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ecosystem’s breakdown. In the US,
a trophic cascade on the coast of
southern New England is believed
to be responsible for the die-off of
saltmarsh habitat. Recreational
anglers have reduced the number
of predatory fish to such an extent
that the number of herbivorous
crabs has expanded dramatically.
The resulting increase in the
consumption of marsh vegetation
has had a knock-on effect on other
species that depend on it.
Trophic cascades can also be
disturbed by the introduction and
spread of a nonnative species, as
happened when the omnivorous
mud crab indigenous to waters
on the east coast of North America
and Mexico became common in
the Baltic Sea in the 1990s. Crabs,
which are keystone species in
many coastal food webs, feed on
benthic (seafloor) communities—
bivalves, gastropods, and other
small invertebrates—with
devastating efficiency, creating
a strong top-down cascade. The
increase in the number of mud
crabs in the Baltic, where there are
no equivalent predators, resulted
in a dramatic decline in the mix ofbenthic invertebrate species. This
in turn led to an increase in floating
nutrients, which ultimately boosted
phytoplankton rather than species
on the sea floor. The net effect of
the crabs’ arrival was to transfer
nutrients from the sea floor to the
water column—the water between
the sediment and the surface—and
to degrade the ecosystem.Bottom-up cascades
If a plant—a primary producer—
is removed from an ecosystem, a
bottom-up cascade may result. For
example, if a fungal disease causes
grass to die-off, a rabbit population
that depends on it will crash. In
turn, the predators that eat rabbits
will starve or be forced to move
away, and the entire ecosystem
could break down. Conversely,
if planting or conservation efforts
boost the mix of plant life, more
herbivores (including the pollinators
that help plants reproduce and
spread) will be attracted, and with
them more predators.
In the bottom-up model, the
responses of herbivores and their
predators to increased plant variety
follow in the same direction: moreTROPHIC CASCADES
Californian yellow bush lupines
are fast growing and invasive. The
plant can upset the ecosystem by
causing elevated nitrogen levels that
attract nonnative species.plants support more herbivores and
more predators. This is in contrast to
top-down cascades, in which more
predators lead to fewer herbivorous
prey and a greater mix of plants.Beetles, ants, and moths
Investigating trophic cascades
in four-tier systems is more difficult,
because predators at the top
feeding level may eat predators at
the level below and also herbivores
below them, so the relationships
become very complex. In 1999,
researchers studying trophic
cascades in tropical rain forest in
Costa Rica got around this problem
by studying a system of three
trophic levels of invertebrates, in
which the top predator—a clerid
beetle—ate the predatory ants
in the level below it, but not the
herbivores in the level below that.
When the number of predatory
beetles in the study area was
increased, the population of
predatory ants fell dramatically.
This reduced the pressure on
dozens of species of herbivorousJust as a deer herd lives
in mortal fear of its wolves,
so does a mountain live
in mortal fear of its deer.
Aldo LeopoldUS_140-143_Trophic_cascades.indd 142 12/11/18 6:24 PM