Distance Decay in Geography

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The further apart two places or populations are, the lower the probability it is that they will interact. This phenomenon is known as distance decay.

Distance decay, in its simplest form, shows how commonalities degrade with increasing distance. This may result in cultural disparities between two remote versus nearby communities or between an urban core and its surroundings.

From a geography standpoint, distance decay can be explained by Waldo Tobler’s First Law of Geography. This is the assertion that “Everything is related to everything else, but near things are more related than distant things,” and can be applied to a range of different geographical concepts, from human settlements, to geolinguistics, to ecology.

An simple example of distance decay using blue points and yellow arrows.
With distance decay, things that are geographically closer tend to be more related than things that are farther apart. Diagram: Caitlin Dempsey.

Though the word “decay” has a negative connotation, distance decay isn’t always a terrible thing. Distance decay between two groups of people gives rise to things like languages and regional customs, and distance decay across biomes contributes to the earth’s natural diversity.

An example of distance decay in geography

The most straightforward method to consider distance decay as it relates to human culture is as follows:

Assume you have A, B, and C for your three different ethnic groups. Along a river that facilitates trade and transit, Groups A and B dwell near to one another. Group C is located far from both group B and group A. As a result, groups A and B will interact with one another frequently, groups B and C will occasionally interact with one another, and groups A and C will seldom interact with one another.

When it comes to the history of these groups, this can have significant ramifications: although groups A and B may start to establish shared traditions and a common dialect, group C will likely diverge greatly and maintain its linguistic distinctiveness. These groups interact less frequently the farther apart they are from one another.

A simply diagram showing the location of three populations with red dots.  The background is light gray with a blue line for river and green shaded areas for parks.
Populations that are close will tend to have more in common than populations that are farther away geographically. Diagram: Caitlin Dempsey.

Factors Affecting Distance Decay

Distance decay between human settlements is somewhat offset by things that enhance interactions among remote groups.

For example, if the populations in groups A, B, and C are connected by a bullet train, then their proximity to each other matters a lot less than if they have to rely to traveling on foot.

If a city can afford to provide well-maintained roads and adequate transit from the center to the outskirts, then the city will experience more even development, and there might be less disparity between the city center and the outskirts.

Inventions like widespread wireless technology and high speed internet have helped cut down on distance decay even more. Now, an individual in France can easily contact an individual in Brazil in seconds via a cable internet connection, allowing for an easy, rapid exchange of ideas.

The affect of distance decay on ecosystems

Biomes are likewise affected by the distance decay phenomenon. The physical distance between two geographic sites is closely proportional to their loss of ecological and biodiversity similarities.

The greater the physical distance between two regions, the more likely it is that climatic and geological features such as lakes and mountains may influence weather patterns in one area but not the other, influencing which species can inhabit them.

Longer distances make seed distribution more difficult for plants. As a result, a grove of maple trees in one place is unlikely to have cousins in a forest a hundred miles distant, and certainly not in a desert a thousand miles away.

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