Download Community Ecology – Succession

© 1995 Saint Mary's College
of California
© 2000 Joseph Dougherty
Beatrice F. Howitt © California Academy of Sciences
Charles Webber © California Academy of Sciences
Photos are used with permission, courtesy of CalPhotos, the Digital Library Project of UC Berkeley.
Community Ecology – Succession
Introduction
Communities are not static; they are constantly changing. As organisms die, new ones are
born. Nutrients cycle within communities as energy passes through them. The process by which
communities develop is called succession, or ecosystem development. The end point of the
successional process is a climax community in which species diversity and composition are in some
quasi-equilibrium and in which whole ecosystem production is nearly matched by whole system
respiration. The community is at its maximum biomass at this point. And it will remain at this state
until disturbed. An ecological disturbance is broadly defined as an event that removes biomass from
a system (e.g. a timber harvest, a fire, a lightning strike in a forest, etc.). Disturbances may be
localized or cover large areas. Disturbances vary greatly in their frequency of occurrence and
intensity. Both factors determine how “hard hit” a community will be by a particular disturbance.
The effect of disturbance is to [relatively] instantaneously move the community to an earlier
successional stage. From there, the community again develops via the processes of succession and
ecosystem development.
The process of succession or ecosystem development is orderly and relatively predictable.
It begins with pioneer species or colonizing species that are fast-growing, short-lived, and prolific
(we often call them “weeds”!). These species are steadily replaced by longer-lived and slower
growing species as more and more biotic structure is added to the community. One very important
characteristic of succession and ecosystem development is that, in the process, the community
actually modifies its local environment. Soil is an excellent example of something that is a product
of a community developing from nothing. There are numerous other examples of ways that
communities actually “tailor” their local environment as they develop.
Succession follows two primary routes: Primary succession is when a community “starts
from scratch”, on new habitat. Nothing precedes the community—not even soil. The best examples
of this are lava flows, where the community literally begins with nothing but the cooled volcanic
rock. Most communities develop along a secondary succession route, however. This involves a
disturbance that merely “resets” a community’s succession clock to an earlier time. Some biomass is
removed, but the community does not start over completely. As an example, a clear-cut or fire that
removes all of the trees and shrubs from an area of forest will probably not remove all of the soil
(unless erosion completely decimates the area afterwards). That soil contains organic matter,
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nutrients, and seeds that will help fuel the redevelopment process. The community is thus not
starting over from scratch.
The process of community succession was one of the first ecological concepts. It came from
observations by the phytosociological scientists of the late 19th century. By the turn of the century,
the search was on for processes that explained differences in plant species distributions and
dynamic change was considered. From this came the idea we know today as succession. Some
classic early successional research was done in sand dune habitats along the southern shore of Lake
Michigan and on coastal barrier islands. The former studies involved plant species distributions and
succession in a dune habitat where wind movement of sand is the key disturbance element. In barrier
island settings, it is the ocean (with its tides, salt, and salt spray) that is the key disturbance element.
Answer/address all of the following questions.
1. What are the primary sources of disturbance in the barrier beach dune system you studied in the
succession lab? How does this disturbance act to reduce biomass in your dune system?
2.
What relationship between species diversity and ocean-uplands gradient did you expect to see
along your transects? Do your data support this hypothesis?
3.
What did you notice about differences in morphological adaptations by the plants along your
disturbance gradient?
4.
Can you come up with any kind of zonation along your disturbance gradient? What
demarcations did you use to identify this zonation?
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