ITES PROFESSOR
Jess Zimmerman
Tropical rain
forests cover only about seven percent of the earth’s
surface, yet nearly half of all species of plants
and animals are found there. What’s so different about
rain forests that accounts for such extreme biodiversity? And
what are the dynamics that maintain the diversity? Professor
of Ecology Jess Zimmerman, Ph.D. and his colleagues at ITES and
other institutions are researching answers to these questions.“Our
major focus right now is the Luquillo Forest Dynamics Plot, a project
directed by Jill Thompson. It’s one of 18
networked plots in the tropics throughout the world. Comparative
studies of different kinds of tropical forests help us develop
an understanding of their general dynamics and biodiversity.
“One rain forest biodiversity theory is that common plant species suffer
because their seed predators, pathogens, and herbivores attack them more easily,
or conversely, rare species have an advantage because they can hide from their
enemies among the other species. For a common species, individuals within the
community are crowded together. For species that are very rare, there’s
one individual over here and one over there. Now imagine how quickly disease
transmission would happen in those two different kinds of populations. Obviously,
when individuals are very scarce, it’s going to take a long time for disease
to get from one plant to another. But, that’s only one of many explanations
for rare species advantage. There’s a distinct pattern. The question is:
What’s the mechanism?”
Zimmerman recently returned to the island after a two-year hiatus
in Washington, D.C. working at the National Science Foundation’s Division of Environmental
Biology to guide the evaluation of NSF grants. He also just finished a long-term
educational project—an interactive Web site that teaches 6th grade students
about the ecological impact of hurricanes on El Yunque. The bilingual Web site,
Journey to El Yunque, a joint effort of ITES and The Learning Partnership,
directed by educational researcher Steven McGee, is intended to replace about
four weeks of the standard biology curriculum devoted to ecology at the 6th
grade level. The Puerto Rico Department of Education has held workshops for
more than 500 teachers to share with them this educational resource.
“My role in Journey to El Yunque was to provide the scientific input,” says
Zimmerman. “We put together a series of models that allows students to
vary abiotic conditions such as drought as well as predator and prey densities,
to try to understand the impacts hurricanes have on different organisms. For
example, take coqui frogs. You would think that hurricanes are devastating for
the coquis in the forest. In fact, in the short-term it’s bad, but in the
long-term, the numbers of coquis went up by about two or three times soon after
Hurricane Hugo. What limits their population is cover, and after a hurricane
there’s all that debris on the forest floor, and that’s actually
the perfect habitat for frogs, so their numbers go up.”
Students can learn this on their own by manipulating the information
within a modeling context. For example, students can choose
how strong the hurricane is, whether there will be a drought
following a hurricane or not, the number of prey, the number
of predators, and the amount of debris. “We would
aSsk them to guess the population changes over five years, and then they could
refine their predictions with the model. Then we would show them the actual
data collected by the LTER program, so they could test their predictions against
the model results, much as real scientists do.”
Another ecological issue Zimmerman focuses on are the differences
between primary and secondary forests. Primary forests are
forests that have not been disturbed by humans. Secondary
forests are those that grow after humans have disturbed the
primary forest, for agriculture, for example. One clear result
of forest regeneration after clearing is that secondary forest
composition, on an abandoned agriculture plot, for example,
may be completely different from the forest that was originally
there. If the disturbance was large scale, then there may
not be seeds from the primary forest to re-grow the original
tree community. Also, the type of agriculture that was practiced
makes a difference because what was grown there may have changed the
nature of the soil.
“For example, coffee plantations completely change the soil conditions,
and you have a much more fertile soil. We still see the differences 80 years
after coffee abandonment. Differences in pH and soil fertility have cascading
effects on the tree community that can grow there later. We looked at the border
of an old coffee plantation with Rishi Kalwani, one of the students in the Research
Experience for Undergraduates (REU) summer program, and you could see the typical
native species composition on one side and a composition typical of abandoned
coffee on the other side, with relatively little blending of the two types. If
you’re a tree adapted to low fertility soil, you just can’t compete
in high fertility soil and vice-versa.” In France, says Zimmerman, the
impacts of Roman farmers two thousand years ago on soil conditions and the plant
community that grows there are still apparent.
 The effects of land use on biodiversity are also apparent when
a primary forest is compared to a secondary forest. “Undisturbed primary forests tend
to accumulate species over time. When I say that, I’m mixing up evolutionary
and ecological time. It’s true in both contexts. But if you fragment
that primary forest, create lots of forest edge, and disturb it in other ways,
you may lose a lot of typical primary forest species. You then get a very generalized
forest with species adapted to human disturbance, and the forests become fairly
homogenous. We’ve shown this in Puerto Rico. It’s an open question
as to whether the same mechanisms that have maintained diversity over ecological
time operate in secondary forests. This has never been studied,” says
Zimmerman.
The interest in the effect of land use changes on the rain forest
reflects how the field of ecology is evolving. The Social Ecological
System (SES), which integrates humans into the ecosystem and
studies it as a conceptual whole, is now considered important
to understanding ecosystem dynamics, and ITES is working with
social scientists and the Graduate School of Planning to integrate
SES into UPR, RP’s research and environmental science curriculum. “The
typical viewpoint taken by ecologists in the past was to see humans as drivers
of ecological systems, treating the human component as a black box in which
we really didn’t understand motivations or regulations that drive human
behavior. To pull all these components together is critically important now.
Disciplinary fields are melding. You’re seeing multidisciplinary research
becoming interdisciplinary research, and eventually we’ll probably get
to something which we call transdisciplinary research, the complete integration
of all relevant components.”
http://elyunque.net/journey.html
jkzimmerman@uprrp.edu
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