The Living World: Ecosystems

A reservoir stores matter; a flux moves matter between reservoirs. A food web tracks feeding relationships, while an energy budget tracks transfers over a specified area and time.

An ecosystem includes organisms and the physical conditions with which they interact. Producers capture energy; consumers obtain energy from other organisms; decomposers return nutrients from organic matter to forms that can reenter biological use. A food web represents multiple feeding pathways, while a food chain is a simplified sequence.

Competition, predation, herbivory, parasitism, and mutualism can affect population sizes and community composition. The ecological niche includes resource use and environmental requirements, not merely the place an organism lives. Resource partitioning can reduce competition by separating use in space, time, or type.

Temperature and precipitation patterns influence terrestrial vegetation, decomposition, and growing seasons. Annual averages alone can hide seasonal drought or cold. Tropical forests, grasslands, deserts, temperate forests, and tundra differ in water availability, productivity, and nutrient storage.

Aquatic systems depend on salinity, light penetration, dissolved oxygen, flow, and depth. Estuaries mix freshwater and saltwater and can be highly productive. Coral reefs require particular temperature, light, and water-quality conditions. Wetlands can store water, filter some pollutants, and provide habitat, but their functions depend on hydrology and surrounding land use.

Carbon enters living tissue through photosynthesis and returns through respiration, decomposition, and combustion. Oceans, soils, organisms, atmosphere, and rocks store carbon on different timescales. Fossil-fuel burning transfers carbon from long-term geological storage into the active atmospheric–ocean system.

The water cycle includes evaporation, transpiration, condensation, precipitation, infiltration, groundwater movement, and runoff. Urban pavement can reduce infiltration and increase rapid surface runoff. Removing vegetation can alter evapotranspiration, erosion, and local water storage rather than merely “using less water.”

Most organisms cannot use atmospheric N₂ directly. Nitrogen fixation produces biologically available forms; nitrification, assimilation, ammonification, and denitrification move nitrogen among chemical forms and reservoirs. Fertilizer and combustion can increase reactive nitrogen in places where it becomes a pollutant.

Phosphorus cycles through rocks, soil, water, and organisms without a major gaseous atmospheric reservoir in the usual model. Weathering releases phosphate, while erosion and runoff move it into waters. Excess nitrogen or phosphorus can stimulate algal production when that nutrient limits growth.

Gross primary productivity is the total energy captured by producers. Net primary productivity subtracts producer respiration and represents energy retained as new biomass. State an area and time basis when comparing productivity measurements.

Trophic transfer is inefficient because energy is used in metabolism, released as heat, excreted, or left uneaten. A 10% transfer is a common approximation, not a universal constant. Matter can cycle; energy continually enters and ultimately leaves as dispersed heat. Biomass and energy pyramids should not be confused with counts of organisms.

Trace carbon from atmospheric carbon dioxide into plant sugars through photosynthesis, then into consumers through feeding. Respiration returns some carbon to the atmosphere; decomposition processes dead material and waste. Burial can move carbon into long-term reservoirs, while combustion transfers stored carbon back rapidly. The atom is conserved even though its chemical form changes.

For nitrogen, identify the transformation rather than saying it simply cycles. Fixation makes atmospheric nitrogen biologically available; nitrification oxidizes ammonium; assimilation incorporates inorganic nitrogen into biomass; ammonification releases ammonium from organic material; denitrification returns nitrogen to gaseous forms. Human inputs alter the sizes and rates of these flows.

Specify the arrow convention before interpreting a food web. Arrows often point from food to consumer to show energy transfer. Removing a predator may release an herbivore from predation, reducing producer biomass, but omnivory and alternative prey can weaken or redirect the response. A defensible prediction names the intermediate interactions.

Field evidence often combines counts, biomass, nutrient measurements, and experiments. Counts alone can be misleading when body sizes differ greatly. Compare equivalent sampling effort and time periods, and distinguish a stock such as standing biomass from a rate such as annual productivity.

Begin with a boundary: a pond, forest stand, or watershed. Energy crosses that boundary as sunlight, chemical energy in food, and heat; matter crosses in water, organisms, gases, and sediment. A budget separates inputs, outputs, and changes in storage. If nitrogen input exceeds output, nitrogen may accumulate, but you still need evidence about the reservoir receiving it. A food web cannot by itself show every storage pool.

