• A group of individuals of the same species in one region comprise a population.

Populations of several species living and interacting in one area comprise a community of living organisms.

The biological community and the abiotic (nonliving) environmental components make up an ecosystem, such as a forest.

A collection of ecosystems characterized by similar climate and other environmental factors define a biome.

All the biomes on Earth belong to a single biosphere.

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A species is a group of similar organisms that can reproduce similar offspring, e.g., Sulphur-crested Cockatoo.	A habitat is the physical environment where individuals of a particular species lives, e.g., a woodland.	A niche is the role a particular species plays in its habitat, including its food source and reproduction needs.

• Terrestrial biomes are determined by temperature and precipitation patterns.

On the wet end, tropical rain forest requires high precipitation (more than a year) and temperatures and is dominated by trees.

Moderate rain characterizes mediterranean scrub and savanna; high temperatures during dry spells allow grasses to compete with trees.

Moderate moisture and temperature yields temperate forest and temperate grassland.

As moisture and temperature drop further, boreal forest covers the landscape, dominated by conifers.

Very dry and very cold conditions support tundra with low-growing vegetation and no trees.

• Energy flow and nutrient cycles

Energy can neither be created nor destroyed, but can be changed from one form to another.

Energy enters the biosphere as light (solar radiation) and leaves as heat (terrestrial radiation).

Thus energy flows in one direction through all ecosystems; living organisms exploit light energy by changing it to other forms.

Matter such as carbon and nitrogen are nutrients; the earth has a finite amount of these, and they cycle through organisms and abiotic components of ecosystems.

• Biochemical cycles

Energy does not cycle in an ecosystem, but molecules are reused in biochemical cycles.

1. Carbon cycle

2. Nitrogen cycle

3. Phosphorus cycle

• The carbon cycle consists of photosynthesis and cellular respiration.

During photosynthesis, producers such as plants use energy from light to convert CO₂ from the atmosphere into energy-rich sugar.

During cellular respiration, both producers and consumers (such as animals) use sugar for their energy needs, releasing CO₂ back to the atmosphere.

• Carbon cycle

Photosynthesis by producers removes CO₂ from the atmosphere and yields carbon compounds (sugar) in ecosystems.

Carbon returns to the atmosphere as CO₂ from cellular respiration, as various organisms, including producers, consumers, and decomposers, use the sugar for their energy needs.

Human activity can upset this cycle by burning fossil fuels (releasing CO₂) and cutting down trees (reducing photosynthesis).

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• Nitrogen cycle

Nitrogen is needed to make proteins and DNA.

Nitrogen fixation by bacteria converts atmospheric nitrogen (N₂) to the organic form of ammonia (NH₃), which then cycles through producers and other organisms in the ecosystem.

Other bacteria complete the cycle through denitrification, restoring the atmospheric nitrogen.

Humans often accelerate the production of organic nitrogen by using fertilizers, some of which escape to aquatic ecosystems, upsetting their nitrogen balance.

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• Phosphorus cycle

Phosphorus is a component of DNA.

The major source is calcium phosphate in rocks; slow weathering releases them into ecosystems, so phosphorus is often a limiting nutrient and cycles slowly.

Excessive levels of phosphorus in aquatic ecosystems can lead to unbalanced growth.

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