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Biology library

Course: Biology library > Unit 28

Lesson 8: Biogeography
Science
Biology library
Ecology
Biogeography
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Climate

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Global, regional, and local factors that influence climate. How climate affects where species are found.

Key points:

  • Each species has a unique range, the set of locations where members of that species are found on Earth.
  • A species' range depends on the biotic (living) and abiotic (non-living) conditions it needs for survival and on geography.
  • The ranges of species and the distribution of biomes (types of ecosystems) are shaped by climate.
  • A place's climate depends on global patterns of solar energy input and air flow, as well as features like mountains and bodies of water.

Introduction

Let's start off with a question: Where would you find a polar bear?
Like me, you may forget whether polar bears live near the North or South pole. (I looked it up: the answer is North!) Even so, you probably wouldn't look for one in, say, the rainforest or desert.
Polar bear walking in a snowy arctic landscape.
Image credit: Polar bear, by Patrick Kelley, U.S. Coast Guard, USGS, public domain
Let's think about why that's the case. Polar bears need certain conditions in order to live, thanks to the way their bodies are built and function. These conditions are found only in certain places. For instance, the thick fur coat that helps a polar bear survive in the cold would be useless (and even harmful) on hot day in the desert.
This is a general rule in ecology: each species is found only in a certain set of habitats out of the many on Earth. This occupied region is called the species' range. Some organisms have broader ranges than others, but no species is found everywhere. That's because different species have different needs, as well as different histories of dispersal, or how they've spread from place to place.
One of the most important factors determining where different species are found is climate, or long-term, typical weather conditions. In this article, we'll take a look at biogeography (the study of why different organisms are found in certain locations, in certain numbers) and how species ranges are affected by climate .

Each species has a range

The range of a species is the set of locations where that species is found on Earth. For instance, the diagram below shows the range of polar bears (looking down on the Earth from above the North pole):
Green highlight marks the regions in which polar bears are found. This map is a view of the globe looking down from above the North pole. Image credit: Polar bear range map by Fabio B., public domain
What determines a species' range? Historical chance and geographical barriers can play important roles. For instance, maybe polar bears could survive at the South pole as well as the North pole. But they were never introduced to the South pole, and have had no way to disperse, or spread, across the oceans in between.
Once a species has been introduced to an area, it can only survive in that area if the conditions are right. Some of the conditions that must be "right" are biotic, meaning that they're directly related to living organisms. For instance, a species may not be able to get a foothold in a given area because a competing species, predator, or pathogen is already there, or because no food supply is available.
Many factors that determine whether a species can live an an area are abiotic, or non-living. Examples of important abiotic factors include temperature, sunlight, and moisture level. These factors sometimes determine whether a species can live in a place in a very direct way. For instance, a plant species will only take root and spread in a place where it's getting enough sunlight and water.
However, abiotic factors can also affect where species are found in less direct ways. For instance, climate and soil quality directly affect the type and number of plants that can grow in a particular area. Since energy enters ecosystems via plants and other primary producers, climate and soil quality indirectly determine what other trophic levels, or "food chain links," the ecosystem can support.

Global distribution of biomes

Abiotic factors shape the ranges of individual species, such as our friend the polar bear. At a more zoomed-out level, though, they also determine where different types of biomes are found on Earth.
What exactly is a biome? Basically, it is a type or category of ecosystem. One familiar example is the desert biome. Each desert is in a different place and has its own unique set of plants and animals. Still, Earth's deserts are all distinctively deserts and share common features. They tend to have little rain, high daytime temperatures, and sparse plants adapted to the harsh conditions.
Climate is the key abiotic factor that determines where terrestrial (land) biomes are found. Each biome has a characteristic range of temperatures and level of precipitation (rainfall and/or snowfall). If we know what temperature and precipitation are like in a location, we can often predict what type of biome will be found there.
This diagram represents the eight major terrestrial biomes, along with mountains and polar ice (which are not formally considered biomes). Image credit: Biomes: Figure 2 by OpenStax College, Biology, CC BY 4.0
Certain types of biomes tend to fall in rough bands along Earth's north-south axis. For instance, there is a big band of tropical forest (green in the diagram above) that encircles Earth's midline, or equator, including parts of Central and South America, Africa, and Southeast Asia. However, Earth's biomes also don't form a strict "stripe" pattern, as you can see from the bumpy shapes on the map.
We can explain both the general pattern of bands and variations from this pattern by looking at different factors that affect climate.

What is climate?

