APES 1.1-1.2: Introduction to Ecosystems and Biomes

Net primary production (NPP)

The organic matter or energy remaining after producers use some GPP for respiration and maintenance. The relationship is $NPP = GPP - R_{producer}$.

Ecosystem

A system consisting of living organisms, the nonliving environment, and the interactions linking them through energy flow and nutrient cycling.

What are the two major components of an ecosystem?

Biotic components are living organisms, such as plants, animals, fungi, and bacteria. Abiotic components are nonliving factors, such as climate, soil, water, light, temperature, and minerals.

How are organisms and abiotic components connected within an ecosystem?

They are connected by energy flows and nutrient cycles. Matter is transferred among organisms and the physical environment, while energy enters mainly as light and is eventually released as heat.

Ecosystem process

A transfer of energy or matter from one pool to another within an ecosystem, such as photosynthesis, feeding, respiration, or decomposition.

How do external factors differ from internal factors in an ecosystem?

External factors, such as climate, parent material, topography, time, and the regional species pool, influence an ecosystem but are not controlled by it. Internal factors, such as decomposition, competition, shading, disturbance, succession, and species composition, both affect and are affected by ecosystem processes.

Why is climate considered a major external control on ecosystem structure?

Climate determines broad conditions such as temperature and precipitation, which influence photosynthesis, water availability, decomposition, and the amount of energy available. Climate also helps determine the biome in which an ecosystem occurs.

How can topography influence ecosystem processes?

Topography affects microclimates, soil development, drainage, and water movement. For example, a depression and a nearby steep hillside can support different ecosystems even under the same regional climate.

Primary production

The formation of organic matter from inorganic carbon, mainly through photosynthesis by autotrophs. It introduces both chemical energy and biologically usable carbon into an ecosystem.

What is the balanced overall equation for oxygenic photosynthesis?

A simplified equation is $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$. Photosynthesis stores light energy as chemical energy in organic molecules.

Gross primary production (GPP)

The total amount of organic matter or chemical energy produced by photosynthesis in an ecosystem before subtracting the producers' respiration.

Why is net primary production important to the rest of an ecosystem?

NPP represents the chemical energy and biomass available for growth of producers and for consumption by herbivores and decomposers. It is therefore the main energy supply for higher trophic levels.

What environmental factors can limit photosynthesis and primary production?

Important limitations include light intensity, leaf area and shading, carbon dioxide supply, water availability, and temperature. A shortage of any one of these can reduce photosynthetic output.

How does energy move through an ecosystem?

Energy enters primarily as sunlight, is stored in organic molecules by producers, and is transferred through feeding. Respiration releases much of it as heat, so energy flows through ecosystems rather than cycling indefinitely.

Trophic level

A feeding position in an ecosystem. Producers occupy the first trophic level, herbivores are primary consumers, and organisms that eat consumers occupy higher trophic levels.

How does a food web differ from a food chain?

A food chain shows one pathway of energy transfer, such as plant to herbivore to carnivore. A food web represents interconnected feeding relationships in which organisms commonly consume multiple food types and may occupy more than one trophic level.

Ecosystem respiration

The combined respiration of all organisms in an ecosystem, including plants, animals, fungi, and other decomposers.

What is net ecosystem production (NEP), and what does it indicate?

Net ecosystem production is $NEP = GPP - R_{ecosystem}$. In the absence of major disturbance, positive NEP indicates net accumulation of carbon in the ecosystem, whereas zero NEP indicates approximately balanced carbon uptake and respiration.

Why do decomposers play a central role in both energy flow and nutrient cycling?

Decomposers obtain energy by breaking down dead organic matter, releasing heat through respiration. They also convert nutrients in dead biomass into inorganic or otherwise usable forms that can return to soil, water, and producers.

Detritus-based trophic system

A feeding pathway that begins with dead organic matter rather than living plants. Fungi, bacteria, and detritivores process detritus and transfer its energy and nutrients through the ecosystem.

What are the three broad processes of decomposition?

Leaching dissolves and transports water-soluble substances, fragmentation breaks dead material into smaller pieces, and chemical alteration changes organic molecules through microbial enzymes and other reactions.

How does leaching affect decomposition?

Water dissolves soluble compounds such as sugars, amino acids, and mineral nutrients and carries them into soil or away from the ecosystem. Leaching is generally more important in wet environments.

How does fragmentation accelerate decomposition?

