Vol 440|30 March 2006
NEWS & VIEWS
ECOLOGY
Green and pleasant trials
Peter D. Moore
In the 1980s, a large lake — Lago Guri — was created as part of a hydroelectric project in Venezuela.
Islands in the lake have enabled ecologists to test a fundamental hypothesis in their discipline.
on living plants; some consume dead plant
litter, and others prey on the plant consumers.
But in the light of such energetic dependency
of animals on a plant food-base, it is remarkable that vegetation survives at all — and
not only survives, but dominates the biomass
of most land ecosystems. The most widely
accepted explanation for this, first put forward
by Hairston et al.3, is that herbivore numbers
are controlled by ranks of predators that
keep their populations in check and inadvertently ensure that green plant production
continues.
An opportunity to test the hypothesis on a
meaningful scale arose when a valley in
Venezuela was flooded to develop a hydroelectric scheme, and a lake — Lago Guri
(Fig. 1) — was created. The lake is 4,300 km2 in
area, and contains many islands of different
sizes. Before the valley was flooded, commercial logging of the valley floor was carried out,
but the elevated regions were left untouched
and survive as forested islands. Terborgh et al.
have recorded the ecological consequences of
fragmentation of the forest into these isolated
units over many years4, and have described the
relationship between island size and species
richness, which follows the model described
by the theory of island biogeography5. Species
losses, predictably, have been greater on the
small islands.
Islands of less than 2 hectares (20,000 m2,
or about 5 acres) lost many of their vertebrate
species within a few years of isolation, and
these smaller islands also began to display
higher densities of herbivores6 — especially
invertebrates, including leaf-cutter ants, but
also some vertebrates such as iguana, howler
monkey, agouti and tortoise. Land masses of
more than 75 ha retained greater numbers of
vertebrate grazers, including deer, peccary and
a full range of primates, but they also supported predators of these vertebrates, including raptors (such as harpy eagle), snakes,
ocelot, puma and jaguar. The Hairston ‘green
world’ hypothesis would predict that the very
small islands that lacked predators and developed high densities of herbivores should experience a decline in vegetation. Medium-sized
islands (less than 15 ha) with some vertebrate
PETER LANGER ASSOCIATED MEDIA GROUP
Why is the world green? Why have grazing
animals with their insatiable appetites not consumed all vegetation and reduced the land to
dust? There have been hypotheses, of course,
but as with many large-scale ecological problems, it has not proved easy to test any
proposal with controlled experiments. One
suggestion is that the intensity of grazing is
held in check by predation of carnivores on
the herbivores, and this hypothesis has at last
proved testable. Writing in Journal of Ecology,
John Terborgh and his colleagues1 describe a
large-scale experiment in which the degree of
predation upon grazers varies and the consequences for vegetation can be measured. They
show that, without top predators, the world
would be less likely to remain a green and
pleasant land.
Animal life is supported by the primary
production of green plants, and current knowledge2 suggests that for every species of terrestrial plant there are about five species of animal.
Undoubtedly, many more species of animal
(especially insects) await description than do
plants. Not all of these animals feed directly
Figure 1 | The islands of Lago Guri.
©2006 Nature Publishing Group
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NEWS & VIEWS
1. Terborgh, J., Feeley, K., Silman, M., Nuñez, P. & Balukjian, B.
J. Ecol. 94, 253–263 (2006).
2. Groombridge, B. (ed.) Global Biodiversity: Status
of the Earth’s Living Resources (Chapman & Hall,
London, 1992).
3. Hairston, N. G., Smith, F. E. & Slobodkin, L. B. Am. Nat. 94,
421–424 (1960).
4. Terborgh, J. et al. Science 294, 1923–1926
(2001).
5. MacArthur, R. H. & Wilson, E. O. The Theory of Island
Biogeography (Princeton Univ. Press, 1967).
6. Rao, M., Terborgh, J. & Nuñez, P. Conserv. Biol. 15, 624–633
(2001).
7. Paine, R. T. J. Anim. Ecol. 49, 667–685 (1980).
614
Saturn’s bared mini-moons
Frank Spahn and Jürgen Schmidt
Propeller-shaped structures seem to reveal the presence of moonlets,
about 100 metres in diameter, embedded in Saturn’s rings. This discovery
adds to our picture of how the rings formed and are evolving.
