In The Characteristics of Life you learned how to tell whether something is alive. Now zoom out to a whole pond, forest or field. Ecologists split every place into two kinds of parts and then ask how the parts affect each other.
Key idea
An ecosystem is all the biotic (living or once-living) parts of a place together with its abiotic (never-living) parts: sunlight, water, air, soil minerals, rock and temperature.
Why it works
The prefix bio- means life and a- means without. Dead leaves, a log or a bone are made of matter that living things built, and that matter still feeds decomposers. That is why ecologists count them as biotic. Abiotic parts were never alive.
The two kinds of parts work on each other. Water and temperature decide which plants can grow. Plants shade the ground, hold soil in place and add oxygen to the air. Earthworms mix the soil.
Worked example: Sort a forest floor
A hiker lists: moss, a granite boulder, rainwater, a fallen pine cone, a beetle, the shade under the trees. Which are biotic?
- Moss and the beetle are alive now: biotic.
- The pine cone grew on a pine tree. It is once-living: biotic.
- The boulder and the rainwater were never alive: abiotic.
- Shade is less sunlight reaching the ground, so it is an abiotic condition (even though trees cause it).
Answer: Biotic: moss, pine cone, beetle. Abiotic: boulder, rainwater, shade.
Common mistake
Many students put dead leaves, logs and bones with the abiotic parts because they are not alive now. Ask one question instead: was it ever part of a living thing? If yes, it is biotic. Abiotic means never living, like rock, water, sunlight and air.