Material production and material cycling in ecosystems

Introduction: "Matter" and "Energy" Flowing Through Ecosystems

Hello everyone! In this chapter, we will explore the "Ecology and Environment" section of Senior High School Biology, focusing on ecosystem production and nutrient cycling.
At first glance, seeing all the formulas and specialized terms might make you feel like, "This looks tough...", but don't worry! The underlying rules are actually very simple—just like keeping track of your allowance or managing a household budget.
Once you master this topic, you'll clearly understand how plants on Earth harness sunlight to create organic matter, how that energy and matter are passed along to animals, and how essential elements like carbon and nitrogen circulate throughout the globe. Let's take it step by step and build a rock-solid understanding!

[Fundamental Premise: The Biggest Difference Between Matter and Energy]
When thinking about ecosystems, there is one ultra-important rule you must never forget:
・Matter (such as carbon and nitrogen): Cycles continuously within the ecosystem.
・Energy (such as sunlight and chemical energy): Does not cycle; it is eventually converted into heat energy and escapes the ecosystem, meaning it flows in one direction and dissipates.
Let’s kick off our studies with this core concept in mind: "Matter cycles; energy flows through and dissipates!"

1. Production of Matter by Plants (Producers)

The foundation of any ecosystem is made up of producers (such as plants), which produce organic matter from inorganic substances via photosynthesis. The amount of organic matter created by producers through photosynthesis over a specific period (usually one year) is called primary production. It is typically expressed in units of weight or energy per unit area per unit time (e.g., \(\text{g}/(\text{m}^2\cdot\text{year})\) or \(\text{kJ}/(\text{m}^2\cdot\text{year})\)).

Producer Balance Sheet (The Core Formulas of Production)

Not all of the organic matter a plant makes via photosynthesis remains as part of its own body. Plants are living organisms too, so they consume organic matter through their own cellular respiration.
The total amount of organic matter produced by a plant through photosynthesis is called gross primary production (gross production). When you subtract the respiration used to keep the plant alive from this gross production, the remainder is the net primary production (net production).

[Formula 1: Net Production]
\((\text{Net Production}) = (\text{Gross Production}) - (\text{Respiration})\)
*Think of it like a part-time job: "Gross Production = Gross Pay (total earnings)," "Respiration = Living Expenses," and "Net Production = Take-Home Pay (what's left in your pocket)!"

Furthermore, plants cannot use all of this leftover net production solely for increasing their body size.
A portion of net production is lost as dead leaves and fallen branches, known as death / shedding (death), while another portion is eaten by herbivores, known as grazing (grazing loss). What remains after subtracting these losses is the growth that actually accumulates as new plant tissue (trunks, branches, roots, leaves, etc.).

[Formula 2: Growth]
\((\text{Growth}) = (\text{Net Production}) - ((\text{Grazing Loss}) + (\text{Death/Shedding}))\)
Rearranging this formula gives us another way to express net production:
\((\text{Net Production}) = (\text{Growth}) + (\text{Death/Shedding}) + (\text{Grazing Loss})\)

Biomass vs. Annual Production

The total amount of living organisms present in a given area at a specific point in time is called biomass (standing crop).
Here, it is crucial to clearly distinguish between "biomass" and "annual production":
・Biomass: The "stock" (stored quantity) of plant matter present at any given moment. Units are typically \(\text{g}/\text{m}^2\), etc.
・Annual Production: The "flow" (amount newly generated) over the course of a year. Units are typically \(\text{g}/(\text{m}^2\cdot\text{year})\), etc.

[Comparing Forests and Grasslands]
・Forests: Because trees accumulate thick, woody trunks and branches over many years, they are characterized by an extremely large biomass. However, because these woody parts do not perform photosynthesis yet still respire, the proportion of total respiration is high. Consequently, the ratio of annual production relative to biomass (the turnover rate) is small.
・Grasslands: Because herbaceous plants do not possess massive woody trunks, their biomass is small. However, because most of the plant tissue consists of photosynthetic leaves, they carry out photosynthesis very efficiently relative to their biomass, resulting in a fast production rate (high turnover rate).

[Common Pitfall]
It's easy to mistakenly assume that "an area with a larger biomass always has a vastly higher annual production!" But biomass refers to the "weight present right now," which is completely different from the "rate of production" over a year. While a forest has a massive biomass because it supports huge woody trunks, grasslands are actually much more efficient at generating new organic matter per unit of biomass because a higher proportion of their tissue consists of photosynthetic leaves.

2. Consumer Energy Budgets and Energy Flow

The organic matter produced by plants is eaten by herbivores (primary consumers), which in turn are eaten by carnivores (secondary and tertiary consumers). Within the bodies of consumers, a similar balance sheet of matter and energy applies.

