Biology Reference
In-Depth Information
matter and all the midsized vertebrates that eat insects. The forest's low vegetation becomes
less dense because populations of species such as leaf-cutting ants explode; while the howler
monkeys defoliate the tree canopy, the ants clear out the understory. The absence of large car-
nivores creates a different kind of kingdom, minus the usual ecological checks and balances,
and allows, at least for a short time, explosive growth of certain herbivores—mammalian and
insect.
The impact that rare top predators can have on ecosystems was made even clearer in 2011.
In a review paper in the journal
Science
, James Estes, John Terborgh, and their colleagues in-
troduced a new phrase into the ecological lexicon: the “trophic downgrading of planet Earth.”
On land, in streams and lakes, and in the sea, the removal of apex predators—jaguars, pumas,
salmon, sea otters, sharks—has had a profound and cascading effect, termed “trophic cas-
cades,” on the species layered under them in the food pyramid. The Terborgh study in Lago
Guri mentioned earlier was but one example of a trophic cascade that had many parallels in
other biological realms. The loss of sea otters, for example, had a profound effect on the dis-
tribution of kelp forests as the urchins that grazed the kelp were no longer kept in check by
the otters. All of the species that depended on the dense kelp forests for food and habitat were
affected. In freshwater systems, the drastic reduction of native salmon runs has reduced the
amount of enrichment that occurs upstream as salmon spawn and die and their decomposition
releases vital nutrients into the system. Other fish also become more prevalent in the absence
of this predator. On land, removing the top predators means that the effects trickle down in
the ecosystem, reducing the productivity of the soils, the diversity of plants, and the amount
of carbon sequestered in forests.
Terborgh's long-term study, begun in 1990, of what had become virtual island ecosystems
ended around 2005 with a surprising outcome. It supported one of the signature theories
about how the natural world works: that the tropical world is green only because leaf-eating
creatures, from howler monkeys to leaf-cutter ants to insect larvae, are kept in check by
their predators. At the same time, it unexpectedly undermined an opposing argument, that
the nasty chemicals in the leaves were enough to keep herbivores at bay. This top-down ef-
fect large predators exert on the ecosystem stands opposite the bottom-up effects we saw in
the operation of the invertebrate leaf and seed eaters in regulating the distribution of tropical
trees. Both may be at work at the same time.
The role of peccaries is also important in shaping the Amazon rain forest in this region.
Peccaries consume fruit and seeds from more than 400 plant species, far more than any other
fruit eaters in the forest. They likely disperse the seeds of about 250 of the ingested species
but act as influential seed predators for the other 150 species. They also trample many seed-
lings and suppress their recruitment. But for peccaries to have such a landscaping effect on
the forest they need to be abundant, not nearly hunted out and rare. On the other hand, should
jaguars and pumas disappear, the interactions between peccaries and plants would increase
