Individuals, populations, communities and ecosystems

An organism is one individual living thing. Example: one dog, one sunflower, or one bacterium.

No. A herd is many organisms. One elephant is one organism.

A species is a group of organisms that can breed together and produce fertile offspring.

Fertile offspring means the babies can grow up and have babies of their own.

Yes. They can breed and produce fertile puppies, so they are the same species.

No. They do not normally produce fertile offspring, so they are different species.

Classification helps scientists identify organisms and organise the huge variety of life.

If you know the group an organism belongs to, you can predict features. Example: if it is a mammal, it likely has hair and feeds milk to young.

A binomial name is a two-part scientific name: Genus then species. Example: Homo sapiens.

Write Genus with a capital letter and species in lower case, and put both in italics (or underline). Example: Homo sapiens.

A genus is a group of closely related species. Example: Canis includes dogs, wolves, and coyotes.

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Some organisms have mixed features. Example: a platypus has fur like a mammal but lays eggs.

New evidence, especially DNA evidence, can show organisms are more or less related than we thought.

Can they breed and produce fertile offspring? If yes, they are the same species.

Homo sapiens.

It makes biodiversity and population data accurate, so scientists can make correct conclusions and conservation decisions.

Examples include leaf shape, flower colour, number of petals, or presence of thorns.

Examples include number of legs, wings, antennae, or body segments.

A dichotomous key is an identification tool that uses a series of paired choices to identify an organism.

It means “two choices”. At each step you must choose between two contrasting options.

1 Read both choices. 2 Pick the choice that matches your organism. 3 Follow to the next step until you reach a name.

It is quick and low cost, and it can be used in fieldwork without lab equipment.

If the organism is damaged, very young, or looks similar to other species, you may choose the wrong path and get the wrong identification.

A caterpillar is an immature stage and may not have the adult features the key expects, so it can be misidentified.

Always read both choices carefully before deciding.

A population is a group of the same species living in the same area at the same time. Example: wolves in one national park.

A species is all organisms of that type worldwide. A population is one local group of that species in one place.

Births and immigration increase population size. Deaths and emigration decrease population size.

An abiotic factor is a non-living condition. Examples: temperature, light, water, pH, or salinity.

A biotic factor is a living influence. Examples: predation, competition, disease, or availability of food.

Low water can limit plant populations because photosynthesis and growth slow down.

An increase in predators can reduce prey population size by increasing deaths.

A limiting factor is something that restricts the size, growth, or distribution of a population.

A tolerance curve shows how well a species survives as one abiotic factor changes, such as temperature.

The optimum is the best condition where the species does best (highest survival or growth).

The zone of stress is near the limits: the species may survive but grows or reproduces poorly.

A fish may grow best at about 22°C. It may survive from about 10°C to 35°C. Outside that range it may die.

Abiotic factors are non-living conditions. Biotic factors are living interactions.

A community is all the different species living together in the same place.

A community is all the populations of different species living and interacting in the same area.

A community is all the different species living together in the same area. For example, fish, plants, insects, and bacteria living in a pond.

An ecosystem is a community of living things and the non-living environment they interact with.

An ecosystem includes living organisms and the non-living environment. For example, a forest with trees, animals, soil, sunlight, and rain.

An ecosystem is a community of organisms interacting with the abiotic environment.

All the animals and plants in a coral reef community, without including the water or sunlight.

A community includes only living things. An ecosystem includes living things plus abiotic (non-living) factors such as water, soil, light, and temperature.

No. A community includes only living organisms.

Yes. An ecosystem includes non-living factors such as water, sunlight, soil, and temperature.

Sunlight warming a lake, soil nutrients in a forest, or water temperature in the ocean.

Abiotic means non-living parts of the environment, such as sunlight, temperature, water, soil, and rocks.

Abiotic means non-living parts of the environment.

Trees in a forest, fish in a lake, grass in a field, or bacteria in soil.

Biotic means living components of an environment, such as plants, animals, fungi, and microorganisms.

Biotic means living parts of the environment.

A habitat is where an organism lives. For example, a frog living in a pond or a bird nesting in a tree.

A population is a group of individuals of the same species living in the same area at the same time.

A habitat is the place where an organism lives.

A lake ecosystem where sunlight enters, rain adds water, and fish and nutrients move in and out.

Examples include predation, competition, parasitism, mutualism, or herbivory between different species in the same area.

Because energy and matter can move in and out of the ecosystem.

Ecosystem, because temperature and rainfall are abiotic (non-living) factors.

Energy enters as sunlight, moves to plants, then to animals, and is lost as heat. For example, Sun → grass → rabbit → fox.