Climate is just the weather, right? Well...sort of. In ecology (unlike in everyday life), these terms have slightly different meanings:
  • Climate refers to long-term, typical atmospheric conditions in an area, such as temperature and rainfall. "It's usually hot in Dallas during the summer" is a description of climate.
  • Weather refers to the same types of conditions, but on a shorter timescale. For instance, "The high was 100 start superscript, start text, o, end text, end superscriptstart text, F, end text in Dallas yesterday" describes weather, not climate.
Basically, you can think of climate as a place's “average” weather.

How climate changes with latitude

In general, temperatures on Earth’s surface drop as we move from the equator to the poles. That's not a big surprise—we tend to think of the Arctic as chillier than then tropics! But why is it the case?
The basic answer is that the equator gets more insolation, or solar energy per area per time, than the poles do. Rays of sunlight hit the Earth directly near the equator, but at an angle near the poles, so the same amount of energy is spread over more area in the polar regions, as you can see in the diagram below:
Diagram illustrating that rays of sunlight hit the earth directly (more or less straight on) near the equator, but obliquely (at an angle) near the poles. The same amount of solar energy is spread out over more surface area when the rays strike the Earth at an angle near the poles. Also, sunlight entering at the poles must travel a longer path through the atmosphere before hitting the earth's surface. This longer path allows more solar energy to be deflected back into space by the molecules of the atmosphere, further reducing insolation at the surface.
Image modified from Oblique rays by Peter Halasz CC BY-SA 2.5. The modified image is licensed under a CC BY-SA 2.5 license
Also, at the poles, sunlight travels a longer path through the atmosphere before reaching the surface. That means more light is deflected into space by particles in the atmosphere (and thus never reaches the surface) at the poles than at the equatorstart superscript, 1, end superscript.
The strong sunlight at the equator (and weak sunlight at the poles) makes the tropics warmer than the Arctic. Not only that, but this difference in solar input also generates major global patterns of air circulation. Because air is heated by the sun most strongly at the equator, it has the greatest tendency to rise there. This rising of air at the equator drives large-scale patterns of air flow and rainfall.
What do these large-scale patterns look like? Earth's atmosphere contains six rotating cells of air are found (three north of the equator, three south of the equator). Each of these cells encircles the Earth like a giant "air donut," as shown in the figure below.
Illustration of the Earth's air circulation patterns and how they generate characteristic air circulation patterns and climate bands at different latitudes.
Around the equator: Air rises and releases water. There is lots of rainfall here. The air proceeds away from the equator to the north and south at high altitudes
Around 30 degrees N/S: The air that rose at the equator falls here. It is very dry and absorbs moisture, so deserts are usually found around these latitudes. Some of the air cycles back to the equator along the surface, while some of it moves poleward along the surface. The air returning from the 30 degrees N and 30 degrees S meets near the equator, in a band called the intertropical convergence zone. (This is the same region where the air originally rose and released water.)
Around 60 degrees N/S: The air the at moved along the surface from the 30-degree latitudes rises again here, releasing some rain. The air may return towards the equator at high altitude, or may continue poleward at high altitude.
Around the poles: Air descends here. It is again dry and absorbs moisture, creating desert-like conditions. The air returns poleward along the surface.
The white arrows show the major wind paths (patterns of air flow along the surface due to circulation of air in cells). The winds curve due to Earth's rotation. Image modified from Earth global circulation by Kaidor, CC BY-SA 3.0. The modified image is licensed under a CC BY-SA 3.0 license
In this six-celled pattern of air flow, air rises in low-pressure zones: one at the equator (under the influence of the strong equatorial sun) and two more at 60, start superscript, start text, o, end text, end superscript start text, N, end text and start text, S, end text. As it rises, the air cools and drops much of its moisture as rain or snow. This leads to regions of high precipitation (rain or snowfall) at the equator and at 60, start superscript, start text, o, end text, end superscript start text, N, end text and start text, S, end text.
Having already dropped its moisture, the air that rose in the low-pressure zones is dry as it flows towards the poles (traveling high up in the atmosphere). When it comes down again in high-pressure zones (which are found at 30, start superscript, start text, o, end text, end superscript start text, N, end text and start text, S, end text and at the poles), the dry air sucks up moisture from the surface, resulting in bands of desert at 30, start superscript, start text, o, end text, end superscript start text, N, end text and start text, S, end text and in dry regions at the north and south poles.

