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Topics in Depth

The Theme of Community Succession in Ecology: Organisms and Their Environments

Competition, predation, and symbiosis are especially important in structuring a community as it recovers from major disruptions. These disruptions are often the same things we explored as density-independent factors in the previous section on population ecology. They include fires, volcanoes, earthquakes, and so on.

  • When most of the members of a community are destroyed, as often happens near the base of active volcanoes, then the community immediately begins to recover through a process called succession.
     
  • If all life is eradicated from an area, which is not common, a community must start from scratch with the process of primary succession.
     
  • If some parts of the community are spared, which is more common, then the community recovers through a process called secondary succession.
     
  • If disturbances don’t occur often, then a community can mature from primary (or secondary) communities all the way to stable climax communities.

As the final component on community ecology, let’s look a bit more closely at the process of succession.

Primary to Climax Communities

When a massive disturbance occurs and wipes out all life in an area, a community must start from scratch. But how? Where do the first species come from? Usually, the first species to arrive in a newly uninhabited area are those from the surrounding environment that are able to produce a lot of offspring that can disperse over long distances. These species are called colonizer species, because, well, they are good at colonizing new areas. They are also called "pioneer species" for the same reason.

Colonizer species often include

  • Fungi
     
  • Algae
     
  • Bacteria
     
  • Lichens
     
  • Plants that have wind- or water-dispersed seeds
     

As you read in the population ecology section on life history trade-offs (if you didn't read that section, you might want to scoot back there really quick), organisms that are good at one thing, like fast and prolific reproduction, are often not good at other things, like long-term survival. These trade-offs play a huge role in ecological succession. Huge. Whoppin' big.

Colonizers are good at what they do—colonization—but they are poor competitors. As the newly arrived species begin to grow and reproduce, they change the environment in ways that allow other species to survive. These later-arriving species are often not as good at colonizing, but are better at persisting for long periods of time and quickly out-compete the colonizer species. This process continues for many years, until eventually, the community is composed mostly of highly stable persister species like tall, slow-growing trees. This final, stable community is often called a climax community because, unlike communities earlier on in the successional process, it doesn’t change much in composition from year to year—succession has reached its climax. If you couldn't stop laughing the whole time you read that last paragraph, good. We thought it was hilarious, too.

Secondary succession occurs in exactly the same way as primary succession, except that it begins with a partially disturbed community, rather than a lifeless wasteland. Teenage Wasteland? No, not really. This partially disturbed community recovers more quickly than the completely disturbed community because colonization can occur over much shorter distances. In addition, because some representatives of the original community are still nearby, secondary succession results in communities that look more similar to the original community than those that result from primary succession. In the end, though, both primary and secondary succession can result in stable climax communities.

The tragedy in succession, if you feel comfortable calling it that, is that the success of earlier species is what leads directly to their eventual demise and replacement. Despite this, understanding the process of succession is important to people interested in conserving and restoring areas disturbed by human activities. In fact, conservation efforts rely heavily on the concepts and theories associated with ecological succession. So, if this section disproportionately whet your palette, you might consider taking a class in or reading more about conservation biology.

Brain Snack

There are many organisms that live near active undersea volcanoes. In these environments, the community is constantly changing—the lava flows kill the lifeforms around the volcano and allow community succession by larvae in the water column. Indeed, completely different organisms can appear after a volcanic eruption. Read a scientific study on this kind of succession here.

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