Describe how the carbon, nitrogen, and water cycles move nutrients through ecosystems, and explain primary and secondary ecological succession (Virginia 2018 Biology SOL BIO.8.b and BIO.8.c).

What this topic is asking

Virginia Biology SOL standard BIO.8 is about how ecosystems are dynamic and change over time. Substandard BIO.8.b covers how nutrients cycle with energy flow through ecosystems (the carbon, nitrogen, and water cycles), and BIO.8.c covers ecosystems having succession patterns. The EOC expects you to trace the main steps of each cycle, to know the organisms that drive them (especially decomposers and bacteria), and to tell primary from secondary succession. Unlike energy, matter is recycled, so these cycles are how ecosystems reuse the same atoms.

Matter cycles, energy flows

The key idea linking this topic to energy flow: energy enters as sunlight and leaves as heat, so it cannot be reused, but matter (the atoms in nutrients) is not lost and is recycled between organisms and the environment. Nutrient cycles are the loops that move carbon, nitrogen, and water through producers, consumers, decomposers, the soil, the water, and the air.

The carbon cycle

Photosynthesis and respiration are opposite processes that move carbon in opposite directions, so they are the core of the carbon cycle. Burning fossil fuels adds extra carbon dioxide to the atmosphere faster than it is removed.

The nitrogen cycle

Nitrogen is needed to build proteins and nucleic acids (DNA and RNA), but the nitrogen gas ($N_2$) that makes up most of the air cannot be used directly by plants and animals. Bacteria do most of the work:

• Nitrogen-fixing bacteria (in soil and in the roots of legumes) convert atmospheric $N_2$ into usable forms such as ammonia and nitrates.
• Plants absorb nitrates and use the nitrogen to build proteins; consumers get nitrogen by eating plants or other animals.
• Decomposers break down dead organisms and wastes, releasing nitrogen back into the soil.
• Denitrifying bacteria convert nitrates back into $N_2$ gas, returning it to the atmosphere.

The takeaway the EOC wants is that bacteria make nitrogen available and return it, so the cycle depends on them.

The water cycle

Water moves through the environment by physical processes: evaporation (liquid water to vapor, driven by the Sun), transpiration (water vapor released from plants), condensation (vapor to droplets, forming clouds), precipitation (rain and snow), and runoff and infiltration (water flowing over and into the ground, returning to rivers, oceans, and groundwater). The water cycle distributes fresh water that organisms and the other cycles depend on.

Ecological succession

In primary succession, pioneer species such as lichens and mosses colonize bare rock; as they grow and die, they break down the rock and add organic matter, slowly building the first soil. Once soil forms, larger plants can grow, and the community develops over a long time. Secondary succession is faster because the soil (and often seeds and roots) is already present, so a disturbed area can regrow more quickly. Both processes tend toward a relatively stable climax community that persists until the next disturbance.

Try this

Q1. Name the two processes that move carbon between living things and the atmosphere in opposite directions, and state the direction of each. [2]

• Cue. Photosynthesis removes carbon dioxide from the air (into producers); cellular respiration releases carbon dioxide back to the air.

Q2. State the difference between primary and secondary succession. [2]

• Cue. Primary succession begins where there is no soil (bare rock or a new lava flow) and pioneer species build soil first; secondary succession follows a disturbance where soil remains, so it is faster.