Download This article discusses the various hypotheses proposed to explain

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A study of the Influence of Connell’s Classic Paper: Diversity in Tropical Rain
Forests and Coral Reefs on the Field of Ecology
Rebecca Deal
November 18, 2005
Biology 112
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Joseph H. Connell’s classic paper Diversity in Tropical Rain Forests and
Coral Reefs has made a major impact on our understanding of diversity in local
ecosystems and has directed research in the field of Ecology since its publication in 1978.
This article discusses the various hypotheses proposed to explain how local diversity is
produced and maintained. Using tropical rainforests and coral reefs as examples, Connell
concludes that local diversity is maintained primarily by disturbances that are both
moderate in size and frequency. This classic paper has served as the base from which
other researchers have been able to branch and many researchers following Connell have
found further support for his theory.
In his article, Connell outlines a few theories regarding how diversity is
maintained, all of which fall into either the category classified by equilibrium and stable
state or the category of constant change. These included the intermediate disturbance
hypothesis, which states that diversity is high when species composition is changing due
to disturbances which are intermediate in frequency and intensity; the equal chance
hypothesis, which states that when all species are equal in their competitive abilities,
diversity is determined only by the number of different species available in the region to
colonize the local area; the gradual change hypothesis, which states that diversity is
higher when the environmental conditions are gradually changing so that the relative
competitiveness of each species changes (eliminating competitive exclusion); the niche
diversification hypothesis, which states that diversity depends on the heterogeneity of the
habitat and the degree of specialization of different species to their niche; the circulatory
networks hypothesis in which competitive adaptations are beneficial in regards to
competition with one species but disadvantageous with respect to another species; and,
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finally, the compensatory mortality hypotheses by which the most competitive species
will be most affected by non-competition related interactions. Although Connell settled
on the intermediate disturbance hypothesis as the best explanation for the maintenance of
diversity, the other hypotheses he described have been studied and supported by later
researchers. Later researchers also picked up where Connell left off: Connell did not
address the effect of disturbance and the maintenance of diversity of mobile species or of
large-scale geographic regions and his study focused only on rainforests and coral reefs
leaving the question as to whether his hypothesis could be supported in other biomes.
In 1981, Runkle examined the processes of succession and regeneration after
disturbance in old growth forest systems in the Appalachian Mountains by studying forest
regeneration in gaps created by windfalls. Runkle found that in most cases the gaps were
filled in by the dominant tree in the canopy. He further concludes that small disturbances
are the primary forces of forest regeneration. His observance of self-regeneration after
disturbance can be contrasted with Connell’s theory that disturbance is what maintains
diversity.
Other deviations from Connell’s conclusion can be exemplified with Sarmiento’s
1983 paper. Sarmiento further examined Connell’s theory but moved the research field
to the grasslands of Venezuela. He found that diversity in the grass community is
maintained by variations in the availability of resources, such as the unpredictable
extension of the rainy season, as well as annual fires and while disturbance is an
important contributor to diversity, niche specialization also plays a role in maintaining
diversity.
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Pollock and his colleagues’ study of flooding in the wetlands of Alaska pointed
out that while disturbances must be intermediate in order to maintain diversity, they
should also be well spaced (1998). Pollock found a correlation between plant species
richness, frequency of flooding and spatial heterogeneity of flooding events: sites that
had intermediate flood frequency and high spatial heterogeneity of flooding events had
greater species richness than sites which were frequently or rarely flooded or in which the
location of the flooding was consistent. Pollock’s conclusion points out yet another
adaptation to Connell’s original conclusion.
A 2002 study by Collins and his colleagues further examined the major
contributors to species richness and noted that local diversity is strongly influenced by
regional diversity. In addition, they observed that when dominate grasses are
experimentally removed from an area in the Kansas Prairie, the local species richness
increases. Similarly, more heavily grazed regions also tended to have higher local
diversity. Their results offer further support for the influence of disturbance on diversity
as well as emphasize that disturbance is not the only factor in the maintenance of
diversity.
In addition to studies which seem to follow directly from the classic paper,
researchers have also attempted to fill in the gaps where Connell left off. Townsend and
colleagues offered further support for the intermediate disturbance hypothesis that was
proposed by Connell both in a new environmental setting and with respect to both sessile
and mobile organisms. In their 1997 paper, they compares the effects of flooding at
different frequencies and magnitude on the streambeds at different stream sites.
