Tropical dry forests are warm year-round, but they have strong seasonal variation in precipitation levels, leading to predictable wet and dry seasons. The forest canopy is less dense than the canopy found in a tropical rainforest. They are also known as tropical deciduous forests; many plants are deciduous and will lose their leaves during the dry season. The organisms that live here are adapted to the strong fluctuations in precipitation that comes with this biome.

The rain during the wet season can be extremely heavy. This, coupled with the fact that the soil is erosion-prone, can lead to significant soil loss in regions where the forests have been cleared, as the root systems of plants help to hold the soil in and prevent erosion.

Tropical rainforests are characterized by constant, warm temperatures and high levels of year-round rain, resulting in very high productivity and far greater biodiversity than other biomes. The communities are rich, with large numbers of tree and other plant species. The interiors of the forest are dark and damp with lush vegetation.

Despite the extremely large number of plant species that can be found scattered throughout the biome, it actually has fairly poor soil that is acidic and low in nutrients. Most nutrients in the biome are found within the plants, not the soil, which leads to very unfortunate consequences when the forests are cleared by humans for other purposes.

As a temperate biome, the temperate rainforest has seasonal variations in its temperature. It has high levels of precipitation and in forest interior is shaded and damp due to the tall-growing coniferous trees that characterize the biome, which supports the growth of a great deal of moss. The temperate rainforest has fertile soil, helping to support the growth of these very tall trees.

Many of the old-growth temperate rainforests have been cleared and harvested, putting the species present within at risk and making the biome more susceptible to erosion and landslides.

A deciduous forest is sometimes also referred to as a seasonal forest. They are sometimes called this because of the strong differences they undergo throughout different seasons. This biome is characterized primarily by fairly stable precipitation levels, temperatures that vary with seasons, and broad-leafed deciduous trees. A deciduous tree is one that loses its leaves in the fall and remains dormant in the winter, a mechanism that has evolved to protect the leaves from damage due to freezing.

This biome has fairly fertile soil, and therefore a high productivity level, but it has far fewer tree species than a tropical rainforest does.

The taiga has moderate precipitation, long and cold winters, and short and cool summers. The forest is dominated by a few species of evergreen trees.

The soils are acidic and nutrient-poor. The summers are very productive, so many organisms do most feeding and breeding during them rather than the long winters - either through the use of hibernation or migration.

The taiga has a broad, continuous distribution. However this is being threatened and lost due to logging, fossil fuel extraction, and climate change.

Tropical regions with warm temperatures but that are even drier than the tropical dry forest will lead to savannahs. A savannah is a biome with significant seasonal variation in rainfall. This biome's precipitation pattern supports the growth of grasses over trees. It is a tropical grassland that does have trees, such as acacias, but they are interspersed throughout the biome and are not the dominant plant species.

Chaparrals are highly seasonal - they have mild, wet winters and warm, dry summers. They are influenced by marine weather and are characterized by dense evergreen shrubs (which is why they are also called shrublands). 

This biome is particularly interesting due to its dependence on fire. Fires are a natural occurrence in this biome and many plants are adapted to resist fire, or even to require it. Some species have seeds that are activated by fire and will not germinate without it.

The temperate grassland, also known as a steppe or prairie, is characterized by limited precipitation (which supports the growth of grasses rather than trees) and seasonal temperature differences that are more extreme than one would find in a temperate deciduous forest.

Deserts are the driest biome on Earth; they occur where precipitation is very rare. The low levels of vegetation and humidity result in stark temperature differences between day and night - the days can be very hot while the nights much cooler or even very cold. The organisms that live in deserts are adapted to survive in such a harsh climate - plants are adapted to have thicker cuticles on leaves to prevent water loss, many animals are active at night when the temperatures are cooler, some animals (such as the Kangaroo rat) have evolved mechanisms to reserve water, such as a longer loop of Henle in the kidney.

The key feature of the desert is its lack of precipitation, the temperature can vary - deserts are not always hot. They can actually be categorized into three types of desert - tropical, temperate, and cold.

A tropical desert is hot and dry most of the year. There are very few plants and there is mainly rocks and sand.

A temperate desert has high day-time temperatures in the summer, but low in the winter. Despite still having very low precipitation, it does have more than a tropical desert, which results in sparse shrubs, cacti, and succulents that are adapted to these dry conditions.

A cold desert has cold winters, but warm summers. Precipitation is low and vegetation is sparse.

Tundras are a cold and dry biome, being almost as dry as a desert. Their winters are very cold with little daylight, while their summers have very long spans of daylight. There are no trees in the tundra, but there is vegetation - lichen and other low-to-the-ground plants, such as shrubs, grasses, mosses, and sedges. Few animals are adapted to survive year-round in the tundra.

