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What is ecology?

Overview of ecology. Biotic and abiotic factors. The different levels of ecology.

Key points

  • Ecology is the study of how organisms interact with one another and with their physical environment.
  • The distribution and abundance of organisms on Earth is shaped by both biotic, living-organism-related, and abiotic, nonliving or physical, factors.
  • Ecology is studied at many levels, including organism, population, community, ecosystem, and biosphere.

Welcome to ecology!

Have you ever hiked through a forest and noticed the incredible diversity of organisms living together, from ferns to trees to mushrooms the size of dinner plates? Or taken a road trip and watched the landscape change outside the window, shifting from oak forest to tall stands of pine to grassy plains? If so, you’ve gotten a classic taste of ecology, the branch of biology that examines how organisms interact with each other and with their physical environment.
Ecology isn't just about species-rich forests, pristine wilderness, or scenic vistas, though. Have you, for instance, ever found cockroaches living under your bed, mold growing in your shower, or even fungus creeping in between your toes? If so, then you’ve seen equally valid examples of ecology in action.
Images illustrating interactions among organisms and between organisms and their physical environment.
Upper left: mushrooms growing on a mossy log. Upper right: rolling green hills covered with wildflowers, grasses, and occasional trees. Lower left: rolling fields of dry, yellow grass with scrub-covered hills and snowy mountains in the distance. Lower right: cockroach on floorboards.
Image credit: upper left, Forest by Jens Hellermann, public domain; upper right, Konza Prairie Preserve by Kgwo1972, public domain; lower left, San Raphael Grasslands by Dominic Sherony, CC BY-SA 2.0; lower right, Cockroach closeup by 邰秉宥, CC BY-SA 2.0

Biotic and abiotic factors

One core goal of ecology is to understand the distribution and abundance of living things in the physical environment. For instance, your backyard or neighborhood park probably has a very different set of plants, animals, and fungi than the backyard of a fellow Khan Academy learner on the opposite side of the globe. These patterns in nature are driven by interactions among organisms as well as between organisms and their physical environment.
As an example, let's go back to our shower mold. Mold is more likely to appear in your shower than, say, your sock drawer. Why might this be the case?
  • Maybe the mold needs a certain amount of water to grow, and this amount of water is found only in the shower. Water availability is an example of an abiotic, or nonliving, factor that can affect distribution of organisms.
  • Maybe mold feeds off of dead skin cells found in the shower, but not in the dresser. Availability of nutrients provided by other organisms is an example of a biotic, living-organism-related, factor that can influence distribution.

Case study: the red panda

Let's apply the idea of biotic and abiotic factors to another organism, one that a field ecologist might be likely to study. Red pandas are distant relatives of raccoons and are found only in the eastern Himalayas. They spend most of their time in trees and eat a primarily vegetarian diet. In recent years, the red panda population has dropped significantly, leading conservation groups to classify it as a vulnerable or endangered speciesstart superscript, 1, comma, 2, end superscript.
Red panda hanging from a tree branch. It's a cute animal that looks roughly similar to a raccoon with reddish fur.
Image credit: Red panda almost falling off by Sander van der Wel, CC BY-SA 2.0
What are the main factors behind this change in abundance? Ecologists have found that biotic factors, such as logging of trees and introduction of diseases from domestic dogs, played a major role in the decline of red panda populationssquared. Abiotic factors have been less important to date, but changing temperatures could cause further habitat loss in the futurecubed.
Understanding the main factors responsible for the decline in red panda numbers helps ecologists form conservation plans to protect the species.

How do ecologists ask questions?

To ask questions about the natural world—such as, "Why is the red panda declining?"—ecologists draw on many areas of biology and related disciplines. These include biochemistry, physiology, evolution, behavioral biology, and molecular biology, as well as geology, chemistry, and physics.
Natural historians were arguably the first ecologists—dating back to the Greek philosopher Aristotle! However, today's ecologists are rigorous, quantitative scientists. They run controlled experiments, use statistics to find patterns in large datasets, and build mathematical models of ecological interactions.

Ecology at many scales

Within the discipline of ecology, researchers work at five broad levels, sometimes discretely and sometimes with overlap: organism, population, community, ecosystem, and biosphere.
Let's take a look at each level.
  • Organism: Organismal ecologists study adaptations, beneficial features arising by natural selection, that allow organisms to live in specific habitats. These adaptations can be morphological, physiological, or behavioral.
  • Population: A population is a group of organisms of the same species that live in the same area at the same time. Population ecologists study the size, density, and structure of populations and how they change over time.
  • Community: A biological community consists of all the populations of different species that live in a given area. Community ecologists focus on interactions between populations and how these interactions shape the community.
  • Ecosystem: An ecosystem consists of all the organisms in an area, the community, and the abiotic factors that influence that community. Ecosystem ecologists often focus on flow of energy and recycling of nutrients.
  • Biosphere: The biosphere is planet Earth, viewed as an ecological system. Ecologists working at the biosphere level may study global patterns—for example, climate or species distribution—interactions among ecosystems, and phenomena that affect the entire globe, such as climate change.
A flow chart of three boxes explaining the hierarchy of living organisms.
The top box contains a photograph of tall trees in a forest and is captioned, “Organisms, populations, and communities: In this forest, each pine tree is an organism. All of the pine trees living in the area make up a population. All of the populations of different species in the area form a community."
The second box contains a photograph of a body of water, behind which is a stand of tall grasses developing into more dense vegetation and trees as distance from the water increases. The photo is accompanied by the following text: “Ecosystems: This coastal ecosystem in the southeastern United States consists of a community of living organisms plus their physical environment."
The third box contains a drawing of planet Earth and is labeled, “The biosphere: The biosphere consists of all the ecosystems on Earth, considered together.
Image credit: modified from The scope of ecology: Figure 1 by OpenStax College, Biology, CC BY 3.0
The five levels of ecology are listed above from small to large. They build progressively—populations are made up of individuals; communities are made up of populations; ecosystems are made up of a community plus its environment; and so forth. Each level of organization has emergent properties, new properties that are not present in the level's component parts but emerge from from these parts' interactions and relationships.
The levels of ecological study offer different insights into how organisms interact with each other and the environment. I like to think of these levels as magnifying glasses of different strengths. If you really want to get what's going on in a particular ecological system, you'll likely want to use more than one!

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