Life is found almost everywhere on Earth, but it is not distributed evenly around the planet. Different species are found in different areas; some species have overlapping ranges, others do not. Each species has a set of environmental conditions within which it can best survive and reproduce. Not surprisingly, those conditions are the ones for which it is best adapted. Many different physical, abiotic (non- living) factors influence where species live, including temperature, humidity, soil chemistry, pH, salinity and oxygen levels. 

Just as species have geographic ranges, they also have tolerance ranges for the abiotic environmental conditions. In other words, they can tolerate (or survive within) a certain range of a particular factor, but cannot survive if there is too much or too little of the factor. Take temperature, for example. Polar bears survive very well in low temperatures, but would die from overheating in the tropics.

On the other hand, a giraffe does very well in the heat of the African savanna, but would quickly freeze to death in the Arctic. This example points out an important aspect of tolerance ranges – different types of organisms have different tolerance ranges for the same factor. And in fact, the tolerance range of a single individual may change over time; individuals of a certain species of salmon, for example, start life in a freshwater stream, migrate out to the open ocean, and then come back to their home stream to reproduce. The salmon tolerates huge changes in the salinity (salt content) of the various water it passes through during its journey, and also experiences many changes in water temperature.

Another important aspect is that all organisms have tolerance ranges – microbes, fungi, plants, and animals, including humans. While human technology has allowed us to live and work in more extreme environments, humans still freeze to death, die from heat stroke, drown, suffocate, and die from exposure to acid or lack of fresh water to drink. Our protective technology and our tolerance for too much or too little of these factors only goes so far – beyond the tolerance range, we cannot and do not survive.

Biologists are frequently interested in studying and understanding the tolerance ranges of different species for different environmental factors. If you draw a graph of how many individuals in a population live under which part of the range of any given factor, you almost always get a bell-shaped curve. Take a look at the two tolerance range curves shown below. The horizontal axis could be any of the abiotic factors (environmental conditions), but for now let’s say it is for oxygen levels in freshwater lakes. If you are studying a particular species of fish, let’s say the blackstripe topminnow (Fundulus notatus), you could go out and measure the oxygen level of every lake where you find the topminnow and also count how many topminnows are in each lake. When you make a graph of your data, it might look like Graph 1. That graph is telling you that the majority of the topminnows live in the middle part of the oxygen range; that’s where the curve is highest. As you move from the middle part to lower oxygen levels (to the left) or to higher oxygen levels (to the right), the curve is not as high – there are fewer individuals that live in lakes that have the lower or higher amounts of oxygen. And if the oxygen level is extremely low or high, it is beyond the tolerance range of the species and no topminnows live in those lakes.

Now take a look at Graph 2, which represents the oxygen tolerance range curve for a different species of fish, in this case the blacktail shiner (Cyprinella venusta).

What is Graph 2 telling us about shiners compared to the topminnows? Shiners have a much narrower tolerance range for oxygen than topminnows do. The shiner can only survive and thrive in a narrow band of oxygen levels, so you would expect that its geographical range would be more restricted; it would not be distributed as widely as the topminnow since it wouldn’t do well in stagnant ponds with lower oxygen levels, for example. If you look closely, you’ll also notice that the peak of the curve for the shiner is a little bit to the right of the peak of the curve for the topminnow. This tells us that compared to topminnows, shiners do best in water that is slightly more oxygenated.

Both Graph 1 and Graph 2 are bell-shaped curves. That’s the normal or typical curve you get when graphing tolerance ranges, and interestingly enough, curves shaped like this illustrate what is referred to as a normal distribution. In some ways, you could say it is the "Goldilocks curve" – it shows where conditions are just right for a species: not too hot, not too cold; not too salty, or not salty enough; not too wet, not too dry. These preferences and needs for certain types of conditions greatly influence the distribution of species around the planet, and it can get fairly complex when you consider that multiple abiotic factors are simultaneously influencing any given individual and species.