The cold leads to the presence of permafrost - the underground soil is permanently frozen in this biome. 

Climate change is resulting in the melting of permafrost and sea ice, both of which are having negative impacts on the tundra biome.

Ponds and lakes are bodies of standing water. They can vary greatly in size, but have common zones that have been characterized.

The littoral zone is found near the nutrient-rich edge of the body of water. It has shallow water where plants will grow and extend above the water's surface.

The limnetic zone is in the open water, past the shore, where sunlight is able to penetrate. This area supports phytoplankton, which depend on the available sunlight for photosynthesis.

Beneath the limnetic zone is the profundal zone. This is the open water that sunlight does not reach. Photosynthesis will not be occurring here and therefore there will be less dissolved oxygen.

The bottom of the pond/lake is the benthic zone. This has many invertebrates living in the mud that feed on waste and remains.

Ponds and lakes can be put into one of two categories. An oligotrophic lake/pond is low in nutrients and high in oxygen. A eutrophic lake/pond is high in nutrients and low in oxygen. An oligotrophic body of water can naturally transition to a eutrophic one as streams or rivers bring them nutrients and sediments.

Rivers, streams, and creeks are all names for the same general thing - they are flowing water on Earth's surface. When precipitation falls to the ground, it is not always absorbed - surface water may stay in one area, but some will flow from high elevation to low elevation thanks to gravity.

Water drains into a stream/river from something known as a watershed (also known as a drainage basin). Many of the nutrients in the streams come from the neighboring land, so the ecology of these watersheds is very important to the ecology of the stream. As the water flows downhill, it goes through three distinct aquatic life zones (Figure 3).

The source zone, the region closest to the watershed and the beginning of the stream, typically contains multiple streams that are narrow, shallow, cold, and flow quickly. Due to the quickly flowing water, it has a large amount of dissolved oxygen; faster-flowing streams have higher oxygen levels, while slower-flowing parts have less. The majority of source zones are low in productivity - they lack high nutrient levels and primary producers.

Next, the middle region of the stream, is the transition zone. The multiple streams from the source zone (called headwater streams) merge together. The resulting streams are wider, deeper, and warmer. The warmer water supports more plant life, leading to this region being more productive than the source zone. The slopes are gentler, leading to slower-flowing water, with less dissolved oxygen in it, and usually have a greater turbidity.

This leads to the third region, the floodplain zone. Here, streams join wider and deeper rivers, which will empty into the ocean. This water usually has even higher temperatures and slower flowing water, leading to less dissolved oxygen. The water in this area causes more erosion than that in the other two zones and it is made of water coming from runoff from a larger area. These two factors lead to high silt (sediment suspended in water) concentration and muddier water.  

Rivers are classified by how they flow across the landscape, with the most common types being braided, meandering, and oxbow.

Water will leave the river and enter a standing body of water, such as the sea, at the river mouth. River deltas are landforms that form due to the deposition of sediments from the river.

A wetland is an area where the soil is saturated with water. Generally, there will be shallow standing water with abundant plants. This is typically a very productive and diverse region, and there are multiple types of wetlands.

Freshwater marshes are frequently covered in shallow water and have cattails as the dominant plant. They are found near the edges of rivers and lakes.

Bogs are typically formed as a pond or lake undergoes aquatic succession. They accumulate dead and decaying plant material (peat), typically from mosses. This forms thick, floating masses of vegetation in the water. Most of the water in a bog comes from the collection of rainwater.

Swamps are forested wetlands. They have shallow water and are dominated by water-tolerant trees. Its water comes from rainwater collection as well as seasonal flooding.

Mangrove forests are highly productive wetlands found in coastal intertidal zones at tropical and subtropical latitudes. The mangrove trees protect coasts from erosion, filter water, and provide a habitat for a wide variety of species. 

Worldwide, most mangroves are being destroyed for shrimp aquaculture, with many others (particularly in Myanmar) being destroyed and converted to rice paddies. In addition, mangrove trees have aerial roots which are particularly susceptible to pollution.

Humans will sometimes drain and fill wetlands in order to use them for agriculture or for building cities. Wetlands take in a lot of water and act as a sort of flood control for nearby ecosystems. This effect is lost where they are drained and filled in, causing flooding to become far more common.

In addition, water is commonly diverted for flood control, agriculture, or for drinking water (along with water flow being disrupted by dam construction). This can reduce water flow, which can result in the drying up of wetlands, such as the Everglades in Florida.

Overfishing of wetlands is a common problem and disrupts the food webs of wetlands.

Coral reefs are one of the most diverse ecosystems in the world, and the most diverse marine biome, earning them the name "the rainforest of the sea."

