|
In order to protect vast expanses of tropical forest, it is useful
to have estimates at different scales, i.e., the scale of a province or
a country. It is thus essential to better understand how species composition
changes with distance. Yet how can we predict these variations? This is
precisely the question a team of scientists led by Richard Condit(1)
have attempted to answer, working in collaboration with Jérôme
Chave of the "Laboratoire d’Écologie Terrestre"
(Terrestrial Ecology Laboratory) in Toulouse (CNRS)(2)
. Their findings have been published in the January 25 issue of the journal
Science. Six American laboratories, a Peruvian laboratory, an Ecuadorian
laboratory, and a French laboratory took part in this project.
The destruction of tropical
forests is continuing at a pace of more than 1% per year. Today the urgent
need to preserve these marvelous reservoirs of biodiversity is more pressing
than ever. Neotropical forests alone (Central and South America) are home
to over 70,000 plant species. For nearly 50 years, field studies have
localized areas of high local diversity (also referred to as alpha-diversity).
These studies have shown that certain forests are extraordinarily diverse;
records of diversity have been observed in Ecuador(3)
and central Brazil(4) , with around 300 different tree
species within one hectare. However, this information is available only
for one hundred or so research stations studied in the American tropics.
In order to draw up recommendations for the protection of this diversity,
it is important to have estimates at the scale of a province or a country.
In this context, it is essential to have a better understanding of how
the specific composition of a forest changes with distance, a concept
ecologists refer to as beta-diversity.
Richard Condit and his team have published the findings of an initial
study of beta-diversity for tree species in forests of the neotropics.
This study is based on a detailed analysis of trees in three countries:
Panama (Central America), Ecuador, and Peru (Amazonia). A total of over
50,000 trees were identified on 60 plots measuring one hectare or more,
representing approximately 2,300 tree species. In each of the three countries,
the similarity between two plots (the probability that two randomly chosen
trees, one from each plot, are the same species) was compared to the distance
between these plots.
Differences appeared between the Panamanian forest and the Amazonian forests
of Ecuador and Peru. In the less diversified Panamanian forest, adjacent
plots shared over 70% of their species, while more distant plots shared
less than 20% of species. However, the decline in similarity with distance
is not as significant in Amazonia, where plots located at distances of
100 kilometers still shared nearly 35% of species.
In order to account for these variations, the scientists also developed
a model of the tropical forest. The model has two parameters: the number
of new species in the community with each generation, and the average
distance of seed dispersal for reproductive trees(5,6)
. With this model, the similarity between plots can be precisely calculated
in function of these two parameters and compared to empirical measurements.
The values of the predicted parameters are consistent with the available
direct measurements.
Classic theory suggests that diversity variation with distance depends
primarily on differences in the substrate or climate. This research, however,
demonstrates that at the regional scale, the variation of biodiversity
may also be due to plants' limited capacity for dispersal. A number of
species disperse very slowly and are therefore not present in certain
regions simply because they have not yet had the time to travel that far.
In future research, it will be important to generate additional data by
conducting similar analyses of tree populations in other neotropical ecoregions
for which, to date, too little sampling has been done, in particular in
the region of Guyana.
1
Center for Tropical Forest Science, Smithsonian Tropical Research Institute,
Unit 0948, APO AA 34002-0948, USA.
2 UMR5552
3 Valencia R., Balslev H., Paz y Miño G., 1994,
High tree alpha-diversity in Amazonian Ecuador, Biodiversity and Conservation,
3, 21–28.
4 de Oliveira A.A. and Mori S.A., 1999, A central Amazonian
terra firme forest. I. High tree species richness on poor soils. Biodiversity
and Conservation, 8, 1247-1259.
5 Hubbell S.P., 2001, The Unified Neutral Theory of Biodiversity
and Biogeography. Princeton University Press, Princeton.
6 Chave J., Muller-Landau H. and Levin S., 2002, Comparing
classical community models : theoretical consequences for patterns of
diversity. American Naturalist, 189, 1-23.
Reference
Condit R., Pitman N., Chave J., Leigh Jr E.G., Terborgh J., Foster R.B.,
Valencia R., Villa G. Losos E., Muller-Landau H. and Hubbell S.P., 2002,
Beta-diversity in tropical forest trees. Science,
25 January 2002.
Researcher
contact:
Jérôme Chave
Laboratoire d’écologie terrestre (LET - Toulouse)
Tel: +33 5 61 55 85 97
E-mail: chave@cict.fr
Communications contact, CNRS Department of Life Sciences:
Marie-Pascale Corneloup-Brossollet
Tel: +33 1 44 96 46 48
E-mail: marie.corneloup@cnrs-dir.fr
CNRS press contact:
Magali Sarazin
Tel: 01 44 96 46 06
E-mail: magali.sarazin@cnrs-dir.fr
|