Mechanisms of Succession in Natural Communities and Their Role in
Community Stability and Organization
Joshua Lobe
Succession and climax communities are terms used very often in ecology. These
terms have spread to an even wider crowd than ecologists. Occasionally, climax and
succession will slip into the mainstream media’s coverage of science news. Sometimes,
they are models taught in high school science classes.
Despite these signs of
establishment, these ideas are misunderstood often enough to drive good ecologists nuts.
Many ecologists have tried to answer the hard questions about what these terms mean
exactly, but few have done as well as J.H. Connell and R.O. Slatyer did in 1977.
The idea of succession leading to climax communities was a popular idea in
ecology for much of the 20th century. R.H. Whittaker stated in 1953, “The succession of
populations on disturbed or newly exposed sites is one of the best-established phenomena
of ecology…”(p.43) However, there was much confusion as to what exactly a climax
community was and how succession led to a mature ecosystem or climax community. As
Whittaker pointed out in 1953, “A brief review of the development of climax theory in
different schools of ecology indicated the confusion existing and the need for
reconsideration of the concept.” (p.61) He was referring primarily to the American and
British views on climax and succession. The American view, according to Whittaker was
a belief in the monoclimax.
The British view was one of polyclimax.
Whittaker
attempted to mesh all evidence of climax communities available at the time and create an
all encompassing climax theory. The idea of climax communities was still a prominent
idea in the 1970’s. In 1969, Odum published a paper in Science illustrating a way to
characterize climax communities en route to describing a world where humans would use
this knowledge to support a mushrooming population. His paper incorporated ideas,
namely those of F.E. Clements, that were suspect early in the century. The assumptions
he used about climax communities were argued against much more forcefully in the
1960’s and 1970’s than climax theories had been argued against before. The concept that
was most controversial was the idea that only certain species could colonize a disturbed
area and, in turn, only a certain other type of species could follow.(Connell and Slatyer
p.1119)
Connell and Slatyer’s classic paper about succession and community organization
was published in American Naturalist in 1977. The authors contrast three models of
succession that they present in the paper. The first is the facilitation model proposed by
Clements.
This model describes a community where the colonizers change the
environment in a way that inhibits the recruitment of their own offspring and at the same
time, makes the environment suitable for another species. In turn, that species does the
same thing for another species in the line of succession.
The second model they
described was the based on life-history characteristics of the species in the community.
In this model, the tolerance species’ have for certain conditions either limit or allow them
to grow and thrive in the environment. The third model Connell and Slatyer portrayed
was the one they favored for most types of succession.
“In contrast to the other two [models], in model 3 the species of individual that
replaces a dying resident need not have life-history characteristics different from the
original resident…This being the case, it is possible that a resident may be replaced by
another of the same species or of a different species also having ‘early succession’
characteristics. Then the traditional sequence won’t occur. If, on the other hand, the
replacement happens to be a species having ‘late succession’ characteristics, then the
traditional successional sequence will be observed. Since the early-succession species
are shorter lived, they will be replaced more often than would the longer-lived late-
succession species. If propagules of these later species are available for invasion, then
after several years of transitions the latter species will tend to accumulate, with the result
that the early species will gradually decrease in relative abundance…In this way the
operation of the inhibition model 3 will produce a succession of species leading from
short-lived to long-lived species, as is commonly observed.”(p.1123)
This elegant description of their inhibition model made it obvious that they were
onto a bright idea. Besides describing this new mechanism of succession, Connell and
Slatyer also bring up the question of climax communities. They state that since their
inhibition succession model is based on life-history and resistance to dying, that a climax
community is simply the species most resistant to damage and elimination. They also
concluded that, since they never found a sexually reproducing community in steady-state
equilibrium, that succession never stops.
These clear and thoughtful descriptions of a muddled and complex topic in the
field made this an important paper, but its influence in the future of ecological studies
came also from the clearly described needs for future work in particular areas. The
authors described experiments and field tests that they claimed would support some of
the models and discount others for the communities being investigated. Overall, the
paper provides a good base for any investigation into community structure and succession
as it is clear and provides very solid models and arguments supporting them.
