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Ecological Succession*

Miranda Dudzik

Based on Community Ecology� by

OpenStax

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 3.0�

Abstract

By the end of this section, you will be able to:

• Describe community structure and succession

1 Community Dynamics

Community dynamics are the changes in community structure and composition over time. Sometimes thesechanges are induced by environmental disturbances such as volcanoes, earthquakes, storms, �res, andclimate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance,the community may or may not return to the equilibrium state.

Succession describes the sequential appearance and disappearance of species in a community over time.In primary succession, newly exposed or newly formed land is colonized by living things; in secondarysuccession, part of an ecosystem is disturbed and remnants of the previous community remain.

1.1 Primary Succession and Pioneer Species

Primary succession occurs when new land is formed or rock is exposed: for example, following the eruptionof volcanoes, such as those on the Big Island of Hawaii. As lava �ows into the ocean, new land is continuallybeing formed. On the Big Island, approximately 32 acres of land is added each year. First, weathering andother natural forces break down the substrate enough for the establishment of certain hearty plants andlichens with few soil requirements, known as pioneer species (Figure 1). These species help to furtherbreak down the mineral rich lava into soil where other, less hardy species will grow and eventually replacethe pioneer species. In addition, as these early species grow and die, they add to an ever-growing layer ofdecomposing organic material and contribute to soil formation. Over time the area will reach an equilibriumstate, with a set of organisms quite di�erent from the pioneer species.

There are traits that make an organism well suited to be a pioneer species. Particularly in the case ofprimary succession, a pioneer species must be well suited to live and thrive in a harsh environment, whereresources such as nutrients may be di�cult to obtain. Often times, pioneer species are capable of obtaining

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nutrients in unique ways that are not typical of other species. Pioneer species typically also have a relativelyshort life cycle, have a high growth rate and fecundity. Plant species that have a very narrow set of conditionsthat they can survive in, have a large root system or can't adapt to changing conditions would not make agood pioneer species, Lichen and moss on the other hand, are ideal pioneer species after a primary successionevent because they can thrive in nearly every habitat, have very low nutrient and water requirements andno root system that requires soil.

Figure 1: During primary succession in lava on Maui, Hawaii, succulent plants are the pioneer species.(credit: Forest and Kim Starr)

1.2 Secondary succession

A classic example of secondary succession occurs in oak and hickory forests cleared by wild�re (Figure 2).Wild�res will burn most vegetation and kill those animals unable to �ee the area. Their nutrients, however,are returned to the ground in the form of ash. Thus, even when areas are devoid of life due to severe �res,the area will soon be ready for new life to take hold.

Before the �re, the vegetation was dominated by tall trees with access to the major plant energy resource:sunlight. Their height gave them access to sunlight while also shading the ground and other low-lying species.After the �re, though, these trees are no longer dominant. Thus, the �rst plants to grow back are usually

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annual plants followed within a few years by quickly growing and spreading grasses and other pioneer species.Due to, at least in part, changes in the environment brought on by the growth of the grasses and other species,over many years, shrubs will emerge along with small pine, oak, and hickory trees. These organisms arecalled intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point wherespecies composition is no longer changing and resembles the community before the �re. This equilibriumstate is referred to as the climax community, which will remain stable until the next disturbance.

Figure 2: Secondary succession is shown in an oak and hickory forest after a forest �re.

Secondary succession usually follows a disturbance of an existing community that removes or damagesthe vegetation, but does not remove, destroy, or cover the soil. Pioneer species in this type of successionalevent are likely to be plant species that have roots or seeds that survived the disturbance. This means that,typically speaking, unlike primary succession events, communities that come out of a secondary successionare most likely going to be the community that was in place before the disturbance.

The time progression of each type of successional event is really variable depending upon the circumstancessurrounding the disturbance, but for the most part, primary succession takes much, much longer thansecondary succession. This is because there is a lot that needs to happen after a primary event to make thehabitat suitable for habitation by a wide variety of species. In most cases, even soil needs to be replenishedbefore any more than lichen can grow there. The process of soil development happens by two processes.Firstly, the pioneer species that grow, reproduce and die quickly are a major contributor to soil production,both in terms of breaking down rock ans sediment, but also through decomposition after death. Secondly,the carcasses of organisms, such as insects and spiders, get blown in by the wind, but quickly die from lackof food will also contribute to soil buildup. For secondary events, the time frame is really no more than ittakes for the organism that lived there before to move back in and reestablish themselves.

2 Intermediate Disturbance Hypothesis

Succession events can really be measured in terms of how stable a community is. And stability, in turn, is ameasurement of biodiversity and species richness. The intermediate disturbance hypothesis is the idea thatthere are three aspects that in�uence the stability of a community in relation to disturbances that alter thelandscape. How intense the disturbance, how often they occur and the amount of time that has passed sincethe last. For all three aspects, a community is considered to be most stable when all of these aspects arenot at either extreme. In other words, Species richness greatest between disturbances of moderate intensity

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and frequency. An example of a community that bene�ts from secondary disturbances would be a redwoodforest in northern California. Certain species of redwood trees, such as the giant sequoia, actually requirethe intense heat of a forest �re to activate seeds and trigger germination events.

3 Section Summary

Communities include all the di�erent species living in a given area. The variety of these species is calledspecies richness. Many organisms have developed defenses against predation and herbivory, including me-chanical defenses, warning coloration, and mimicry, as a result of evolution and the interaction with othermembers of the community. Two species cannot exist in the same habitat competing directly for the sameresources. Species may form symbiotic relationships such as commensalism or mutualism. Community struc-ture is described by its foundation and keystone species. Communities respond to environmental disturbancesby succession (the predictable appearance of di�erent types of plant species) until a stable community struc-ture is established.

4 Review Questions

Exercise 1 (Solution on p. 5.)

The �rst species to live on new land, such as that formed from volcanic lava, are called ________.

a. climax communityb. keystone speciesc. foundation speciesd. pioneer species

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Solutions to Exercises in this Module

to Exercise (p. 4)D

Glossary

De�nition 2: climax community�nal stage of succession, where a stable community is formed by a characteristic assortment of plantand animal species

De�nition 2: environmental disturbancechange in the environment caused by natural disasters or human activities

De�nition 2: pioneer species�rst species to appear in primary and secondary succession

De�nition 2: primary successionsuccession on land that previously has had no life

De�nition 2: secondary successionsuccession in response to environmental disturbances that move a community away from its equi-librium

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