Module 8: Decomposers: The End and The Beginning

Decomposers: The End and the Beginning

By Dr. James A. Danoff-Burg, Columbia University

In today’s module, we complete our tour through the players in ecology by looking at the decomposers. In Module 6 we looked at the producers who produce their own food and in Module 7 we looked at the consumers who consume tissue from live organisms.

The decomposers differ from these other two ecological classes in that they consume dead tissue. In so doing, they complete the cycle of energy by recycling resources back to the community. These recycled resources, mostly in the form of nitrogen, carbon, and other minerals, are then absorbed by plants and thereby reintroduced into the environment.

The group of organisms that are classified as decomposers are much more taxonomically diverse (but still less species rich—see Module 5) than the producer and consumer ecological classes. Producers are mostly plants, and consumers are mostly animals. In contrast, decomposers include organisms from all five kingdoms - saprophytic (feeding on dead tissue) plants, fungi, bacteria, protista, and a great diversity of animals.

The sequence of organisms arriving at a decomposing plant or animal follows a generally predictable pattern. Although there are variations within this general pattern, at the carcass of a large mammal, the decomposer community may begin with vertebrates (macrofauna). Very often the macrofauna do not arrive at a carcass and the community then begins with large invertebrates (mesofauna) like flies and then beetles, then proceeds to smaller invertebrates and fungi, and culminates with protists and bacteria (microfauna). For a carcass of any given animal species, the arrival times and community composition of decomposer species are consistent within a species. However, they can and often do differ between carcass species.

Interestingly, the arrival sequence of decomposers is usually inverted at a dead tree. Bacteria and fungi (microfauna) begin the process of weakening the wood, which makes it possible for mesofauna to invade. The process culminates with the arrival of small mammals and other macrofauna that may also use the decomposing log as shelter. As with animal carcasses, the processes are relatively consistent within certain host (dead tree) species although the specifics of this process differ between tree species.

The ecological process that we have been discussing is called degradative succession. During this and all other types of succession (allogenic, autogenic, and others—see Begon, Harper, and Townsend pp. 692-710 for more info on these types), each wave of arrivals modifies the environment as a consequence of their actions. In this manner, each wave facilitates the arrival of subsequent successional species. Eventually they change the resource so that they can no longer exist there. Thus, most processes of succession are almost always irreversible.

Population control of decomposers occurs primarily with availability of resources. Their population sizes typically undergo exponential population growth, only when they encounter a carcass of the appropriate tree or animal species. Decomposers do not control the availability of their resources (as do predators) but are instead controlled by them. They are therefore prime examples of organisms whose populations can grow explosively initially (exponential growth, as discussed in Module 4) , but then must rapidly stop growing as resources become scarce. Eventually they will have to leave the local area because the carcass is used up.

Scientists have been able to take advantage of these predictable events to determine plot age, health, tree harvesting rates, and similar applications. One of the most interesting applications of degradative succession is forensic entomology. This specialized field of law enforcement determines the date, time, and location of a crime by analysis of the organisms present on a corpse when it is found. This primarily involves inventorying and identifying which flies and beetles are present, in what relative abundance they are present, and where they are concentrated on the corpse. The scientists then use these data to exonerate the innocent and convict the guilty.

All processes of decomposition are slower and decomposers are less common and diverse in very arid environments. Instead of decomposing, organic matter merely dries out and mummifies. As we will explore in several ecosystems (in this module or Module 13), we can see this same tendency in some edge habitats as well.

Decomposers play an essential role in the structure and function of ecosystems, as well as being worthy of study in their own right.

Additional Relevant Online Resources

NOAA's site about Otter Island, South Carolina's estuaries has an interesting listing and discussion of the ecological players and their ecological roles in the Estuarine ecosystem.

J. A. Holmes of the Environmental Research and Teaching Center at the University of Toronto has an enlightening page on the various types and mechanisms of succession.

Forensic-Entomology.com has several very informative pages on the application of succession to solving crimes using insects.

The American Board of Forensic Entomology has a rigorous discussion of the science and the applications of forensic entomology to solve crimes.

Dr. Morton Staerkeby of the University of Oslo has a very interesting set of pages he calls The Forensic Entomology Pages, International.

All Materials Copyright © 2000 by James Danoff-Burg

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