Energy enters ecosystems mainly as sunlight.

Dead plants and animals decompose and nutrients return to the soil, where plants reuse them.

A habitat is the place where an organism lives and finds the resources it needs to survive.

Ecosystem, because it includes living organisms plus non-living factors like soil, air, and sunlight.

A habitat is where a particular species lives. An ecosystem includes many species plus abiotic factors and their interactions.

No. Energy flows through ecosystems and is lost as heat.

Yes. Matter such as nutrients and water is recycled.

An open system is a system where both energy and matter can enter and leave across the system boundary.

Because energy (sunlight, heat) and matter (water, nutrients, organisms) move in and out of the ecosystem.

Scale is the size or level at which a system is studied, such as a pond, a forest, a biome, or the whole planet.

At small scale you see local interactions. At large scale you see wider patterns and flows across regions.

Community = only living things (different populations of different species in the same area).

Ecosystem = community + abiotic environment interacting together.

Sustainability is using resources at a rate that allows them to be replaced so the system can continue long term.

Sustainability means using resources at a rate they can be replaced, so the ecosystem can keep going in the future.

Sustainability is long-term continued functioning; resilience is ability to recover after disturbance.

Redundancy is when multiple species perform similar roles, so ecosystem functions continue if one species is lost.

A storage is a place where energy or matter is held for a period of time within a system.

Low biodiversity and small storages reduce the ability to recover after disturbance.

A disturbance is an event that disrupts ecosystem structure or function and changes populations or resource flows.

With little buffering and few backups, disturbances push the system past thresholds more easily.

Resilience is the ability of an ecosystem to resist disturbance and recover after it.

If one species declines, others can replace its role, reducing the chance of function collapse.

Large storages buffer change by releasing resources slowly, reducing extremes after disturbance.

Sustainable fishing means catching only as many fish as can be replaced by reproduction each year.

Natural: wildfire, storm, flood, drought. Human: deforestation, pollution, overfishing, oil spill.

High biodiversity, large storages, and redundancy (multiple species doing similar roles).

Disturbance causes change; resilience determines recovery; recovery shows how fast the system returns towards its previous state.

Cutting down forest faster than it can regrow is unsustainable because the resource gets depleted.

Bees, flies, butterflies and beetles can all pollinate; if one declines, others may still pollinate many plants.

Changes are more extreme because there is little buffering; recovery is slower and collapse risk is higher.

More biodiversity creates more pathways and backup species, so ecosystem functions continue even if one species declines.

Coral reefs under repeated heat stress can shift to algal-dominated states and recover slowly or not at all.

Forests and soils store carbon in biomass and organic matter, reducing rapid carbon release to the atmosphere.

Resilience is how well an ecosystem can recover after a disturbance and keep functioning.

Habitat destruction, pollution, overexploitation, and invasive species can reduce resilience by simplifying the ecosystem.

Redundancy.

Wetlands and lakes store water, reducing floods and providing water during dry periods.

After a disturbance, a resilient ecosystem recovers faster and is more likely to maintain key functions and services.

Resource use does not exceed renewal, so ecosystem functions and services continue over time.

High biodiversity often increases redundancy because more species means more chances that roles overlap.

Maintaining storages prevents rapid depletion, keeping ecosystem services available for the long term.

Sustainability is long-term continued functioning; resilience is short-term ability to recover after disturbance.

A tipping point is a threshold where small extra change causes a large shift to a new state that may be hard to reverse.

No. Redundancy protects function, but losing species still reduces biodiversity and can weaken the system over time.

Increase biodiversity, protect or restore storages (forests, wetlands, soils), and reduce chronic human pressures.

Resilience.

After a fire, plants regrow and animals return over time. The ecosystem returns to a working state.

A disturbance is an event that disrupts an ecosystem. Natural: hurricane or fire. Human: oil spill or deforestation.

More species means more “backup” organisms. If one species declines, others can still keep ecosystem jobs going.

If bees decline, other pollinators like butterflies, flies, and beetles can still pollinate many plants.

A storage is a place where a resource is kept in an ecosystem, like water in a wetland or carbon in a forest.

Wetlands store extra water during heavy rain, so less water rushes downstream at once.

Forests store carbon in tree biomass and in soils, which slows how fast carbon enters the atmosphere.

Redundancy means several species do the same job, so the system still works if one species is lost.

Dead leaves can be broken down by fungi, bacteria, earthworms, and beetles. If one is missing, others still decompose.

Low biodiversity and small storages reduce resilience. Heavy human pressure (pollution, habitat loss) also lowers resilience.

A tipping point is a point where a small extra change causes a big shift, and the ecosystem may not return to the old state.

