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+