Ecology and ecosystems

01 / UNDERSTAND & APPLY

Organisms, populations and ecosystems

Ecology examines relationships among organisms and their surroundings. A population consists of members of one species within a defined area and interval; a community includes populations of different species and their interactions. An ecosystem includes that living community together with its physical and chemical environment. A pond study therefore needs more than a fish count: water, dissolved substances, light and exchanges with surrounding land also matter.

The boundary is a choice made for the question being studied. A pond can receive nutrients from its catchment and lose water downstream; calling it an ecosystem does not make it a closed system. State the boundary before interpreting changes, because an apparent gain inside it may be an inflow from outside.

Habitat refers to where an organism lives. Its niche concerns how it uses resources, tolerates conditions and interacts with other organisms. Two species sharing a pond may feed at different depths or times and on different foods. Shared habitat therefore does not establish an identical niche or prove direct competition for every resource.

02 / UNDERSTAND & APPLY

Resources, tolerance and population change

Organisms have physiological tolerances. Temperature can affect biological reaction rates, water availability constrains many terrestrial organisms, and light or nutrients can limit production. More of one resource may make little difference when another remains limiting. A factor that limits growth at one site or season need not be limiting everywhere.

For a defined population, final size equals initial size plus births and immigration minus deaths and emigration. In the fictional accounting interval, 200 organisms plus 40 births and 10 arrivals, less 25 deaths and 5 departures, leave 220. This is a 10% increase. Immigration is explicitly allowed; the population is not assumed closed.

Exponential growth is a simplified model in which a constant per-capita net rate generates increases proportional to the existing population. Logistic growth introduces a resource-related constraint: growth slows as size approaches a model carrying capacity. These are explanatory models, not promises that every observed population follows a smooth curve.

Carrying capacity depends on resources, habitat and interactions, so drought, habitat change or altered food supply can change it. Competition or some disease transmission can strengthen as density rises. A disturbance such as a severe storm can occur independently of population density, although its consequences still depend on circumstances. Neither category implies that every organism is affected equally.

03 / UNDERSTAND & APPLY

Interactions and indirect effects

Interaction signs describe effects relative to an appropriate comparison. Competition for a limiting resource imposes costs on both participants; predation, herbivory and parasitism benefit one participant at another’s expense. Mutualism benefits both, while commensalism benefits one with no appreciable effect on the other in the context examined. These labels summarize an interaction, not the complete relationship under all possible conditions.

Direct effects can produce indirect effects. If birds eat leaf-eating insects, a reduction in birds could release insects from predation and increase plant damage. But birds may also consume seeds, other predators may compensate, and food or weather may constrain insects. A food web identifies plausible pathways; their strengths require evidence.

In the fictional web, grass feeds insects and rabbits, insects feed birds, and birds also consume seeds. Removing birds can reduce insect predation, potentially harming grass, while reducing seed consumption could favour plant recruitment. Without additional information about these pathways, an inevitable grass increase is not a defensible prediction.

04 / UNDERSTAND & APPLY

Producers, consumers and detrital pathways

Autotrophic producers build organic matter from inorganic carbon using an energy source. Photosynthetic producers use light; chemoautotrophic producers use energy from chemical reactions. Thus sunlight supports many food webs, but it is not a universal requirement for all primary production. Consumers obtain organic matter by feeding on organisms or their products.

Dead material and wastes connect organisms to detrital pathways. Detritivores consume such material, while decomposers, especially many bacteria and fungi, chemically break down organic material and release nutrients. These roles are not confined to the end of one linear chain: material from producers and consumers at several levels can enter the pathway.

This lesson’s food arrows run from resource or prey to consumer, showing transfer. Phytoplankton → zooplankton means zooplankton feed on phytoplankton, not the reverse. Omnivores feed at more than one trophic level, and a species can have several prey and predators. Check arrow conventions before drawing conclusions from any unfamiliar diagram.

05 / UNDERSTAND & APPLY

Energy flow and production rates

Gross primary production, GPP, is the total rate at which producers fix energy or carbon into organic matter. Producers use some of that production in respiration. Net primary production, NPP, equals GPP minus producer respiration. Use compatible units, area and time: subtracting annual respiration from a daily production estimate would not give a meaningful result.

NPP is available for producer growth and reproduction and can enter grazing or detrital pathways. It is not identical to the amount actually eaten by herbivores. Some material remains unconsumed, some ingested material is not assimilated, and consumers themselves respire. Each distinction affects how much production can appear at the next trophic level.

The fictional calculation uses GPP 1,200 and producer respiration 500 energy units per square metre per year, giving NPP 700. With supplied production-transfer efficiencies of 15% and then 20%, primary-consumer production is 105 and secondary-consumer production 21 in the same units. These are assumptions for this example. There is no universal rule requiring every transfer to equal exactly 10%.

Energy transformations ultimately dissipate usable energy as heat. Decomposers also respire; they do not convert that metabolic heat back into sunlight for producers. Material nutrients can return to forms that organisms use again, while sustained biological activity requires renewed usable energy input. The native figure separates these two accounts rather than drawing one ambiguous recycling loop.

06 / UNDERSTAND & APPLY

Numbers, biomass and energy

Standing biomass measures material present at an instant, commonly as mass per area. Production measures an amount generated over time. A small stock that is replenished rapidly can support consumption over a much longer interval. Comparing these quantities without their time dimensions is like confusing water in a tank with water flowing through it during a year.