Biodiversity gives more species. Redundancy gives backup species doing the same job. Storages provide reserves (water/carbon/nutrients). Together they help the ecosystem recover after disturbance.

A trophic cascade is a chain reaction of population changes through a food web after a species is added or removed.

Keystone species.

An ecosystem engineer is a species that modifies the physical environment and creates or maintains habitats for other species.

A keystone species is a species with a disproportionately large effect on ecosystem structure or function relative to its abundance.

Trophic cascade.

They help maintain food-web balance by controlling populations or supporting key interactions, which keeps biodiversity higher.

Herbivore numbers can increase, plant biomass can decrease, and biodiversity may fall as habitats become simplified.

Because habitat changes can affect many other populations, increasing biodiversity and altering community structure.

Food-web links weaken and key functions fail, so the ecosystem is less able to recover after disturbance.

If removing one species causes major changes across many other species (food web shifts, biodiversity drops), it is likely a keystone species.

Beavers build dams that create wetlands, increasing habitat for fish, birds, insects and plants.

They control dominant populations and maintain habitat/food-web structure, allowing more species to coexist.

State the keystone has a large effect, then describe knock-on impacts on other populations and biodiversity/food-web stability.

Step 1: remove keystone → immediate population change. Step 2: knock-on effects spread → community structure and biodiversity change.

They can change water flow, soil moisture, light levels or sedimentation, which reshapes the habitat.

A top predator can prevent one prey species from becoming too abundant, protecting plant communities and keeping habitats diverse.

Look for “creates habitat”, “builds”, “digs”, “modifies environment”, or “changes water flow/soil structure”.

Keystone species maintain stability, supporting faster recovery after disturbance.

Protecting a keystone species can protect many other species and maintain ecosystem services by keeping the system stable.

Keystone species increase resilience by keeping key ecosystem functions and food-web relationships stable after disturbance.

A niche is the role of a species in an ecosystem, including how it uses resources and interacts with other species.

True. It includes role, resource use and interactions, not just location.

Resources are things organisms need to survive, such as food, water, light, space or shelter.

Niche overlap is when two species use the same resources in the same way and place/time.

Habitat is where a species lives; niche is how it lives (its role and resource use).

True. Habitat is the place or physical environment where a species lives.

Competition (often caused by niche overlap).

If resources are limited, both species demand the same resource, reducing growth, survival or reproduction for at least one.

Food type and feeding method; activity time; abiotic tolerances; interactions (predator, competitor, pollinator).

State feeding role, key interactions, and the abiotic conditions needed for survival.

One species may be outcompeted and decline locally, reducing biodiversity.

By resource partitioning: using different food types, locations, or activity times (different niches).

More niches usually allow more species to coexist, increasing biodiversity.

More available niches allow species to specialise and coexist with less direct competition.

Their niches overlap, so competition is likely unless resources are abundant or they separate by time/place.

Carrying capacity is the maximum population size an environment can support sustainably over time.

Predation is an interaction where a predator hunts, kills and eats a prey organism.

The maximum population size the environment can support sustainably over time.

Because counting every individual is usually impossible; sampling estimates population size from a representative subset.

Births, deaths, immigration and emigration.

Competition is the demand by two or more organisms for the same limited resource.

The prey population peaks first; the predator peak usually lags behind.

A limiting factor is an environmental factor that restricts population growth, size or distribution.

Quadrats are used to sample non-mobile organisms (mainly plants) to estimate density, frequency or percentage cover.

Prey peaks first; predator peaks later due to time lag.

Mutualism benefits both species; parasitism benefits the parasite while harming the host.

Negative feedback is a process that reduces change and returns a population towards balance (for example predators increase when prey increase).

Liebig’s Law of the minimum.

A transect is used to show how species or abundance change across an environmental gradient (for example shore to land).

Population growth is limited by the factor in shortest supply, even if other resources are abundant.

Pathogens spread faster when population density is high because individuals contact each other more often.

Density-dependent: competition, disease, predation. Density-independent: drought, flood, fire, storm.

Non-mobile organisms, mainly plants (and very slow animals).

A time lag is a delay between a change in one population and the response of another population.

N = (n1 × n2) / m, where n1 is marked first, n2 is caught second, and m is recaptured marked.

Describe the pattern (rise, fall, oscillation, time lag) before explaining the cause.

Identify the lowest bar and state it is the limiting factor because it caps population size.

The population is closed (no immigration/emigration) and marks are not lost and do not affect survival or capture.

N = (n1 × n2) / m.

Name the interaction and state who benefits and who is harmed (or how resources are affected).