What is an ecosystem?

Learn what an ecosystem is, how energy and matter move through ecosystems, and what makes an ecosystem stable.

Key points

  • An ecosystem consists of a community of organisms together with their physical environment.
  • Ecosystems can be of different sizes and can be marine, aquatic, or terrestrial. Broad categories of terrestrial ecosystems are called biomes.
  • In ecosystems, both matter and energy are conserved. Energy flows through the system—usually from light to heat—while matter is recycled.
  • Ecosystems with higher biodiversity tend to be more stable with greater resistance and resilience in the face of disturbances, disruptive events.

Introduction

What do a tide pool on the California coast and the Amazon rainforest of South America have in common? Despite being many orders of magnitude different in size, both are examples of ecosystems—communities of organisms living together in combination with their physical environment.
Image credits: left, Tide pools at Half Moon Bay by Brocken Inaglory, CC BY-SA 4.0; right, Aerial view of the Amazon rainforest by Neil Palmer/CIAT, Center for International Forestry Research, CC BY 2.0
As a reminder, a community consists of all the populations of all the species that live together in a particular area. The concepts of ecosystem and community are closely related—the difference is that an ecosystem includes the physical environment, while a community does not. In other words, a community is the biotic, or living, component of an ecosystem. In addition to this biotic component, the ecosystem also includes an abiotic component—the physical environment.
Ecosystems can be small, such as the tide pools found near the rocky shores of many oceans, or very large, such as the Amazon Rainforest in South America. It's basically up to the ecologist studying the ecosystem to define its boundaries in a way that makes sense for their questions of interest.

What are ecosystems like?

The short answer: incredibly diverse! Not only can ecosystems vary in size, but they can also differ in just about every imaginable biotic or abiotic feature.
Some ecosystems are marine, others freshwater, and others yet terrestrial—land based. Ocean ecosystems are most common on Earth, as oceans and the living organisms they contain cover 75% of the Earth's surface. Freshwater ecosystems are the rarest, covering only 1.8% of the Earth's surface. Terrestrial, land, ecosystems cover the remainder of Earth.
Terrestrial ecosystems can be further grouped into broad categories called biomes, based largely on climate. Examples of terrestrial biomes include tropical rain forests, savannas, deserts, coniferous forests, deciduous forests, and tundra. The map below shows the broad distribution of biomes on Earth.
Image credit: Biomes: Figure 2 by OpenStax College, Biology, CC BY 4.0
Even within a biome, there can be great diversity. For example, both the Sonoran desert, on the left, and the interior of the island of Boa Vista, on the right, can be classified as deserts, but they have very different ecological communities. Many more species of plants and animals live in the Sonoran desert.
Image credits: left, Sonoran desert by Highqueue, public domain; right, Rock desert (hamada) inside the island of Boa Vista by Ingo Wölbern, public domain

Energy and matter in ecosystems

Ecosystem ecologists are often most interested in tracing the movement of energy and matter through ecosystems.
We’ll take a closer look at the movement of energy and matter when we consider food webs, networks of organisms that feed on one another, and biogeochemical cycles, the pathways taken by chemical elements as they move through the biosphere. The organisms found in an ecosystem tend to have adaptations, beneficial features arising by natural selection, that help them get energy and matter in the context of that particular ecosystem.
Before we get into details, though, let’s look at the key features of how energy and matter travel through ecosystems. Both energy and matter are conserved, neither created nor destroyed, but take different routes through ecosystems:
  • Matter is recycled; the same atoms are reused over and over.
  • Energy flows through the ecosystem, usually entering as light and exiting as heat.

Matter is recycled.

Matter is recycled through Earth’s ecosystems—though it may move from one ecosystem to another as it does when nutrients are washed away into a river1^1. The same atoms are used over and over again, assembled into different chemical forms and incorporated into the bodies of different organisms.
As an example, let’s see how chemical nutrients move through a terrestrial ecosystem. A land plant takes in carbon dioxide from the atmosphere and other nutrients, such as nitrogen and phosphorous, from the soil to build the molecules that make up its cells. When an animal eats the plant, it uses the plant’s molecules for energy and as building material for its own cells, often rearranging atoms and molecules into new forms.
When plants and animals carry out cellular respiration—break down molecules as fuel—carbon dioxide is released into the atmosphere. Similarly, when they excrete waste or die, their chemical compounds are used for energy and building material by bacteria and fungi. These decomposers release simple molecules back into the soil and atmosphere, where they can be taken up anew in the next round of the cycle.
Image credit: based on similar image by J. A. Nilsson2^2
Thanks to this recycling, the atoms that make up your body right now have long, unique histories. They’ve most likely been part of plants, animals, other people, and even dinosaurs3^{3}!