Consumer Energy Budgets

The amount of food ingested by a consumer is called its ingestion (or intake). Not everything that is eaten is absorbed into the body; the undigested portion is excreted as feces. This is called egestion (undigested waste).
The amount actually absorbed into the body after subtracting egestion from ingestion is called assimilation.

[Formula 3: Consumer Assimilation]
\((\text{Assimilation}) = (\text{Ingestion}) - (\text{Egestion})\)
A portion of this assimilated organic matter is consumed via respiration to sustain the animal's life (for movement, maintaining body temperature, etc.). The remainder is allocated to growth (forming new body tissues), mortality (death due to old age or disease), and predation (being consumed by higher-level consumers).
\((\text{Assimilation}) = (\text{Respiration}) + (\text{Growth}) + (\text{Predation Loss}) + (\text{Mortality})\)

*Difference in Assimilation Efficiency Between Herbivores and Carnivores:
The plants eaten by herbivores contain a lot of difficult-to-digest cellulose, leading to a high amount of egestion and relatively low assimilation efficiency (assimilation / ingestion). In contrast, the meat consumed by carnivores is rich in proteins and lipids, which are easily digested, resulting in a higher assimilation efficiency.

Energy Flow and Trophic Efficiency

In an ecosystem, the feeding relationships ("who eats whom") along a food chain are structured into hierarchical steps called trophic levels. Solar light energy is converted into chemical energy within organic matter by producers and is then transferred to primary consumers, secondary consumers, and so on.
When energy transfers from one trophic level to the next, the efficiency of this transfer is called trophic efficiency (or energy transfer efficiency).

[Formula 4: Trophic Efficiency]
\((\text{Trophic Efficiency}) = \frac{\text{Energy at a given trophic level}}{\text{Energy at the preceding trophic level}} \times 100\quad [\%]\)

Generally, trophic efficiency is estimated to be around \(10\sim 20\%\). Why is it so low?
Because organisms at each trophic level carry out respiration to stay alive—releasing energy as heat—and because substantial energy is lost through unconsumed biomass, egestion, and natural mortality, the amount of energy passed on to the next level drops drastically with each step.
As a result, higher trophic levels can support fewer individuals and less biomass, giving ecological pyramids their characteristic tapering, upright shape. This also explains why food chains rarely exceed 4 to 5 trophic levels: beyond that point, there simply isn't enough energy left to sustain another level.

[Key Takeaway: Energy Does Not Cycle!]
The heat energy generated by organisms through respiration radiates out (dissipates) into space. Plants cannot recapture this lost heat energy to perform photosynthesis. An ecosystem can only continue to function because it receives a continuous supply of fresh light energy from the sun.

3. Biogeochemical Cycles (The Pathways of Carbon and Nitrogen)

In contrast to energy, the chemical elements that build living bodies (such as carbon and nitrogen) never leave the Earth; instead, they constantly cycle between the abiotic environment and living organisms. Let’s break down the two most frequently tested cycles in secondary-school biology: the carbon cycle and the nitrogen cycle.

(1) The Carbon Cycle

Carbon (\(\text{C}\)) is the most fundamental element that forms the backbone of all biological organic molecules, including proteins, carbohydrates, lipids, and nucleic acids.

・Form in the Atmosphere and Water: Exists primarily as carbon dioxide (\(\text{CO}_2\)).
・Uptake by Plants: Producers absorb \(\text{CO}_2\) from the atmosphere or water via photosynthesis to synthesize organic matter (such as carbohydrates).
・Transfer via Food Chains: The organic matter synthesized by plants is passed along to herbivores and subsequently to carnivores.
・Release via Respiration: All living organisms—producers, consumers, and decomposers—carry out respiration, breaking down organic matter and returning \(\text{CO}_2\) to the atmosphere or water.
・Action of Decomposers: Organic matter in dead organisms and animal waste is broken down (mineralized) into carbon dioxide through the cellular respiration of soil fungi and bacteria (decomposers).
*Fossil fuels, such as coal and petroleum, are formed from the remains of ancient organisms that accumulated and underwent fossilization over millions of years.

(2) The Nitrogen Cycle and Nitrogen Assimilation

Nitrogen (\(\text{N}\)) is an indispensable element for building proteins (amino acids) and nucleic acids (DNA, RNA, ATP, etc.). Although nitrogen gas (\(\text{N}_2\)) makes up about \(78\%\) of the atmosphere, its chemical bond is extremely strong, meaning plants and animals cannot directly absorb and use gaseous \(\text{N}_2\) in their bodies.
Instead, nitrogen circulates by continually changing into various inorganic nitrogen compounds.

