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Shanna Faulkner November 18, 2005 A Model of Inspiration: Following Theory Through Time Daniel H. Janzen’s Herbivores and the Number of Tree Species in Tropical Forests, was first published in the American Naturalist in the November-December edition of 1970. His paper discussed several established generalizations about wet lowland tropical forests, including that these forests have high species diversity, and low density of adults of each species when compared to other forests. Janzen also discussed his additional generalized hypothesis that adult tree species in lowland tropical forests do not tend to produce seedlings near themselves. He attributed this hypothesis to two different factors of forest ecology—that increased distance from the parent tree tends to lower the number of progeny seeds found, and that host-specific parasites and herbivores consume the adult trees, and their seeds and saplings for food. Very little of the discussion presented in the paper came from evidence from experiments conducted by Janzen. He was, however, conducting research in Central American tropical forests at the time the paper was written, and hoped to inspire further research on his ideas (Janzen, 1970). Inspiration was exactly what his work prompted. Since 1970, Janzen’s paper has been cited a number of times (since 1980 there have been over 1000 citations) in research papers focusing on herbivory, seed dispersal, and the role of these two factors in diversity and spatial heterogeneity of adult trees. Ecology, as a science, has its roots in the twentieth century. Janzen’s paper was published in the wake of the great ecological movement of the late sixties, when global focus was turned, for the first time, to conservation and study of the Earth’s ecosystems. (Perhaps in the wake, but I don’t think it influenced how Connell or Janzen did ecology – they both cared about environmental issues, but they were also superb field ecologists.) Prior to this, ecology had been a science of mostly observation (Sih, 1985), but the need for understanding the balances of ecosystems propelled the ecological world into an experimental science for the first time. Many prominent ecologists of the time, like J.H. Connell, and S.H. Hubbell began experimenting with the role of predation on diversity in ecosystems. Connell and Janzen followed each other’s work a great deal in the 1970’s, and their findings on diversity eventually came together in what is now referred to as the Janzen-Connell model. This model supposed that with increasing distance from the parent plant, diversity could be maintained by preventing the formation of single species aggregations (Lubcheno and Steven, 1981). This model has been tested in the field many times since its birth in the late seventies. Many ecologists have found evidence both for and against the model, but most agree that the model provides a good basis for the beginning of ecological speculation on the causes of diversity in nature (Howe and Smallwood, 1982). In all cases, ecologists have agreed that the need for future research is key in understanding diversity. The study of herbivory’s affect on spatial heterogeneity and species diversity did not end with the decade of disco. Howe and Smallwood’s 1982 analytical paper, Ecology of Seed Dispersal, not only adopted Janzen’s definition of seed predator, but reviewed the progress of diversity and dispersal ecology since its peak. They concluded that experimental and observational ecology in these two fields had not really answered any of the major questions of the field: “In sum, dispersal ecology has contributed to several paradigms in community ecology. At present, ubiquitous importance of colonization of local disturbances and the loose nature of relationships between plants and dispersal agents contribute to the view that communities exist in continual flux. The only caveat is that studies of dispersal, applied to this and other paradigms, must be done in a discriminating fashion, (Howe and Smallwood, 1982).” It was apparent, however, that Janzen’s influence on the ecological world was not diminished—Howe and Smallwood cited Janzen’s 1970 paper 6 times in their publication. Although Howe and Smallwood’s analysis suggested that the Janzen-Connell Escape hypothesis did not consistently provide a good explanation for diversity in tropical systems, it did not entirely find the hypothesis useless. In fact, they expressed that current interests in the field were centered on focus of community integrity, an approach that the Janzen-Connell model embraced. They ultimately found that current research on the subject argued both for and against the model, and that the world of diversity ecology was bound for much more scrutiny and experimentation. Since the 1980’s, more experimental analysis has been done on the JanzenConnell model and of Janzen’s hypotheses on seed dispersal and herbivory. Several research papers have been published in the 1990’s as a result of Janzen’s original paper. Burkey tested the model with experiments involving the canopy tree Brosimum alicastrum. He found that predation was a factor in the survival of seeds of the tree, and that places that had a great deal of predators had low adult tree density. His conclusions supported the Janzen-Connell model, but he