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Shallow tropical coral reefs have declined by 50% since 1950, partly because they are sensitive to water conditions.

Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician, displacing the microbial and sponge reefs of the Cambrian.

== Formation == Most coral reefs were formed after the Last Glacial Period when melting ice caused sea level to rise and flood continental shelves.

Sometimes called rainforests of the sea, shallow coral reefs form some of Earth's most diverse ecosystems.

Most coral reefs are built from stony corals, whose polyps cluster in groups.

Coral reefs flourish in ocean waters that provide few nutrients.

== History == The concept of the keystone species was introduced in 1969 by zoologist Robert T.

In his 1966 paper, Food Web Complexity and Species Diversity, Paine had described such a system in Makah Bay in Washington.

Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.

The role that a keystone species plays in its ecosystem is analogous to the role of a keystone in an arch.

Some keystone species, such as the wolf and lion, are also apex predators.

While the keystone is under the least pressure of any of the stones in an arch, the arch still collapses without it.

Animals like bees, beetles, and butterflies are prime pollinators.

Wind pollination occurs when wind carries pollen from one plant to another.

Water pollination happens when water transports pollen from one plant to another.

Self-pollination occurs when a flower produces pollen and receives it from the same flower.

Pollination between species can lead to hybrid offspring in nature and plant breeding.

Pollination is crucial for the survival of many plant species.

Tardigrades live in diverse regions of Earth's biosphere: mountaintops, the deep sea, tropical rainforests, and the Antarctic.

The earliest known fossil is from the Cambrian, some 500 million years ago.

Tardigrades are usually about 0.5 mm (0.02 in) long when fully grown.

== Description == === Body structure === Tardigrades have a short plump body with four pairs of hollow unjointed legs.

There are no lungs, gills, or blood vessels, so tardigrades rely on diffusion through the cuticle and body cavity for gas exchange.

Tardigrades have been suggested to be eutelic, with a fixed number of body cells at maturity, but there is evidence that they are not.

In 2023, the World Bank published a report proposing a non-deforestation based economic program in the region.

Orellana derived the name Amazonas from the Amazons of Greek mythology, described by Herodotus and Diodorus.

Archeologist Betty Meggers was a prominent proponent of this idea, as described in her book Amazonia: Man and Culture in a Counterfeit Paradise.

The Upano Valley sites in present-day eastern Ecuador predate all known complex Amazonian societies.

Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the centre point of the eight limbs.

An octopus can radically deform its shape, enabling it to squeeze through small gaps.

Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse invertebrates.

All octopuses are venomous, but only the blue-ringed octopuses are known to be deadly to humans.

Octopuses appear in mythology as sea monsters such as the kraken of Norway and the Akkorokamui of the Ainu, and possibly the Gorgons of ancient Greece.

A battle with an octopus appears in Victor Hugo's book Toilers of the Sea.

In plants, algae, and cyanobacteria, sugars are synthesized by a subsequent sequence of light-independent reactions called the Calvin cycle.

Some organisms also perform anoxygenic photosynthesis, which does not produce oxygen.

In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas.

Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the centre point of the eight limbs.

An octopus can radically deform its shape, enabling it to squeeze through small gaps.

Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse invertebrates.

All octopuses are venomous, but only the blue-ringed octopuses are known to be deadly to humans.

Octopuses appear in mythology as sea monsters such as the kraken of Norway and the Akkorokamui of the Ainu, and possibly the Gorgons of ancient Greece.

A battle with an octopus appears in Victor Hugo's book Toilers of the Sea.