Allochthonous Material in Ecology: Definition & Impact

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

In nature, when you notice something such as branches or leaves floating in a stream, you are looking at allochthonous material. Continue reading to learn about allochthonous materials and their impact in ecology.

How Can Material Be Allochthonous?

Think about a beaver dam. The construction of the dam is a very interesting process. Created along bodies of water such as lakes and streams, beaver dams are typically composed of matter such as leaves on the ground, mud, branches and twigs. In ecology, we can classify some of these items as allochthonous. What does allochthonous mean?

A beaver dam
beaver dam

Allochthonous refers to material that has been imported into an ecosystem. While an ecosystem involves both organic (plants, bacteria, animals) and inorganic things (rocks, soil, water), the allochthonous material that enters an ecosystem refers to organic matter and its nutrients, such as nitrogen and phosphorous. Allochthonous material commonly resides in an aquatic ecosystem, such as a stream or river.

Living and non-living things

If we peek inside an aquatic ecosystem, we will see many different types of allochthonous material, usually of arboreal origin. This simply means of or relating to a tree. That fallen tree log toppled into a river, or leaves and twigs floating downstream, are all examples of allochthonous material you may encounter within the aquatic ecosystem.

How Allochthonous Material is Transported

Man-made transport of allochthonous material can happen in a variety of different ways. For example, let's say you are hiking on one of your favorite trails. You pick up a branch and use it while walking. After completing your hike you toss the branch into a nearby stream. This branch is now considered to be a type of allochthonous material. Why? Well, from the action of tossing, this branch was transported (or imported) to a different location.

Wind and water are also methods by which allochthonous material can be transferred from one location to another. For example, a moving stream may dislodge a tree log, causing it to float downstream and rest in a different location. Leaves may fall from a tree only to be swept to a different location because of the wind.

Whether it is man-made transport by a human or mechanical transport by wind and water, remember that material can only be classified allochthonous if it has been imported from a different location. This begs the question, if we go back to our beaver example, what allochthonous material did the beaver use? If those branches, twigs and leaves originated from a different location, we can classify those materials as allochthonous.

Impact to the Aquatic Ecosystem

One impact of allochthonous material on the aquatic ecosystem is its support to the food web. How do dead leaves and branches impact an aquatic food web?

Example of an aquatic based food web
food web

Well, when leaves fall to the ground, bacteria feed on this matter, obtaining the energy needed to thrive in the aquatic ecosystem. Numerous creatures in the food chain, benefit from the presence of bacteria. That means our pal allochthonous material serves an important role essential to the continuation of a food web.

What about a tree branch that falls into a nearby stream? Well, after being tossed around in the water, this battered tree becomes a great food source for algae to grow on or aquatic insects to chew on. Once again, various species can benefit from the presence of this allochthonous source in the aquatic ecosystem. That is, fish can prey on these aquatic insects and birds can eat the fish.

Just as there are positive benefits to the presence of allochthonous material in an aquatic ecosystem, negative impacts also exist. At the bottom of the aquatic food web we have algae. If we have a large accumulation of algae in our streams, this can pose serious threat to aquatic wildlife. How is this possible?

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