9.1.1 Levels of organisation in ecosystems

Organisation: Individuals Build Ecosystems

1. An individual organism is one living organism, such as one oak tree, one rabbit or one bacterium.
2. A population is all the organisms of one species living in the same area at the same time.
3. Population size changes when organisms are born, die, enter the area or leave it.

Communities Contain Many Populations

1. A community is all the populations of different species living and interacting in the same area.
2. A woodland community may include populations of oak trees, bluebells, earthworms, fungi, insects and birds.
3. One species is a population, while every species considered together is a community.

Ecosystems Include the Environment

1. An ecosystem is a community of organisms and the non-living parts of their environment interacting together.
2. Biotic components are the living organisms, while abiotic components include light, temperature, water and soil conditions.
3. The levels increase in scale: individual organism, population, community, then ecosystem.
4. Interactions connect the levels: a change in one population can affect other populations and can alter the whole ecosystem.

9.1.2 Abiotic and biotic factors

Abiotic Factors Change Communities

1. An abiotic factor is a non-living condition that affects where organisms can live and how large their populations can become.
2. Temperature affects enzyme-controlled reactions, so temperatures outside an organism's tolerated range reduce growth, reproduction or survival.
3. Light intensity affects the rate of photosynthesis, so low light can reduce glucose production, plant growth and the food available to consumers.
4. Water availability affects hydration, transport and photosynthesis, so drought can reduce plant growth and the populations that depend on those plants.
5. A pollutant is a substance released into the environment that causes harm, and pollutants may kill organisms, reduce reproduction or change the conditions in a habitat.
6. Abiotic factors affect distribution: organisms are most common where the conditions match their adaptations and tolerances.

Biotic Factors Change Populations

1. A biotic factor is a living influence that affects organisms in a community.
2. Competition occurs when organisms require the same limited resource, so some obtain less and are less likely to survive or reproduce.
3. Plants compete for light, water, space and mineral ions, while animals compete for food, water, territory and mates.
4. Predation reduces the prey population because predators kill and eat prey.
5. Predator and prey populations are linked: more prey can support more predators, then increased predation lowers prey numbers, followed by a fall in predator numbers as food becomes scarce.
6. Disease can spread more readily when organisms live close together, which can increase deaths and reduce population size.

Several Factors May Act Together

1. A limiting factor is a factor in shortest supply that restricts population growth or distribution.
2. The limiting factor can change: after rainfall increases water availability, light, minerals or competition may become the factor that restricts growth.
3. Correlation does not prove cause: two factors may change together, so an investigation should measure other relevant variables before identifying one as the cause.

9.1.3 Importance of interdependence

Interdependence Links Species

1. Interdependence is the way organisms in a community depend on other species for resources or services needed for survival and reproduction.
2. Food relationships create dependence because consumers need their food organisms, while predators depend on enough prey being available.
3. Shelter relationships create dependence when one species uses another as a place to live, nest or hide from predators.
4. Pollination depends on animals transferring pollen between flowers, which allows many flowering plants to reproduce.
5. Seed dispersal depends on animals carrying seeds away from the parent plant, which can reduce competition between the parent and seedlings.

One Change Can Affect Many Species

1. A fall in one population affects every species that feeds on it, shelters in it, pollinates it or disperses its seeds.
2. Food-web effects spread: fewer flowering plants can mean less nectar for insects, fewer insects can mean less food for insect-eating birds, and bird populations may then fall.
3. An increase can also spread: more prey may support more predators, which can later increase the rate at which the prey is eaten.
4. The effect depends on alternatives: a consumer with several food sources may be less affected than a specialist that relies on one species.

Interdependence Affects Community Stability

1. A stable community has population sizes that remain within a limited range because interactions and environmental conditions are balanced over time.
2. Stability does not mean no change: births, deaths, predation and seasonal changes continue, but populations do not rise or fall without limit.
3. Removing one species can disturb this balance and cause increases or decreases in several other populations.

9.1.4 Parasitism and mutualism

Parasitism Harms the Host

1. Parasitism is a relationship in which a parasite gains resources from a host and the host is harmed.
2. A parasite lives on or inside another organism and takes nutrients or other resources from it.
3. A host is the organism from which the parasite obtains resources.
4. The parasite benefits because it gains food, water, shelter or a place to reproduce.
5. The host is harmed because resources are removed, tissues may be damaged, or disease may be transmitted.

Mutualism Benefits Both Species

1. Mutualism is a relationship between two species in which both organisms benefit.
2. Each benefit must be specific: one species may gain food, while the other gains pollination, protection, cleaning or seed dispersal.
3. Both species may survive or reproduce more successfully because of the resources or services exchanged.

Compare the Two Relationships

1. Parasitism has one beneficiary and one harmed organism.
2. Mutualism has two beneficiaries.
3. The identity of each organism matters: state which species gains, what it gains, and whether the other species gains or is harmed.

Survival Depends on Other Species

1. A parasite depends on a suitable host for the resources and conditions needed to survive or reproduce.
2. Mutualists may depend on each other when the exchanged food or service is difficult to obtain from another species.

9.1.5 Determining number of organisms in an area

Sampling Estimates Abundance

1. Abundance is the number of organisms of a species in an area, or how common that species is.
2. Sampling measures part of a habitat because counting every organism is usually too slow, impractical or damaging.
3. A representative sample has the same general pattern as the whole area, so the estimate is less affected by selection bias.
4. Raw data are the original counts or percentage-cover readings recorded during fieldwork before a mean or estimate is calculated.

Quadrats Estimate Population Size

1. A quadrat is a square frame of known area used to sample plants and slow-moving organisms.
2. Mark out the area with two tape measures at right angles, then use a random number generator to choose coordinate pairs.
3. Place the quadrat at each coordinate and count one named species, or estimate its percentage cover.
4. Use a boundary rule, such as counting organisms touching the top and left edges but not the bottom and right edges, and apply it every time.
5. Repeat at many coordinates and calculate the mean so one unusually dense or sparse quadrat has less effect on the estimate.
6. Estimate population size with $estimated\; population=\frac{total\; area}{area\; of\; one\; quadrat}\times mean\; count\; per\; quadrat$.

Raw Data Must Be Processed Carefully

1. Calculate a mean with $mean=\frac{sum\; of\; all\; counts}{number\; of\; quadrats}$.
2. Population density can be calculated with $population\; density=\frac{number\; of\; organisms}{area\; sampled}$.
3. Percentage cover is estimated with $percentage\; cover=\frac{occupied\; grid\; squares}{total\; grid\; squares}\times 100$.
4. Keep units consistent because the total area and quadrat area must use the same area unit before scaling.

Transects Show Change Across a Gradient

1. A belt transect uses quadrats placed at regular intervals along a line to measure abundance across an environmental gradient.
2. A line transect records organisms touching the line, while a belt transect samples a strip and gives abundance as well as presence.
3. Systematic sampling uses fixed intervals because the purpose is to reveal a change with distance, rather than estimate one area's mean abundance randomly.
4. Measure an abiotic factor at each position with suitable equipment, such as temperature with a thermometer or light intensity with a light meter.
5. A relationship in the data supports an association, but it does not prove that the measured factor is the only cause because other conditions may also change along the transect.

Reliability Depends on Sampling

1. Increase reliability by using many quadrats, repeating transects and calculating means.
2. Reduce bias by using random coordinates when estimating abundance in one area and fixed intervals when testing a gradient.
3. Improve validity by measuring the intended factor accurately and keeping other conditions as consistent as fieldwork allows.
4. Report estimates honestly because clumped organisms, identification errors, seasonal change and movement can make a sample differ from the whole habitat.