When most people think of a coral reef, they imagine shallow-water reefs. These are found in warm, shallow tropical waters where the coral has a mutualistic relationship with zooxanthellae (a photosynthetic algae). The corals provide an environment to live as well as carbon dioxide for the algae, which photosynthesizes and supplies a food source, sugar, and oxygen to the coral. 

The corals form a colony and provide a habitat for a wide variety of species. It is estimated that roughly 25% of the ocean's fish depend on coral reefs for shelter, a place to rear their young, finding food, or as a place to reproduce.

There are also deep-water reefs that are located in the deep, dark, and cold waters of the ocean. Less is known about these, as we have fully examined relatively very little of the seafloor, however, we do know that they support a wide variety of life, much like their shallow-water cousins. Unlike shallow-water reefs, deep-water reefs live in dark waters where not much photosynthesis is happening. As such, they feed on microscopic organisms (such as plankton) and other organic matter from the flow of ocean currents.

Coral reefs are being severely threatened by climate change. Rising temperatures (as well as other human-caused things such as increased pollution) lead to stressed corals. When corals are stressed, the algae is more likely to leave the corals' tissues, which causes the coral to become bleached. Bleached coral is more susceptible to disease and loses its major food source - the algae. In addition, ocean acidification, caused by excess carbon dioxide in the atmosphere, leads to a decrease in carbonate ions in the ocean. Corals need these to form their skeletons. This leads to more brittle corals that are less resilient. 

Kelp forests are areas of ocean, typically found along the coast, that are characterized by the highly dense groupings of kelp, a type of fast-growing brown algae seaweed. They typically are found in shallow, cold, nutrient-rich waters. They form a canopy much like trees do in forests and provide food and shelter for thousands of other species. Multiple kelp species can grow and live together in the forest, with some extending past the water's surface and others staying near the seafloor.

These ecosystems contain incredible biodiversity and are considered one of the most dynamic and productive on Earth. 

Overfishing near shore ecosystems has had drastic effects on kelp forests. Many more vulnerable regions have had their kelp forests nearly disappear due to human activities.

An estuary is where seawater and freshwater mix together. They are found where rivers and the sea meet. There are high levels of nutrients that were brought by the rivers, which are circulated due to the flow of the tides.

Estuaries have associated coastal wetlands - coastal areas of land that have their soil saturated with water and that are covered with water during all or large portions of the year.  In temperate zones, coastal wetlands include river mouths, inlets, bays, sounds, and salt marshes. In tropical zones, there are mangrove forests. These areas are highly productive for the same reason as estuaries.

You will find an intertidal zone where the ocean meets the land between high and low tides. It is underwater during high tide, and above the water-level during low tide. There can be several distinct habitats within one intertidal zone due to the nature of the area. They are regions of high biodiversity. Due to the rise and fall of the tide, there are three distinct zones within an intertidal zone.

The high intertidal zone is the furthest inland that is covered by water the least - only during high tide. 

The middle intertidal zone is submerged for roughly half of the day, depending on the turn of low/high tide.

The low intertidal zone is submerged for the majority of the day, only being exposed during low tide.

There is something known as the spray zone that is not technically a part of the intertidal zone, but is closely related to it. It is the part of the beach that gets splashed by waves, but is never submerged by water.

Tide pools can form within the intertidal zone. They are shallow pools of seawater that remain during low tide while the surrounding area is now exposed.

Intertidal zones exist can be found on both rocky and sandy beaches.

The ocean is the largest of all ecosystems on Earth. Like how freshwater bodies of water, lakes and ponds could be split into different regions, so can oceans (Figure 4). 

The euphotic zone is the upper-most layer of the open ocean (the pelagic zone) that is brightly lit from sunlight. It has photosynthetic phytoplankton within it that carry out roughly 40% of the world's photosynthesis. Due to this, there are very high levels of dissolved oxygen in the euphotic zone. Nutrient levels are low, although there are upwells where ocean water from lower in the ocean that is cooler and nutrient-rich will be brought up towards the ocean surface.

The second major zone is the bathyal zone. This area receives some, although not much, light and is dimly lit. As such, photosynthesis does not occur in this region, although it is not without life. Species will live in this zone and migrate toward the surface and feed in euphotic zone at night.

The abyssal zone is the deepest ocean zone and is quite dark and cold. Photosynthesis does not occur here, as there is no sunlight, and there is little dissolved oxygen. There is a lot of life despite this, however, due to chemosynthesis and marine snow (the shower of dead and decaying organisms that drifts down from the above ocean zones). Some species in this zone are deposit feeders, which eat and extract nutrients from mud.

Aside from upwelling areas, productivity is relatively low in the open ocean. Because there is just so much of it, though, it produces the most overall.

Ocean ecosystems are at serious risk due to a variety of problems such as overfishing, pollution, and climate change.