The distribution of life

##Abiotic factors affect where life exists

**Abiotic factors** are non-living parts of the environment. Collectively, abiotic factors affect where life can exist and the types of organisms found in different parts of the biosphere.

##Abiotic factors vary in relation to latitude

Abiotic factors such as temperature and precipitation vary in relation to latitude. **Latitude** describes how far north or south an area is from the equator. Latitude is measured in degrees, which ranges from $0\degree$ at the equator to $90\degree\text{N}$ (north) at the North Pole and $90\degree\text{S}$ (south) at the South Pole, as shown in the diagram below.

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Temperature tends to *decrease* with latitude. At low latitudes, the sun’s rays strike the Earth’s surface most directly. So, equatorial regions tend to be warmer. At high latitudes, the sun’s rays hit the Earth’s surface at an angle and are more spread out. So, polar regions tend to be cooler.

Precipitation also tends to *decrease* with latitude. Temperature affects the amount of water vapor that air can hold; warm air holds more water vapor than cold air. So, the warmer air in equatorial regions can hold more water, which then condenses into rain as it rises up into the atmosphere and cools. This causes equatorial regions to receive more precipitation than polar regions.

##Latitude can explain global trends in the distribution and number of species

Latitude can explain global trends in the distribution and number of species. The number of different species in an area is known as **species richness**.

Species richness tends to be *highest* at low latitudes near the equator. Here, the climate is relatively warm and humid year-round, making it a favorable environment for many species.

Species richness tends to be *lowest* at high latitudes toward the poles. With increasing latitude, the climate becomes cooler and varies more with the seasons, making it more difficult for species to survive.

Species richness usually declines in relation to latitude in both **terrestrial** (land-based) and **aquatic** (water-based) systems. However, in oceans, water currents play a major role in the distribution and number of marine species. So, trends of marine species richness can vary for some types of organisms.

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##Abiotic factors influence the distribution of life

Different abiotic factors influence the distribution of life in terrestrial versus aquatic environments.

##Elevation, temperature, and precipitation in terrestrial environments

In terrestrial environments, elevation, temperature, and precipitation are important abiotic factors. The interaction of these factors affects where terrestrial organisms are found.

Both temperature and precipitation change with elevation. As elevation *increases*, temperatures become *cooler* and precipitation *increases*.

In general, species richness is *higher* in areas with warmer temperatures and more precipitation. However, in higher elevation areas, species richness is often *lower* due to harsher climatic conditions.

However, species richness becomes more complex in mountain ranges because of variation in precipitation. As air moves up and over a mountain range, the air cools and its water vapor condenses into rain or snow. So, one side of a mountain range tends to have more precipitation and the other side tends to be drier. This variation in habitats across a mountain range leads to different patterns of species richness.

##Water depth, temperature, and sunlight in aquatic environments

In aquatic environments, water depth, temperature, sunlight, and dissolved oxygen are important abiotic factors. The interaction of these factors affects where aquatic organisms are found.

Both temperature and sunlight change with water depth. As water depth *increases*, temperatures become *cooler* and sunlight *decreases*.

In general, species richness is *higher* in warmer, sunlit areas near the water’s surface.

However, species richness becomes more complex in oceans because of ocean currents. Ocean currents can change abiotic factors, such as temperature, by moving and mixing water. These currents create varied environmental conditions and result in different patterns of species richness.

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##Important abiotic factors in marine and freshwater ecosystems

Aquatic environments can be further divided into freshwater and marine ecosystems based on water salinity (saltiness of water).

**Freshwater ecosystems** contain freshwater (which has a low salt concentration, usually $<1\%$) and include lakes, rivers, and wetlands. Water velocity (how fast water is moving), clarity, and pH (a measure of acidity) are additional important abiotic factors in freshwater ecosystems.

**Marine ecosystems** contain saltwater and include oceans, coral reefs, and mangroves. Ocean currents, dissolved nutrients, and proximity to land are additional important abiotic factors in marine ecosystems.