Describe the levels of ecological organization (organism, population, community, ecosystem) and explain how biotic and abiotic factors interact to shape an ecosystem (MA STE HS-LS2-1, HS-LS2-2 supporting, systems and system models).

What this topic is asking

The Massachusetts STE framework (HS-LS2-1 and HS-LS2-2) treats an ecosystem as a system of interacting living and nonliving parts. On the High School Biology MCAS, this topic is the foundation of the ecology module: you are asked to identify the levels of ecological organization, sort factors into biotic and abiotic, and explain how the parts of an ecosystem interact. The crosscutting concept is systems and system models: an ecosystem behaves as a connected whole.

The levels of ecological organization

From smallest to largest:

• Organism. A single individual living thing (one fish, one oak tree).
• Population. All the individuals of one species living in the same area at the same time (all the trout in a lake).
• Community. All the populations of different species living and interacting in the same area (the trout, the water plants, the insects, the microbes).
• Ecosystem. The community together with its nonliving surroundings (the community plus the water, soil, light, and air).

This hierarchy parallels the body's levels of organization from levels of biological organization, but at the scale of whole environments. The MCAS frequently asks you to order these levels or to tell a population from a community.

Biotic and abiotic factors

The parts of an ecosystem fall into two groups, and the MCAS expects you to sort them:

• Biotic factors are the living parts: plants, animals, fungi, bacteria, and anything they do (predation, competition, decomposition).
• Abiotic factors are the nonliving conditions: water, light, temperature, soil type, dissolved oxygen, pH, and minerals.

A simple test: if it is or was alive, it is biotic; if it never was alive, it is abiotic. Sorting a list of factors is one of the most common quick MCAS questions.

How the parts interact

The central idea is that biotic and abiotic factors interact, and the ecosystem behaves as a system:

• Abiotic factors shape the community. The amount of water, light, and warmth, and the dissolved oxygen, determine which organisms can survive in a place. A pond low in dissolved oxygen supports fewer fish; a shaded forest floor supports shade-tolerant plants.
• Organisms affect the abiotic conditions. Plants release oxygen and shade the ground; trees change the soil; animals move nutrients around. So the living things modify the nonliving environment too.

Because of these two-way interactions, a change in one part of an ecosystem can ripple through the rest, an idea explored further in ecological interactions and human impact on ecosystems.

Try this

Q1. Explain the difference between a population and a community. [2]

• Cue. A population is all the individuals of one species in an area; a community is all the populations of different species living and interacting there.

Q2. Give one biotic and one abiotic factor in a forest ecosystem. [2]

• Cue. Biotic: a tree, a deer, or a fungus. Abiotic: light, water, temperature, or soil.