Breaking dead material into smaller pieces exposes more surface area and removes protective layers such as bark, cuticle, cell walls, or animal exoskeletons. Microbes can then colonize and chemically alter the material more efficiently.

What conditions generally produce the fastest decomposition rates?

Decomposition is fastest in moist conditions with adequate oxygen and moderate-to-warm temperatures. Very dry conditions slow microbial activity, while waterlogged conditions often limit oxygen and also slow decomposition.

How do temperature and oxygen availability affect microbial decomposition?

Higher temperatures generally increase microbial respiration and decomposition, provided organisms are not stressed by extreme conditions. Oxygen-poor soils, such as waterlogged soils, restrict many decomposers and reduce decomposition rates.

Disturbance

A relatively discrete event that removes biomass or substantially changes ecosystem conditions, such as fire, flooding, storms, herbivore outbreaks, cultivation, or volcanic activity.

What is the difference between ecological resistance and ecological resilience?

Resistance is the tendency of an ecosystem to remain near its original state during a disturbance. Resilience is the ability to absorb disturbance, reorganize, and retain essentially the same function, structure, and identity.

How do primary and secondary succession differ?

Primary succession begins on a surface lacking developed soil and most organisms, so recovery is slow. Secondary succession follows a less severe disturbance where soil or other biological legacies remain, allowing faster recovery.

How do the frequency and severity of disturbances affect ecosystem recovery?

More severe or more frequent disturbances generally cause larger changes and lengthen recovery time. Disturbances can also alter species populations, soil organic matter, and future resource availability.

Why do mineral nutrients cycle while energy does not?

Matter, including carbon, nitrogen, phosphorus, and other minerals, can be chemically transformed and reused among organisms and the environment. Energy is converted between forms but is ultimately dissipated as heat during processes such as respiration.

Why is nitrogen cycling often a major control on terrestrial ecosystem production?

Nitrogen is required for proteins, nucleic acids, and other biomolecules, and many terrestrial ecosystems have limited immediately usable nitrogen. Nitrogen fixation and decomposition therefore strongly influence plant growth.

Nitrogen fixation

The conversion of atmospheric $N_2$ into biologically usable nitrogen compounds, carried out by certain bacteria and cyanobacteria. These organisms may live freely in soil or symbiotically with plants.

What is the sequence from organic nitrogen to nitrate during decomposition and nitrification?

Decomposition and mineralization release ammonium, $NH_4^+$. Nitrifying microbes oxidize ammonium to nitrite, $NO_2^-$, and then nitrate, $NO_3^-$. Plants can absorb inorganic nitrogen forms such as ammonium and nitrate.

Nitrification

The microbial oxidation of ammonium to nitrite and then nitrate: $NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^-$. It is generally an aerobic process.

Denitrification

The microbial reduction of nitrate or nitrite to gaseous nitrogen compounds, ultimately returning nitrogen to the atmosphere as $N_2$. It is favored by oxygen-poor conditions, especially when nitrogen is abundant.

How do mycorrhizal fungi affect plant nutrient acquisition?

Mycorrhizal fungi receive carbohydrates from plant roots and, in return, transfer nutrients such as phosphorus and nitrogen to the plants. Their hyphae increase the effective area over which roots can acquire nutrients.

How do the major nutrient sources for nitrogen and phosphorus differ?

Nitrogen commonly enters ecosystems through biological fixation, atmospheric deposition, and fertilizer inputs. Phosphorus primarily enters through weathering of rocks, and its supply often declines as landscapes become older and weathering-accessible material is depleted.

What are the primary macronutrients required by plants?

The primary macronutrients are nitrogen, phosphorus, and potassium, commonly abbreviated N, P, and K. They are required in relatively large amounts and are often limiting.

Ecosystem biodiversity

The variety of organisms and their relative abundances within an ecosystem. Biodiversity can influence ecosystem functioning because different species perform different ecological roles and may respond differently to environmental change.

Why does adding a species not always increase ecosystem functioning by the same amount?

The effect depends on whether the added species performs a distinct ecological function. A species with a role similar to existing species may add little, whereas an ecologically distinct species may introduce new processes or improve resource retention.

Biome

A broad class or category of ecosystems. Biomes are generally defined by characteristic environmental conditions and biological communities, while individual ecosystems include the specific organisms, abiotic factors, interactions, and physical space in a particular location.