In the fictional pond, producer biomass is 5 g/m² and consumer biomass 20 g/m² at one instant. Fast producer turnover can replenish what consumers remove. The inverted biomass stock comparison therefore does not by itself violate energy conservation. Production, imports, exports and transfer measurements over matching boundaries would be needed for an energy account.

A numbers pyramid can also invert: one large tree may support many small herbivores. In contrast, a trophic energy-production pyramid in the stated transfer model has less production at successive levels because only part is transferred. Comparisons must use consistent areas and intervals and account for external subsidies; a sampled consumer aggregation supported from outside is not a closed local energy budget.

07 / UNDERSTAND & APPLY

Carbon, nitrogen, phosphorus and water

Carbon moves between organisms and physical reservoirs. Photosynthesis transfers inorganic carbon into organic matter; respiration and decomposition return some carbon to inorganic forms; combustion transfers carbon from fuels or biomass; air–water exchange connects reservoirs. A large amount stored in a reservoir is a stock, whereas its annual transfer is a flux. One cannot infer the flux merely from the reservoir’s size.

Nitrogen transformations have distinct names and directions. Fixation converts atmospheric nitrogen into reactive forms that can enter biological use. Nitrification converts reduced nitrogen such as ammonium toward nitrite and nitrate under suitable conditions. Assimilation incorporates available nitrogen into biological material; decomposition and mineralization release inorganic nutrients from organic matter. Denitrification converts nitrate or nitrite toward gaseous nitrogen forms under appropriate conditions. These processes are not interchangeable.

In the basic phosphorus cycle, weathering makes phosphorus from rocks available to soils and waters; organisms take it up, and decomposition and sediment processes redistribute it. Unlike nitrogen, phosphorus has no major atmospheric gaseous phase in this simplified cycle. Transport through dust or droplets does not require inventing such a dominant gaseous reservoir.

Water moves through evaporation and transpiration, condensation, precipitation, runoff and infiltration. These pathways connect ecosystems with the atmosphere, soils and groundwater. Nutrient transport can accompany water movement, while biological transformations change nutrient forms. A substance leaving one local compartment has moved or transformed; it has not necessarily vanished from the wider material account.

08 / UNDERSTAND & APPLY

Succession, disturbance and resilience

Primary succession describes community development where developed soil is absent, such as newly exposed rock. Secondary succession follows a disturbance where soil or biological legacies such as roots, seeds or surviving organisms remain. The difference concerns starting conditions, not simply whether a site currently has many visible plants.

Those starting conditions affect what can establish, but they do not fix a universal timetable. Climate, dispersal, substrate, repeated disturbance and management shape the pathway. A burned field retaining soil may recover differently from bare rock, yet severe repeated disturbances can also redirect either trajectory. Do not assume every site must reach one identical permanent climax community.

Resistance concerns how little a system changes during disturbance; resilience concerns its capacity to recover or reorganize afterward. A system may resist a modest disturbance but recover poorly after a larger one. Observing rapid recovery of one measured variable also does not prove that every species or ecological function has returned. Specify the variable and timescale being assessed.

09 / UNDERSTAND & APPLY

Wetlands, forests, grasslands and services

Forests, grasslands, wetlands and aquatic systems have different dominant organisms, water conditions and disturbance regimes. Natural grasslands can sustain distinctive native communities; they are not necessarily forests waiting to be restored. Converting them to tree plantations can alter habitat and harm grassland values. The appropriate comparison is the actual native ecosystem and its functions, not tree cover alone.

Wetlands connect water, soils and organisms through characteristic wet conditions. They can provide habitat and influence water storage or processing, but their functions depend on location and condition. Treating every wetland as vacant land misses these relationships, while claiming every wetland always prevents every flood would overstate them.

Ecosystem services connect ecological processes with human benefits. Provisioning benefits include food and materials; regulating benefits include aspects of water regulation, pollination or climate regulation; cultural benefits include recreation and cultural meaning. Processes such as nutrient cycling and soil formation underpin many benefits. Benefits can differ among people, and improving one use can reduce another, so evaluation needs a stated location and affected population.

Nutrient enrichment can stimulate algal production when other conditions allow. When additional organic matter is decomposed, microbial respiration can increase oxygen demand and contribute to low dissolved oxygen. Water movement, light and other conditions influence the outcome. More nutrients therefore need not improve the whole ecosystem: a production increase in one group can accompany damage to others.

10 / UNDERSTAND & APPLY

Reading ecological evidence

A plausible mechanism explains how an effect could occur; a correlation alone does not establish that it did occur for that reason. If plant cover and rainfall rise together, rainfall may matter, but other changes could also contribute. Repeated observations, comparisons and knowledge of the system help distinguish explanations.

Prelims statements often become wrong through an unnecessary absolute. All producers require sunlight fails because chemical energy can support autotrophy. All ecological pyramids are upright confuses stocks, numbers and energy flows. Nutrients and energy recycle identically confuses material return with energy dissipation. Carrying capacity never changes ignores its environmental conditions.

All quantitative examples here are fictional. Primary educational and institutional sources checked on 12 October 2026 support the bounded ecological concepts; no current ecosystem extent, legal land designation or universal restoration prescription is asserted. A site-specific management decision needs evidence beyond these foundational models.