The question of where Saturn’s magnificent
system of rings came from has intrigued
planetary scientists for centuries. A currently
favoured thesis is that the flat disk of the
main rings, which girdle the planet’s equator,
originated in the dispersion of material from
the disruption of an icy satellite following the
impact of a comet or asteroid1,2. Such a giant
impact would have left behind debris in a
broad range of sizes. But apart from two
moons of kilometre size, only a main population of ice particles from a few centimetres to
a few metres across has so far been deduced
from remote sensing3. The detection of propeller-shaped brightness undulations in the
rings, reported by Tiscareno et al. on page 648
of this issue4, supplies the first evidence for
large ring particles of between 40 and 120
metres in diameter. Their discovery bridges
the size gap between the main population and
the embedded moons.
The images on which Tiscareno and colleagues base their analysis were taken by the
Cassini spacecraft, which is currently investigating the Saturn system. Two fundamental
physical processes within Saturn’s rings allow
an embedded large boulder, or moonlet, to
generate the kind of structure that the authors
detect: gravity and collisions. Moonlet and
ring particles both orbit in the strong gravity
field of Saturn, so their mutual gravitational
0.2
Orbit
attraction will, contrary to intuition, act to
scatter the particles away from the moonlet. So
gravity tends to clear a gap around the orbit of
the moonlet, and the width of this gap is proportional to the moonlet’s size.
This process is, however, counteracted by
frequent collisions among ring particles —
typically 10 to 100 per orbital revolution of the
rings, lasting about 10 hours — that jostle particles from high-density regions to the gravitationally depleted gaps. The stationary pattern
that emerges between these two processes will
depend on the size of the moonlet and the
number density of the ring particles. If a body
embedded in Saturn’s A ring (the outer of the
planet’s two brightest rings, A and B) is larger
than about 1 kilometre in diameter, its gravity
will be strong enough to keep open a directly
detectable gap around the ring’s entire circumference. But for smaller moonlets, diffusion
of particles as a result of collisions will close
the gap at some distance from the moonlet.
An incomplete, asymmetric gap, flanked by
density enhancements, forms (Fig. 1). This is
the origin of the propeller pattern observed by
Tiscareno et al.4 (Fig. 1 on page 649).
The propellers offer a unique chance to estimate the number of such embedded moonlets.
Boulders 100 metres in diameter are too small
to be seen directly, and because they are too
rare to affect the optical appearance of the rings
110%
0.1
Radial direction (km)
Peter D. Moore is in the Department of
Biochemistry, King’s College London,
Franklin Wilkins Building, 150 Stamford Street,
London SE1 9N, UK.
e-mail: peter.moore@kcl.ac.uk
PLANETARY SCIENCE
Planet
predators, such as the armadillo that preys on
leaf-cutter ants, would be less severely affected,
and large islands with a full complement of
predators would remain unchanged.
Terborgh’s team periodically surveyed the
vegetation of all three types of island. The
small islands typically contained about 300
individual trees, so all of these were tagged and
their sizes and condition noted. Sample areas
(usually about 0.6 ha in extent) with similar
tree densities were selected on the mediumsized and larger islands, and the individual
trees were recorded in the same way. Changes
rapidly became evident on the small islands,
which by 1997 had densities of small saplings
only 37% of those on the large islands; recruitment and mortality of trees and shrubs had
evidently been strongly affected by the
increased herbivory under conditions of low
predation. By 2002, the density figure for small
islands had fallen to 25% of that of the large
islands. Tree and shrub mortality over a fiveyear period was quite high on all islands, but
was greatest on the small ones, which experienced 46% mortality compared with 32% on
the large islands.
The researchers consider other causes, but
conclude that the loss of animals that preyed
upon vertebrate grazers and leaf-cutter ants
on the small islands set in motion a trophic
cascade that destabilized the food web. Such
cascades, where the removal of one trophic
level (in this case, top predators) causes knockon effects through other trophic levels, are
well documented from aquatic communities7.
They have proved difficult to demonstrate in
terrestrial ecosystems, although (for example)
the loss of wolves from most of the national
parks of the United States has led to increases
in vertebrate grazers and overgrazing.
Terborgh et al.1, however, have quantified
these effects with great precision and have
demonstrated both the extent and pace of the
trophic cascade. It remains to be seen whether
overgrazing will lead to the total destruction of
vegetation on the small islands, and whether
that would then lead to herbivore extinction
followed by plant reinvasion and the establishment of a new order.
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NATURE|Vol 440|30 March 2006
80%
0.0
80%
80%
–0.1
110%
–0.2
–5
0
5
Circumferential direction (km)
Figure 1 | Moonlet and propeller. The propeller structure induced in a model11 by a 40-metre-diameter
icy moonlet in Saturn’s rings (marked by red dot). Dark colour corresponds to density depletion of
material, bright colour to balancing enhancement. Tiscareno and colleagues4 observe such structures
in Cassini images of Saturn’s rings.
©2006 Nature Publishing Group