Mountains, elevation, and climate

Latitude patterns in climate give us broad patterns, such as bands of desert and high rainfall at different latitudes. but as you may have guessed, they're only part of the picture. After all, not all places at the same latitude have the same climate or the same type of biome!
Elevation above sea level is one key factor that shapes climate. To give a real-life example, when I was a kid, I went to a school on top of a big hill. My classmates and I sometimes got a snow day (day off from school) when other kids in the area didn't. Why? It was colder on the top of the hill than it was at sea level, so it sometimes snowed at our school when it was raining in the areas below.
To put that idea more generally, places at high elevations tend to have a colder climate than nearby low-lying areas. In general, for each 1000 meters we move upwards (say, hiking up a mountain), the air temperature will drop by roughly 6 start superscript, start text, o, end text, end superscript, start text, C, end text cubed.
Because temperature changes with altitude (along with things like moisture and soil type), a mountain can have different biomes at different altitudes. For instance, a tall mountain may have grassland on its lower slopes, but a zone of alpine tundra, like the arctic tundra biome found near the north pole, at higher elevationsstart superscript, 4, comma, 5, end superscript.
Mountains also affect patterns of rainfall, both on their own slopes and in surrounding areas. Imagine the case where a mountain tends to get hit by winds coming from a certain direction—say, off the ocean. Especially if those winds are damp, the windward (wind-facing) slopes and surrounding areas will tend to get lots of rain.
Diagram illustrating how a rain shadow forms. The prevailing wind blows off the ocean, bringing moisture-rich air over the land. When the air reaches a mountain, it's forced upward and loses its ability to hold as much water, so some water falls as rain. Descending the other side of the mountain, the air is very dry, so it absorbs moisture and produces a rain shadow (desert-like area).
Image modified from "Orographic effect," by Meg Stewart (CC BY-SA 2.0). The modified image is licensed under a CC BY-SA 2.0 license.
Why is that the case? The air loses its capacity to hold water as it rises and cools while moving up the slopes, and it drops the extra moisture as rain. The air that makes it over the mountain is dry, so the other side (the leeward side) tends to have a desert-like climate. This dry region on the leeward side is known as a rain shadow.

Lakes, oceans, and climate

As the example above shows, bodies of water (especially big ones like oceans and lakes) can affect the climate of surrounding regions. In fact, bodies of water influence climate in a variety of ways, even when mountains are not in the picture.
At a basic level, lakes, oceans, and streams play a vital role in climate processes by serving as reservoirs for water, which can evaporate from the surface to fall later as rain or snow. You can learn more about this process in the water cycle article.
Bodies of water also minimize changes in temperature of nearby landmasses. That is, they keep high temperatures from getting as high and low temperatures from getting as low as they otherwise would. You can learn more about how water's unique properties make this possible in the video on specific heat capacity of water.
Finally, ocean currents (which carry water from one place to another) can strongly affect the climate of nearby land. The map below shows some of Earth's major currents:
World map illustrating major ocean currents. The Gulf Stream carries warm water up past the eastern coast of the United States. The North Atlantic Drift then carries the water onward, across the Atlantic ocean and past the western coast of Europe, including the British Isles.
Warm currents are represented by red arrows in the diagram, while cold currents are shown in blue and neutral currents in black. Image modified from "Corrientes oceanicas," by Popadius (public domain).
To see how currents affect climate, let's compare two cities at nearly the same latitude: London, England and Calgary, Canadastart superscript, 6, end superscript. London only gets down to 40 start superscript, start text, o, end text, end superscript, start text, F, end text or so in the winter. Calgary, on the other hand, routinely gets below 10 start superscript, start text, o, end text, end superscript, start text, F, end text—cold enough that a friend of mine had her eyelids freeze shut while she was visiting there!start superscript, 7, comma, 8, end superscript
This difference between London and Calgary can be traced to a current called the Gulf Stream. The Gulf Stream carries water heated at the equator up past the eastern coast of the United States, feeding into another current called the North Atlantic Drift. This current carries warm water past England and the west coast of Europe, making the climate warmer than it otherwise would bestart superscript, 9, end superscript.

Why does this climate stuff matter?

Climate is a key factor that determines where different species can live. This principle holds true across the many branches of the tree of life, from animals (like our friend the polar bear) to plants to microbes. Each species needs its own specific set of conditions for survival, many of which are directly or indirectly related to climate.
If climate conditions in an area change, the species that can live there may also change. For instance, a drop in rainfall may mean that a region can no longer support the plant species it previously did, becoming more desert-like instead. Such changes can have cascading effects on ecological networks, with shifts in plant communities affecting all the animals that depend on them.
This principle holds true for any change in climate, whether it affects a tiny area or a large one. However, it's especially important in light of the global climate change that is now taking place. As a result of human activities, scientists predict a rise in average temperatures of 1
5°start text, C, end text by 2100 start superscript, 11, end superscript. For species sensitive to small differences in temperature, this could be a devastating change.
To learn more about global climate change and how it can affect species ranges and biodiversity, see the climate change and biodiversity video from the California Academy of Sciences.

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