Energy flow is unidirectional, or one-way.

Energy, unlike matter, cannot be recycled in ecosystems. Instead, energy flow through an ecosystem is a one-way street—generally, from light to heat.
Energy usually enters ecosystems as sunlight and is captured in chemical form by photosynthesizers like plants and algae. The energy is then passed through the ecosystem, changing forms as organisms metabolize, produce waste, eat one another, and eventually, die and decompose.
Each time energy changes forms, some of it is converted to heat. Heat still counts as energy—and thus no energy has been destroyed—but it generally can't be used as an energy source by living organisms. Ultimately, energy that entered the ecosystem as sunlight is dissipated as heat and radiated back into space.
Image credit: based on similar image by J. A. Nilsson2^2
This one-way flow of energy through ecosystems means that every ecosystem needs a constant supply of energy, usually from the sun, in order to function. Energy can be passed between organisms, but it cannot be recycled because some of it is lost as heat in each transfer.

Stability and dynamics of ecosystems

Ecosystems are dynamic systems, and a static ecosystem would be a dead ecosystem—just as a static cell would be a dead cell. As we discussed above, energy is constantly flowing through an ecosystem and chemical nutrients are continually being recycled. At higher levels of organization, organisms are dying and being born, populations are fluctuating in their numbers, and climate patterns are varying seasonally and in less predictable ways.

Equilibrium and disturbance

Equilibrium is the steady state of an ecosystem, in which its composition and identity remain generally constant despite fluctuations in physical conditions and the makeup of the biotic community. Ecosystems may be knocked out of equilibrium by disturbances, disruptive events that affect their composition.
Some disturbances are a result of natural processes. For example, fire is a disturbance that can be caused by lightning in a prairie or forest ecosystem. Other disturbances are the result of human activities. Examples include acid rainfall, deforestation, algal blooms, and the introduction of invasive species.
Different ecosystems may respond differently to the same disturbance; one may recover rapidly, and another may recover more slowly—or not at all.

Resistance and resilience

Ecologists sometimes use two parameters to describe how an ecosystem responds to disturbance. These parameters are resistance and resilience. The ability of an ecosystem to remain at equilibrium in spite of disturbances is called resistance. How readily an ecosystem returns to equilibrium after being disturbed is called resilience. Some ecologists consider resistance to be an element of resilience—one that acts on a short timescale4,5^{4,5}.
Many ecologists think that the biodiversity of an ecosystem plays a key role in stability. For example, if there were just one plant species with a particular role in an ecosystem, a disturbance that harms that one species—say, a drought for a drought-sensitive species—might have a severe impact on the ecosystem as a whole. In contrast, if there were several plant species with similar functional roles, there would be a better chance of one of them being drought-tolerant and helping the ecosystem as a whole survive the drought period6^6.
Ecosystem resistance and resilience are important when we consider the effects of disturbances caused by human activity. If a disturbance is severe enough, it may change an ecosystem beyond the point of recovery—push the ecosystem into a zone where it is no longer resilient. A disturbance of this sort could lead to permanent alteration or loss of the ecosystem.

Attribution

This article is a modified derivative of the following articles:
The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited

  1. Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert B. Jackson, "Physical Laws Govern Energy Flow and Chemical Cycling in Ecosystems," in Campbell Biology, 10th ed. (San Francisco: Pearson, 2011), 1234.
  2. Jan A. Nilsson, "Energy Flow Through Ecosystems," General Biology Hub, accessed June 11, 2016, http://www.desertbruchid.net/4_GB2_LearnRes_fa11_f/4_GB2_LearnRes_Web_10Ecol.html.
  3. GeeJo, "How Likely Is It That an Atom That Was Part of a Dinosaur Is Part of My Body? [Answer]," Reddit, accessed June 11, 2016, https://www.reddit.com/r/NoStupidQuestions/comments/3e7qtz/how_likely_is_it_that_an_atom_that_was_part_of_a/.
  4. Brian Walker, C. S. Holling, Stephen R. Carpenter, and Ann Kinzig, "Resilience, Adaptability and Transformability in Social–Ecological Systems," Ecology and Society 9, no. 2 (2004): 5, http://www.ecologyandsociety.org/vol9/iss2/art5/.
  5. "Resistance (Ecology)," Wikipedia, last modified September 5, 2015, https://en.wikipedia.org/wiki/Resistance_%28ecology%29.
  6. Elsa E. Cleland, "Biodiversity and Ecosystem Stability," Nature Education Knowledge 3, no. 10 (2011): 14, http://www.nature.com/scitable/knowledge/library/biodiversity-and-ecosystem-stability-17059965.

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