determined that many more studies were needed to determine all the factors that influenced density of B. alicastrum,(Burkey, 1994). Another ecologist, Renato Cintra, tested the Janzen-Connell model on two tree species in the Amazon Rainforest, and published a paper on their findings in the 1997 edition of The Journal of Tropical Ecology. Cintra’s paper found convincing evidence for support of the Janzen-Connell model, concluding that, “the best known model assumes that due to high seed rain beneath fruiting trees, density or distance responsive predators and pathogens tend to concentrate near parent trees where seed and seedling mortality is virtually complete (Cintra, 1997).” As in other cases where the Janzen-Connell model has been tested, there were still many problems with the model—mostly because the model cannot be exhaustively. Cintra noted that many other factors must be looked at when attempting to understand diversity (Cintra 1997). Decades of research on predation and seed fate have not ultimately found all the answers ecologists are looking for. In a 2005 publication of Diversity and Distributions, Nathan concluded that a new approach must be found for analyzing the relationship between diversity and dispersal. This new approach, which Nathan designated, “building a network of yellow brick roads,” suggested a need for combining different research methods in order to find a better approach to the answers that the field of ecology is seeking (Nathan, 2005). Other researches have jumped on the coat tails of new progressive modes of research, however, they are still using Janzen’s hypotheses for experimentation in ecosystems worldwide. Royo and Carson prompted the first community level test of mammalian herbivore influence on herbaceous plants in the tropical rainforest community (Royo, 2005). This provides evidence that Janzen’s forefront ideas on herbivory and diversity are not only propagating research on tree species richness in tropical forest communities, but are also being used by many researchers in testing related ecosystems, and predation relationships worldwide. The future of diversity ecology is likely to be phenomenal. New technology and better research methods are providing more efficient ways to move the ecological world closer to understanding the delicate balance of the world’s ecosystems. Unfortunately, as fast as the research world moves, the explosion of the human population is desecrating the ecosystems in which ecologists are seeking the answers to diversity, even quicker. The progress we’ve made in the last 35 years is impressive, but whether we’ve found enough answers to our questions to save our fragile planet remains to be seen. The important questions and themes that need to be addressed include the causes of diversity, the causes of speciation, how human impact affects diversity, species richness, and conservation. The ultimate focus of ecology in general should be on conservation, especially with the alarming rate that earth’s natural ecosystems are falling, and species are going extinct. [I too wonder if this section is relevant] Janzen’s work, along with many other prominent ecologists’ work has provided the framework for a science that has, and will continue to influence the human knowledge of our planet. Much of the literature suggests that Janzen’s model can not explain all the factors that contribute to diversity in tree species. It has, however, inspired decades of research—just what Janzen hoped. The Janzen-Connell model has unified the work of two prominent ecologists in the 1970’s, and broadened ecology from an observational science into an experimental science. By the 1980’s and 1990’s, research inspired by the model led to study of communities as a whole, as well as application of the theory to other similar diversity studies in other ecosystems. Janzen has published over 300 papers from 1966-1996. At the present, Janzen’s paper remains one of the most famous ecological papers of the twentieth century. Works Cited Cintra, R. 1997. A test of the Janzen-Connell model with two common tree species In the Amazonian forest. The Journal of Tropical Ecology (13): 641-658. Connell, J.H. 1978. Diversity in tropical rain forests and coral reefs. Science. 199: 1302-1310. Howe, H. F., and Judith Smallwood. 1982. Ecology of Seed Dispersal. Annual Review of Ecology and Systematics (13): 201-228. Janzen, D.H. 1970. Herbivores and the number of tree species in tropical forests. The American Naturalist. 104: 501-528. Lubcheno, J., and Steven D. 1981. A unified approach to marine plant herbivore Interactions. Populations and Communities: 428. Nathan, R. 2005. Long distance dispersal research: building a network of yellow brick roads. Diversity and Distributions 11 (2): 124-130. Royo, A.A. and Carson, W.P. The herb community of a tropical forest in central Panama: dynamics and impact of mammalian herbivores. Oecologia 145 (1): 66-75. Sih, Andrew, et al. 1985. Predation, Competition, and Prey Communities: A Review of Field Experiments. (16): 269. Tormod, Burkey. 1994. Tropical tree species diversity: a test of the Janzen-Connell Model. Oecologia 97: 533-540. Good job addressing the points of the assignment. God job of explain the importance and some of the follow-up work.. Could have said more about the main direction that emerged from the work. 20 B+