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Townsend et al. confirm that the diversity of both sessile and mobile organisms is
greatest when the disturbance is intermediate.
Most of the studies discussed thus far have been closely if not directly related to
Connell’s classic paper and have served as supplements to his original conclusion, but
there have also been studies which follow from Connell’s paper yet deviate from his
central theme. In a 2002 study, Wright examined how high diversity is maintained in
tropical rain forests by asking how competing plant species can coexist. He found that,
rather than disturbance, the main factors that contribute to diversity are related to a state
of equilibrium, similar to the equilibrium hypotheses which Connell mentioned. Wright
proposed that the main contributors to species coexistence in highly diverse systems is
dependent on niche specialization, negative density dependence which regulates the most
competitively successful species, and the fact that rare plants are rarely in competition
with each other. While Wright’s conclusion deviates from that of Connell, his research is
based on a topic covered in Connell’s original paper.
Haeussler and his fellow researchers also branched off of Connell’s paper by
studying the effect of non-natural disturbances on boreal forests of southern Canada in
2002. He examined vegetation diversity and composition after clear-cut logging and
found that 5 to 8 years after the clear-cut, the species richness was increased by 30 to
35%. Haeussler pointed out that this increase in diversity came at a price: the clear-cut
disturbance stripped the land of important soil organic layers causing a shift in the type of
plants colonizing the region. Instead of resproutings of understory species, more ruderal
species growing from seeds and spores colonized the area. In addition, when the
intensity of the disturbance was extreme, there was a significant increase in the number
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of invasions of non-native species. Haeussler’s conclusion exemplifies the continued
revision of and branching off from Connell’s intermediate disturbance hypothesis.
Researchers today have an interest in the same topics of disturbance and diversity
explained by Connell’s classic paper. Like Connell, Adjeroud and his colleagues have
studied coral reef but instead of asking how the diversity is maintained, they study the
general effects of disturbance with the conclusion that the effects of disturbance can be
largely species specific and while in some local regions variation may increase after
disturbance, in other regions it tends to decrease (2005).
Modern scientists are still also asking questions about diversity but in addition to
asking how it is maintained, they also ask why diversity is so important and to what
extent it is important. Tilman, for example, quantified the importance of diversity by
comparing it with plant productivity (1997).
Connell’s paper has had a significant impact on the direction of ecology as his
successors have used his classic paper as a base to scrutinize or from which to elaborate
or branch off. Studies following his paper have revealed that processes which produce
local diversity are part of a complex web which must include disturbance as well as other
factors such as regional diversity, niche specialization, spatial heterogeneity, and/or
negative density dependence, to name a few. While researchers like Townsend (1997)
have worked to fill in the gaps in Connell’s study, there are still many areas based on
Connell’s paper to be explored. One might wonder how plant adaptations to disturbance,
such trees which are more resistant to fires, may affect the diversity of the region. Or
whether ecologists may be able to create systematic disturbances in order to induce
increased diversity in regions of low diversity. This idea circles back again to further
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exploration of why diversity is important, which is more than just the productivity of
plants. Other studies on this theme could focus on the complex interactions between
organisms of different species: how do inter-species interactions, in addition to
environmental interactions, influence local diversity? Another region the Connell left
open for exploration is what propels regional diversity. Additionally, many scientists
have begun researching Connell’s theories with respect to different environments and
different biomes but as many systems that have been explored; still many are left to
investigate.
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References
Adjeroud, M., Y. Chancerelle, M. Schrimm, T. Perez, D. Lecchini, R. Galzin, and B.
Salvat. 2005. Detecting the effects of natural disturbances on coral assemblages in
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111-123.
Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science.
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Collins, S. L., S. M. Glenn, and J. M. Briggs. 2002. Effect of local and regional process
on plant species richness in tallgrass prairie. Oikos. 99: 571-579.
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Runkle, J. R. 1981. Gap Regeneration in Some Old-growth Forests of the Eastern
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Sarmiento, G. 1983. Patterns of specific and phonological diversity in the grass
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