5.2 Matter Cycles and Ecosystem Stability

Matter Is Reused, Not Used Up

Matter-cycle questions are a common place where Regents biology mixes ecology with chemistry. Atoms in carbon dioxide, water, proteins, carbohydrates, fats, and minerals can move from the nonliving environment into organisms and back again. The atoms are rearranged into new molecules, but the atoms are not created from nothing and are not destroyed during normal ecosystem processes. That is why the phrase matter cycles is more accurate than saying matter flows one way.

The carbon cycle is the easiest anchor. Plants, algae, and some bacteria remove carbon dioxide from air or water during photosynthesis and build carbon-containing sugars. Consumers eat organisms and use those food molecules for growth, repair, and cellular respiration. Respiration by producers, consumers, and decomposers returns carbon dioxide to the environment. Decomposition returns carbon compounds from dead bodies and wastes to soil, water, and air. Combustion moves carbon from fuels or biomass into the atmosphere as carbon dioxide.

Main Cycles Regents Students Should Recognize

You do not need to memorize every named bacterial step to answer most Regents ecology items. You do need to recognize that plants cannot use nitrogen gas directly in the same way they use carbon dioxide, and that nitrogen compounds in soil support amino acids, proteins, nucleic acids, and growth. If a question gives data about soil nitrate, plant biomass, and runoff, use the data rather than guessing from vocabulary.

Decomposers and Nutrient Availability

Decomposers are not optional extras in a food web. Bacteria and fungi break down dead organisms and wastes, using some molecules for their own respiration and releasing nutrients into the environment. Without decomposers, matter would remain locked in dead biomass. Producers would eventually lack important materials needed for growth, and the food web would become less productive.

A common trap is to say decomposers return energy to producers. They do not. Decomposers obtain energy for themselves by breaking down organic matter, and some energy is released as heat. What they make available to producers is matter: carbon dioxide, water, and mineral nutrients. A strong answer says decomposers recycle matter and help maintain nutrient availability.

Stability, Resilience, and Disturbance

Ecosystem stability does not mean nothing changes. Populations rise and fall, seasons change, organisms die, and nutrients move. A stable ecosystem maintains key functions, such as energy capture, nutrient cycling, water retention, and habitat support, despite normal variation. Resilience means the system can recover after disturbance.

Biodiversity can support stability in several ways. More species can mean more feeding pathways, so one population decline does not remove the only food source. Genetic diversity can help a population survive disease or changing conditions. Habitat diversity can provide refuges after storms, fires, floods, or droughts. Regents questions often ask for this mechanism, not just the phrase biodiversity is good.

Science Data Example

Suppose a stream study records nitrate concentration, algal density, decomposer activity, and dissolved oxygen. After heavy rain, nitrate increases from farm runoff. Algal density rises for two weeks. Then dead algae increase, decomposer activity rises, and dissolved oxygen drops. The best explanation links matter cycling and stability: extra nutrients increase producer growth, but decomposition of dead algae increases oxygen use, stressing fish and invertebrates.

The trap is choosing an answer that names only one part of the system. Saying fertilizer makes algae grow is incomplete if the question asks why fish died. Saying oxygen disappeared is also incomplete if the question asks for a mechanism. The full reasoning connects nutrient input, algal bloom, decomposition, cellular respiration by decomposers, and lower dissolved oxygen.

How to Answer Stability Questions

First identify the disturbance: nutrient runoff, invasive species, drought, deforestation, disease, overharvesting, or pollution. Then identify which ecosystem function changes: energy capture, nutrient cycling, population regulation, habitat structure, or water quality. Finally connect evidence to resilience. A diverse wetland with many plant species and intact decomposer communities may recover from moderate flooding faster than a simplified, polluted wetland because more biological roles remain available.

When an engineering solution appears, compare criteria and constraints. A buffer strip of native plants near a stream may reduce nutrient runoff and erosion, but it requires land, maintenance, and time to establish. A strong Regents response evaluates trade-offs using data, not opinion.

Succession as a Stability Story

Ecological succession also tests matter cycling and stability. After a fire, flood, abandoned farm, or new bare surface, the community changes as organisms alter soil, shade, moisture, and nutrient conditions. Early species may grow quickly and add organic matter when they die. Later species may be better competitors under the changed conditions. The trap is to treat succession as a fixed ladder that always ends in one perfect community. Regents questions usually want the evidence-based pattern: species composition changes because conditions and interactions change, and nutrient cycling helps make later growth possible.