Chapter Summary and Practice

Ecological systems integrate living communities and physical conditions

An ecosystem is a functional unit of interacting organisms and their nonliving surroundings. Species composition records what species occur, while stratification describes their vertical arrangement, such as tree canopy, shrub and ground layers. A pond contains producers such as phytoplankton, consumers including zooplankton and fish, and decomposers such as bacteria and fungi acting within its water and sediment. Light, temperature, dissolved substances and other abiotic conditions regulate the connections.

The four central functional processes are productivity, decomposition, energy flow and nutrient cycling. New primary production introduces organic matter; consumers redistribute some of it; decomposers transform dead matter and release nutrients; energy flows along food relationships and becomes less available through respiration and heat loss. Nutrient atoms can repeatedly return to organisms, while continuing external energy is required to keep the system working.

Productivity and the importance of time units

Gross primary productivity describes organic matter produced through photosynthesis before producer respiration. Net primary productivity is the gross amount minus respiration: NPP = GPP − R. If GPP is 1,200 and respiration is 450 g dry matter m⁻² yr⁻¹, net production is 750 g m⁻² yr⁻¹. The subtraction is necessary because respiration consumes part of grossly fixed organic compounds. Secondary productivity measures the rate of new biomass formation by consumers.

Production rates must not be confused with standing crop. A tree can hold a large mass accumulated over decades, while phytoplankton may hold little mass but reproduce rapidly. Light, water, temperature, mineral nutrients, species traits and growing-season duration affect production in different habitats. Oceanic surface waters may be nutrient limited, while deeper layers may be light limited; an unqualified single limiting factor is scientifically unreliable.

Decomposition connects dead matter with living producers

Detritus consists of dead plant and animal material and wastes. Fragmentation produces smaller particles, leaching carries soluble substances into water, and microbial catabolism breaks complex molecules down through enzymatic processes. Humification contributes relatively persistent soil organic matter, whereas mineralisation releases inorganic nutrient forms. These processes overlap in soil and sediment. Decomposer activity can release carbon dioxide, water and nutrient ions while retaining some elements in biomass or soil material.

Temperature, moisture, oxygen and residue chemistry influence breakdown. Suitable warmth and moisture often support aerobic decomposition, while cold or oxygen-limited environments can slow it. Tough lignin-rich residues frequently persist longer than easily used sugar-rich substances. The return of mineral nutrients can support further producer growth, but decomposer metabolism also dissipates chemical energy and cannot supply perpetual usable energy to the food web.

Energy enters food webs and declines through successive transfers

Most food webs begin with light captured by producers. PAR is the portion of light useful to photosynthesis, commonly around 400–700 nm and roughly half or less of incident solar radiation in broad descriptions. In a grazing chain, living producer tissues support herbivores and predators. Detritus chains begin with dead organic matter and involve detritivores and decomposers. The chains intersect to form food webs, because organisms can feed on several types of resource.

A trophic level is a functional feeding position: producers occupy level one, primary consumers level two and secondary consumers level three in a simple chain. Energy transfer is not completely efficient because material goes uneaten or unassimilated and respiration dissipates energy. An illustrative 10% model would convert 10,000 kJ m⁻² yr⁻¹ at producers to 1,000 at herbivores, 100 at secondary consumers and 10 at tertiary consumers. The true percentage depends on the organisms and ecosystem.

Ecological pyramids require careful interpretation

Pyramids of numbers compare counts, pyramids of biomass compare standing living mass, and pyramids of energy compare energy throughput per area and time. A grassland numbers pyramid may be upright, while a few large trees can support many small insects and form an irregular count pattern. Biomass pyramids are commonly upright in terrestrial ecosystems but may invert in plankton-based waters because producer biomass is renewed rapidly while being grazed.

A properly defined energy-production pyramid stays upright because the next level cannot receive all production energy from the level below. Biomass and number pyramids can invert because they measure different quantities. Food webs, omnivory, decomposers and changes over time are not shown fully in simple pyramid illustrations. The best explanation always begins by asking what quantity is being measured, in which units and over what period.

Matter cycles while useful energy dissipates

Carbon can move from atmospheric or dissolved inorganic carbon into producer organic matter, on to consumers and decomposers, and back into inorganic reservoirs through respiration and decay. Phosphorus is often stored in rocks and sediments, released by weathering, incorporated into organisms and returned by decomposition. These atom cycles differ from the progressive loss of useful chemical energy during biological transfer.

The ocean is the largest carbon pool in the active atmosphere–ocean–biosphere carbon cycle; sedimentary rocks store far greater amounts when the slower geological cycle is included. Such distinctions illustrate why a short factual answer can depend on the stated time scale or reservoir definitions. Ecosystem services such as nutrient recycling and water filtration depend on continuing interaction among organisms and the physical environment.

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