Gross primary productivity is the total rate at which producers capture energy. Net primary productivity subtracts their respiration; it is the new biomass energy potentially available to consumers and decomposers. Write the subtraction before inserting numbers, and retain area and time units. A forest with high gross productivity can have lower net productivity than another system if its producers also respire much more.

Trophic transfer efficiencies are ratios of production at successive levels, not a universal physical constant of 10%. Use the stated efficiency at each step. Multiplying two 10% transfers gives 1% of the initial production. Matter in uneaten tissue can enter decomposer pathways, while respiratory energy becomes dispersed heat. Decomposers recycle nutrients, not a supply of usable energy that returns to the Sun.

In a carbon-cycle explanation, state both the starting reservoir and the destination. Combustion moves carbon from fuel into atmospheric carbon dioxide; photosynthesis moves inorganic carbon into organic molecules; respiration returns some organic carbon to carbon dioxide. Carbon burial and geological processes operate on different timescales from a daily photosynthesis–respiration cycle. A large reservoir need not have the largest annual flux.

Nitrogen availability depends on transformations. Fixation converts atmospheric nitrogen to usable compounds; nitrification converts ammonium through nitrite to nitrate; denitrification returns nitrogen to the atmosphere under suitable conditions. Leaching can move soluble nitrate into water. Phosphorus usually lacks a major atmospheric gaseous phase in the introductory cycle: weathering, erosion, uptake, and sedimentation are central.

A fertilizer problem may connect several cycles. Runoff supplies limiting nutrients, producer growth increases, dead organic matter increases, and aerobic decomposition consumes dissolved oxygen. Explain the intermediate steps; “fertilizer kills fish” skips the mechanism. Temperature, mixing, flow, and the identity of the limiting nutrient affect the outcome, so avoid claiming that every nutrient addition causes the same bloom.

Read temperature and precipitation across the year, not just annual totals. Two places receiving equal annual rain may support different vegetation if one has a long dry season. Latitude, altitude, rain shadows, and ocean proximity can alter climate. A climograph supports a biome prediction but is not a complete inventory of species or soil chemistry.

For an aquatic system, ask about salinity, light, mixing, flow, and nutrients. The photic zone supports light-dependent production; deeper water can receive sinking organic matter. Dissolved oxygen is influenced by photosynthesis, respiration, temperature, and mixing. High productivity is not synonymous with high oxygen at every depth or every hour.

Connect ecosystem services to a physical or biological mechanism. Wetland vegetation slows water and promotes sediment settling; roots stabilize soil; some microbial processes transform pollutants. A wetland has finite capacity and can be damaged by excessive loading. A strong management claim identifies both the service and the conditions under which it operates.

Food-web arrows usually indicate energy or matter moving from a consumed resource to its consumer; confirm the diagram’s convention. If a predator declines, its prey may increase, potentially reducing the prey’s resources. This indirect sequence is a trophic cascade, but alternative food sources and competing interactions can modify it.

A prediction should specify direction and mechanism at each link. Removing an herbivore can release plants from grazing, while removing a decomposer affects nutrient return through another pathway. Do not assume every species one step away responds identically or immediately.

Distinguish a productivity measurement from standing biomass. A rapidly growing and rapidly consumed producer population can have modest standing biomass but high production over time. Biomass is a stored amount at a time; productivity is a rate. This distinction helps explain why some aquatic biomass pyramids look different from energy pyramids.

State an independent variable that can actually be manipulated and a dependent variable that can be measured. Identify the control condition, important constants, independent replication, and how long measurements continue. A prediction should be testable and related to the mechanism under study.

For a nutrient experiment, several independently treated containers per nutrient level are stronger replication than many water samples from one container. Repeated samples reveal change within that container but do not create additional independent treatment units. Random assignment helps distribute uncontrolled differences.

Interpret an outcome at the level supported by the design. A response to added nitrogen supports nitrogen limitation under those conditions, not a claim that nitrogen limits every ecosystem in every season. A null result may reflect a different limiting factor, inadequate duration, or measurement limitations.