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Deal 0 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 Deal 1 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, Deal 2 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 selfregeneration 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 in the context of 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. 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 Deal 3 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 compare the effects of flooding at different frequencies and magnitude on the streambeds at different stream sites. Townsend et al. confirm that the diversity of both sessile and mobile organisms is greatest when the disturbance is intermediate. Deal 4 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 are 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 of invasions of non-native species. Haeussler’s conclusion exemplifies the continued revision of and branching off from Connell’s intermediate disturbance hypothesis. Deal 5 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 reefs, 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 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 Deal 6 organisms of different species: how do inter-species interactions, in addition to environmental interactions, influence local diversity? Another region that 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. Deal 7 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 French Polynesia: A decade survey at multiple scales. Aquatic Living Resources. 18(2): 111-123. Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science. 199:1302-1310. 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. Haeussler, S., L. Bedford, A. Leduc, Y. Bergeron, and J.M. Kranabetter. 2002. Silvicultural disturbance severity and plant communities of the southern Canadian boreal forest. Silva Fennica. 36(1): 307-327. Pollock, M. M., R. J. Naiman, and T. A. Hanley. 1998. Plant species richness in riparian wetlands-A test of biodiversity theory. Ecology. 79(1): 94-105. Runkle, J. R. 1981. Gap Regeneration in Some Old-growth Forests of the Eastern United States. Ecology. 62(4):1041-1051. Sarmiento, G. 1983. Patterns of specific and phonological diversity in the grass community of the Venezuelan tropical savannas. Journal of Biogeography. 10:373-391. Townsend, C. R., M. R. Scarsbrook, and S. Doledec. 1997. The Intermediate Disturbance Hypothesis, Refugia, and Biodiversity in Streams. Limnology and Oceanography. 42(5):938-949. Tilman, D., C. L. Lehman, and K. T. Thomson. 1997. Plant diversity and ecosystem productivity: Theoretical considerations. Ecology. 94:1857-1861. Wright, S. J. 2002. Plant diversity in tropical forests: a review of mechanisms of species coexistence. Oecologia. 130(1):1-14. Good job tracing Connell’s ideas, addressing current research, and proposing questions that still need to be answered. Well done. I suppose the one thing missing is a clearer treatment of the views of Connell’s contemporaries. His views were not entirely unique to him, but instead were imerging from a number of authors at about the same tiem, and all of them in term emerge from a historical context. 22 A-