① Nitrogen Assimilation (Super Important!)
Plants absorb inorganic nitrogen compounds such as ammonium ions (\(\text{NH}_4^+\)) and nitrate ions (\(\text{NO}_3^-\)) from the soil through their roots. They combine these with organic compounds (carbohydrates) produced by photosynthesis to synthesize organic nitrogen compounds such as amino acids and nucleic acids. This biochemical process is known as nitrogen assimilation.
Because animals cannot synthesize amino acids from inorganic nitrogen compounds on their own, they obtain their nitrogen by consuming other organisms (like plants) in the form of organic nitrogen compounds.

② Nitrogen Fixation
The process of converting atmospheric nitrogen gas (\(\text{N}_2\)) into biological forms such as ammonium ions (\(\text{NH}_4^+\)) is called nitrogen fixation. Only certain specialized prokaryotes (nitrogen-fixing bacteria) can perform this feat.
・Free-living examples: Azotobacter (aerobic), Clostridium (anaerobic), cyanobacteria (e.g., Nostoc)
・Symbiotic examples: Rhizobium (root nodule bacteria), which live symbiotically within the root nodules of leguminous plants

③ Nitrification (Nitrosomonas and Nitrobacter)
Organic nitrogen compounds found in dead organisms and excreta are mineralized into ammonium ions (\(\text{NH}_4^+\)) by soil decomposers.
The process of oxidizing these ammonium ions into nitrate ions is called nitrification, and the chemosynthetic bacteria responsible for this are collectively referred to as nitrifying bacteria.
・Nitrite bacteria (e.g., Nitrosomonas): Oxidize ammonium ions (\(\text{NH}_4^+\)) to nitrite ions (\(\text{NO}_2^-\)).
・Nitrate bacteria (e.g., Nitrobacter): Oxidize nitrite ions (\(\text{NO}_2^-\)) to nitrate ions (\(\text{NO}_3^-\)).
Because plants often absorb \(\text{NO}_3^-\) more readily than \(\text{NH}_4^+\), the activity of nitrifying bacteria is vital for plant growth.

④ Denitrification
The process where soil nitrate ions (\(\text{NO}_3^-\)) and nitrite ions (\(\text{NO}_2^-\)) are reduced back into atmospheric nitrogen gas (\(\text{N}_2\)) by soil-dwelling denitrifying bacteria is called denitrification. This commonly occurs in oxygen-poor soil environments, such as flooded rice paddies.

[Fun Fact: Lightning as Nature’s Fertilizer]
Although atmospheric nitrogen molecules (\(\text{N}_2\)) are extremely stable, the tremendous energy of a lightning bolt can break their bonds, allowing nitrogen to combine with oxygen to form nitrogen oxides. These dissolve in rainwater and fall to the ground as nitrate ions. The old traditional saying that "years with plenty of lightning bring abundant harvests" is actually grounded in real science—lightning literally delivers natural nitrogen fertilizer from the sky!

4. Summary and Concept Check

To wrap things up, let's review the most critical takeaways from this chapter!

[Quick Review of Key Concepts]
1. Producer Formulas:
　・\((\text{Net Production}) = (\text{Gross Production}) - (\text{Respiration})\)
　・\((\text{Growth}) = (\text{Net Production}) - ((\text{Death/Shedding}) + (\text{Grazing Loss}))\)
2. Consumer Formula:
　・\((\text{Assimilation}) = (\text{Ingestion}) - (\text{Egestion})\)
3. Fundamental Difference Between Energy and Matter:
　・Energy decreases across trophic levels through respiration and dissipation; it flows in one direction and does not cycle.
　・Matter (such as carbon and nitrogen) continuously cycles between the abiotic environment and living organisms.
4. Key Players in the Nitrogen Cycle:
　・Nitrogen Fixation: \(\text{N}_2 \rightarrow \text{NH}_4^+\) (Rhizobium, Azotobacter, etc.)
　・Nitrification: \(\text{NH}_4^+ \rightarrow \text{NO}_2^-\) (Nitrite bacteria), \(\text{NO}_2^- \rightarrow \text{NO}_3^-\) (Nitrate bacteria)
　・Nitrogen Assimilation: The process by which plants convert inorganic nitrogen compounds (\(\text{NH}_4^+\), \(\text{NO}_3^-\)) into organic nitrogen compounds (amino acids, proteins, etc.)
　・Denitrification: \(\text{NO}_3^- \rightarrow \text{N}_2\) (Denitrifying bacteria)

Once you can picture these interconnected relationships and flowcharts in your head, you'll be more than ready to tackle any exam question on primary production and nutrient cycles! Tackle some practice questions with confidence!