Ecology, food webs and nutrient cycles

Ecology, food webs and nutrient cycles

Ecology studies interactions among organisms and between organisms and their physical surroundings. A population contains members of one species in an area, while a community includes the interacting populations of different species. An ecosystem includes the community and its abiotic environment. Factors such as temperature, water availability, light and soil conditions can influence where organisms live and how well they reproduce.

A habitat is the place where an organism lives. Its niche describes its role and interactions, including how it uses resources. Organisms sharing a habitat can occupy different niches. Competition occurs when demand for a limited resource overlaps. Predation, parasitism and mutualism describe other interactions, but their effects must be explained from the particular organisms and resources involved, rather than from labels alone.

Food chains represent feeding relationships and energy transfer. Arrows point from the food organism to the consumer, showing the direction in which energy and material move. Producers make organic substances, often by photosynthesis. Primary consumers eat producers, while secondary consumers eat primary consumers. Food webs show the connections among multiple chains and make a more realistic representation of many ecosystems.

Energy flows through ecosystems and is dissipated at each transfer through respiration, movement, waste and uneaten material. Not all the energy in one trophic level becomes new biomass at the next. This limits the energy available to support higher levels. Matter, in contrast, is recycled through processes including decomposition and biogeochemical cycles. Avoid describing energy as being recycled in the same way as carbon atoms.

Decomposers break down dead material and release nutrients that can become available to producers. In the carbon cycle, photosynthesis incorporates carbon dioxide into organic substances, while respiration and combustion return carbon dioxide. In the nitrogen cycle, different microorganisms carry out processes such as nitrogen fixation, nitrification and denitrification. Plants cannot directly use atmospheric nitrogen gas in the same way that they absorb nitrate ions.

Ecological sampling estimates patterns without counting every organism. Quadrats suit many plants and slow-moving organisms; random or systematic placement should match the study design. Repeated samples improve the usefulness of an estimate. Human activities can alter habitats, food webs and nutrient cycles, so conservation decisions should consider habitat protection, sustainable resource use and evidence about the organisms involved rather than rely solely on a single short-term observation.

Energy passes through; nutrients are recycled

Producers convert energy from sunlight or, in some ecosystems, chemical reactions into organic matter. Consumers obtain organic material by feeding, and decomposers break down dead organisms and wastes. At each trophic transfer, much energy is used in metabolism and eventually dissipated as heat; some biomass is not eaten or not assimilated. A food chain is therefore not an efficient conveyor that transfers all energy upward. Nutrients can cycle between living organisms and environmental reservoirs, whereas energy requires a continuing input and does not cycle in the same way.

A pyramid of energy represents energy transfer per unit area over a specified time and decreases along successive trophic levels. A pyramid of numbers may be inverted, such as one tree supporting many insects. A pyramid of standing biomass can sometimes be inverted in aquatic systems where rapidly reproducing producers have small standing biomass at the moment sampled. Distinguish the type of pyramid before explaining its shape. Do not assume that a smaller number of organisms always means less total biomass or lower productivity.

Carbon and nitrogen pathways

Photosynthesis removes carbon dioxide from the atmosphere or water and incorporates carbon into organic molecules. Feeding transfers carbon through food webs. Respiration and decomposition return carbon dioxide, while some carbon enters longer-term stores such as soils and sediments. Burning fossil fuels transfers stored carbon into the atmosphere. A complete explanation identifies a reservoir, a transfer process and its direction. Listing the words “photosynthesis” and “respiration” without connecting them to carbon movement leaves the cycle unexplained.

Nitrogen fixation converts atmospheric nitrogen into biologically usable nitrogen compounds. Some microorganisms, including bacteria associated with legume root nodules, carry out this process. Decomposition releases ammonium compounds from organic matter. Nitrification converts ammonium to nitrite and then nitrate under suitable aerobic conditions. Plants absorb nitrogen compounds and use nitrogen to build amino acids and other molecules. Denitrification can return nitrogen gas to the atmosphere, especially under low-oxygen conditions. Nitrogen-fixing bacteria do not simply turn nitrogen gas directly into plant protein, and legumes still require other nutrients and appropriate growing conditions.

Sampling populations without bias

A quadrat defines an area for counting stationary organisms or estimating cover. Place quadrats using a defensible method, such as random coordinates across a clearly defined study area. Counting only places where a species is obvious produces selection bias. Mean density equals total organisms counted divided by total sampled area. Multiplying mean density by habitat area estimates population size if the sample represents that habitat. The estimate has uncertainty because organisms are often clumped and habitats vary. Report the method, sample size and limitations rather than presenting the estimate as an exact census.

A transect helps investigate change across an environmental gradient, for example from a shaded edge to an open field. A line transect records organisms touching the line, while a belt transect samples a strip with successive quadrats. Keep sampling intervals and identification rules consistent. For mobile animals, mark-release-recapture can estimate population under restrictive assumptions, including mixing, no substantial births or migration during sampling and marks that do not affect survival or capture. Actual animal handling requires approved supervision and humane procedures; examination calculations can be practised using tokens instead.

Interactions, succession and soil

Competition occurs when organisms require the same limited resource. Predation involves one organism consuming another, while parasitism benefits the parasite at the host's expense without necessarily killing it immediately. Mutualism benefits both partners; commensalism benefits one with no clear significant effect on the other. An interaction's classification depends on evidence about effects, not whether two species happen to be found together. Lichens are a familiar symbiotic association, but named examples should be explained rather than memorised as labels.

Succession describes directional change in community composition over time. Early colonisers alter conditions, allowing other organisms to establish, but disturbances can change or interrupt the pathway. Soil provides mineral particles, organic matter, air and water. Texture affects drainage and retention, while decomposers help recycle nutrients. A productive habitat cannot be explained by rainfall alone; soil, temperature, light, disturbance and species interactions also matter. A small school field investigation can compare these factors while avoiding disturbance to protected species or sensitive sites.

Environmental application and self-check

Fertiliser runoff can add nutrients to water, stimulate excessive plant or algal growth and increase decomposition when that biomass dies. Microbial respiration may then reduce dissolved oxygen, harming aquatic animals. The chain of causes matters more than the word “pollution”. Conservation may involve protecting habitats, reducing overharvesting, controlling invasive species and using resources sustainably. A claim that planting any tree anywhere always benefits an ecosystem ignores local species, water availability and land-use needs.

If 36 plants are counted in twelve 0.25 m² quadrats, sampled area is 3 m² and mean density is 12 plants m⁻². A 100 m² comparable habitat would have an estimated 1,200 plants. This calculation is justified only if the quadrats reasonably represent the area. Check that you use area units consistently, and distinguish an estimated population from the number actually observed. For a food-web question, predict direct effects first and then consider compensating changes; real ecosystems need not respond as one simple chain.