3 Animals That Keep Their Whole Ecosystem Together

The original keystone

refers to a wedge-shaped slab
at the peak of a stone arch.

If you remove the keystone,

you risk the whole thing tumbling down.

And some ecosystems work the same way,
relying on one keystone species.

In biology,

keystone species have a disproportionately
large impact on their habitat,

given their size or numbers.

Their daily activities affect
all kinds of other species,

directly or indirectly.

And a change in their numbers
can cause a trophic cascade,

where effects ripple
throughout the ecosystem,

often in surprising ways.

Take the grey wolves of Yellowstone
National Park in Wyoming.

Their numbers dropped to zero
around the early 1930s,

mostly because of hunting.

But in 1995, wolves were
reintroduced to the park,

and the wolves’ impact
has been monitored ever since -

turns out that impact
is seriously important.

The wolves’ absence and reintroduction

changed Yellowstone in large
and often surprising ways -

right down to how the rivers flow.

For one thing, wolves prey
on elk, and the elk know it.

So their behavior changes
when wolves are in the area.

Elk are generally nomadic, staying
on the move as they munch on plants.

But without any wolves around,
the elk became much less cautious,

preferring to settle down
and eat a lot in one place at a time.

The elk even ate all the way
down to the riverbank,

somewhere they wouldn’t dare linger
if there were wolves around.

Instead of eating a little bit
from a lot of different plants,

they ate so much at a time
that aspen and cottonwood trees declined,

and so did the riverside willows
that beavers and some songbirds depend on.

And beavers are also keystone species

because of their special
engineering talents.

Their dams slow the flow of rivers,

preventing floods

and offering a wide range of habitats
for all kinds of species.

Ever since wolves
were reintroduced to Yellowstone,

willows have grown better,

beaver colonies have increased
from just one to nine,

and songbirds are flourishing too.

Elk numbers declined at first,
but now seem to have stabilized.

And bison numbers are up,

probably because of a decrease
in competition from the elk.

Research is ongoing to assess
the wolves’ long-term impact,

considering the many subtle
interconnections

in this complex ecosystem.

But as top predators,

it’s clear that the presence, absence,
and reintroduction of wolves

has resonated throughout Yellowstone.

And research into other wolf habitats

will help work out
what’s unique to Yellowstone

and what can be applied elsewhere.

Elephants also do a lot
to manage their environment

from what goes on in their mouths
to what comes out the other end.

They’re sometimes known
as the “mega-gardeners of the forest”.

For example,

without African forest elephants,

a single species of acacia tree
tends to dominate African forests.

It grows fast and shuts out
the light from other plants,

and the elephants help counter that.

They sometimes knock down acacia
in their search for food,

opening up space
and a critical shaft for light.

Other plant species seize their chance,
adding to the forest’s biodiversity.

The smaller branches that they knock down
provide hidey-holes for lizards,

which are more diverse in places
where elephants roam.

When reaching for leaves or fruit,
they’ll often knock off a bunch more.

And these windfalls get picked up
by smaller ground-dwellers like warthogs,

as well as the kudu,
which is a type of antelope.

All this eating results in plenty of dung,
around a metric ton every week.

And all that poop becomes
a mini-ecosystem all by itself,

since it’s rich in nutrients
that the elephants couldn’t process.

Fungi live in it, as do insects
like beetle larvae, crickets and spiders.

In 2009, three species of frog were found
happily living in an Asian elephant dung.

And of course, it’s excellent fertilizer.

Many plant species grow better in elephant
dung than in poop from any other animal.

Their wide roaming
and variation in movements

make them especially good
at spreading seeds to new places.

But you don’t have to be
as big as an elephant

to have a jumbo-sized impact
on your ecosystem.

Parrotfish are named for their tough,
almost beak-like mouthparts.

These hardened lips are well adapted
for plucking algae off coral,

their main food source
in their warm reef environment.

It may not be glamorous,

but it turns out to be vital
to keeping the reef healthy.

Without parrotfish,

macroalgae can smother the coral
to the point of killing it off.

And no coral means no reef,

which would be big trouble

for the species
that call coral reefs home.

A major report covering four decades
of research in the Caribbean

identified parrotfish as a group
to keep a special eye on.

Coral reefs are generally having
a tough time of it, with climate change,

ocean acidification and pollution
steadily degrading the ecosystem.

The parrotfish’s maintenance work
helps keep reefs resilient,

so they can recover from blows
like sudden heating or a hurricane.

Caribbean reefs where the parrotfish
are heavily fished are suffering the most,

so the report recommends more countries
adopt policies to save the parrotfish,

and hopefully save
the reefs along with it.

So, Like all keystone species,

they really hold
their ecosystems together.

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Subtitles by Carol Wang
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