frugivory.” In: Fenner M, ed. Seeds: the ecology of regeneration in plant communities. Wallingford, UK: CABI Publishing. Jordano P. 2000 []
  9. “Why is Pollination Important?” U.S. Forest Service. []
  10. “Shocked scientists find 400km of dead and damaged mangroves in Gulf of Carpentaria.” Graham Readfearn. BBC News. Oct 3, 2019. []
  11. “Deoxygenation.” IUCN. []
  12. “Past and future global transformation of terrestrial ecosystems under climate change.” Connor Nolan, Jonathan T. Overpeck, Judy R. M. Allen, Patricia M. Anderson, Julio L. Betancourt, Heather A. Binn. Science 31 Aug 2018: Vol. 361, Issue 6405, pp. 920-923. []
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