These ideas that climax communities really do not exist in the world began to take
a hold of the ecological community around the time of Connell and Slatyer’s paper. In
1984, W.P. Sousa wrote about the role of disturbance in natural communities. He also
discussed how disturbance happens too frequently for a community to truly stabilize.
Sousa wrote, “Physical disturbance is one of the major mechanisms that break this
inhibition[of germination] and generate conditions favorable for recruitment, growth, and
reproduction.”
This is very similar to Connell and Slatyer’s inhibition model 3 of
succession.
Another example of their influence on ecology was published in Science in 1984.
The paper by Harris et al. described community recovery after disturbance. Specifically,
how a kelp forest regenerated after a series of storms. During the storms, rocks were
broken creating new surfaces. On the fresh rock surfaces, filamentous brown algae grew
instead of the normal red algae that grew on older rock surfaces.
The researchers
manipulated kelp sporophyte habitats and discovered that these brown algae actually
facilitated the kelp’s growth. Without the brown algae, the kelp was eaten by fish. This
was an example of primary succession allowing facultative growth. It was also important
because it showed the facilitative growth during primary succession due to a disturbance.
The study was perfect evidence of a community that seemed to be a climax community,
but was still going through succession due to disturbance.
Ecology, more and more, must deal with human influences on ecosystems. When
humans do affect ecosystems, it usually creates disturbance. Humans can also introduce
new species into ecosystems that may compete with native species. In a study of a
forest’s change in upper Wisconsin, human influence played a large role. There was
initially logging, which was a large disturbance on the forest. For over 50 years, though,
the forest was not logged. This was a perfect place to study a young forest to see what
kind of succession, if any, was taking place and how it was affecting the species
diversity. As it turned out, there was not a measurable amount of succession taking
place. Instead, there was heavy grazing by large deer populations and exotic species
spreading through the forest. These two factors contributed to a more homogenous
forest. (Rooney et al. p.787) In the paper, the successional models of Connell and Slatyer
were discussed, showing how accepted their theory has become in ecology. In forest
studies now, successional models much like Connell and Slatyer’s are used regularly.
Another interesting place that successional models are being applied are to
microbes. Bacteria in the soil are essential to our earth to break down organic matter.
The questions being studied now are how the communities of microbes are built. In a
study done in 2004 by Manfred Wanner and Willi Xylander, the microbes seemed to
have fairly stable populations. It is also known that microbes in the soil do fill multiple
and can be classified as a community. However, the question of succession probably
needs to be looked into deeper to truly understand the workings of these invisible
communities.
As the theory of succession and climax communities has been adjusted and
augmented over the last century, probably no one has made as big a contribution as
Connell and Slatyer in their classic paper. It has influenced thousands of other papers
directly as evidenced by its being cited more than 1200 times. They were the turning
point in changing from the older, outdated idea of climax communities being the result of
succession to a more accurate theory where disturbance maintains a regime of constant
succession.
Connell, J.H. and R.O. Slatyer. 1977. Mechanisms of succession in natural
communities and their role in community stability and organization. American
Naturalist. 111:1119-1144
Harris, L.G, A.W. Ebeling, D.R. Laur, and R.J. Rowley. 1984. Community Recovery
after Storm Damage – A Case of Facilitation in Primary Succession. Science.
224, (4655): 1366-1338
Odum, E.P. 1969. The Strategy of Ecosystem Development. Science 164: 262-270
Rooney et. al. 2004. Biotic Impoverishment and Homogenization in Unfragmented
Forest Understory Communities. Conservation Biology. 18: 787-798
Sousa, W.P. 1984. The Role of Disturbance in Natural Communities. Annual Review
of Ecology and Systematics. 15: 353-391
Whittaker, R.H. 1953. A Consideration of Climax Theory: The Climax as a Population
and Pattern. Ecological Monographs. 23: 41-78