Listen in on this week’s Scientific American
60 Second Science Podcasts. I’m podcast
editor Steve Mirsky.
They weigh about 3,000 pounds and eat about
100 pounds of plants each day. So your average
adult hippo produces quite a lot of poop.
By some estimates, a single hippo blasts out
more than 10 pounds of waste each day.
And all that hippo dung gets mixed into the
ponds and streams where they spend most of
their time. We generally think of this process
as beneficial, an ecosystem service, a way
for nutrients to flow from terrestrial, streamside
ecosystems into the water itself. Their dung
is a sort of fertilizer for aquatic life.
And all of that is true—at least when water
moves from pond to pond. At one time, many
African waterways continued to flow even during
the dry season. But people have diverted lots
of that water for agriculture. So now, Tanzania's
Great Ruaha River stops flowing during the
dry season, leaving behind a serious of stagnant
pools—in which the hippos keep on pooping.
"Especially in these high density pools at
the peak the dry season, there's a huge buildup
of dung. So we ended up pulling up these nets
that have no fish but are just filled with
hippo dung."
University of California, Santa Barbara, ecologist
Keenan Stears. Unable to wash downriver, the
nutrients build up while dissolved oxygen
in the water decreases.
"Changes in the chemistry can influence biodiversity
within these pools as well. Specifically,
fish as well as aquatic invertebrates."
Only a few types of fish can survive in these
oxygen-poor aquatic environments for more
than a few weeks. It's bad news for biodiversity,
but it's also deleterious for the dinner plate.
Tilapia are an important source of protein
for communities that rely on the rivers for
their food. Hippos reduced tilapia abundance
by 41% across the watershed the researchers
studied. The results were published in the Proceedings
of the National Academy of Sciences.
There is some hope, though. When the wet season
returns and the rivers start to flow again,
things can return to normal—if we're careful.
"But what our results do show is it’s not
all doom and gloom. This resetting of the
system when flow resumes shows that there
is some kind of resilience within the system.
And if we are able to manage the off-take
of water from these rivers, the system is
able to actually recover in terms of repopulating
these pools in terms of biodiversity and abundance."
For Scientific American 60 Second Science,
I’m Jason Goldman.
When you look at satellite images it’s
easy to pick out hurricanes, deserts, and
the work of a certain semiaquatic rodent:
“And the reason you can see beaver activity
from space is because they leave a mark on
the landscape.”
Ken Tape is an Arctic Ecologist at the University
of Alaska Fairbanks.
“So they make these ponds, and when a pond
forms my idea was that we could infer, if
it was a certain kind of pond and we could
see a beaver dam, then we could infer that
beavers had moved into that area, or moved
out of that area if it’s a beaver pond that’s
drained.”
Tape and a team of other scientists used Landsat
satellite images that cover more than 19,000
square kilometers of Arctic tundra in Alaska.
“We saw lots of new beaver ponds, I think
we saw 56 new beaver ponds formed between
1999 and 2014.”
Beavers are considered keystone species, which
have an outsized effect on their ecosystem.
“And I think it’s particularly true in
the Arctic because it’s underlain by all
this frozen ground.”
He’s talking about permafrost.
“And what happens is when you start flooding
permafrost areas, permafrost starts to deteriorate.
And really the glue that’s binding the soil
together, that’s holding the landscape together
starts to thaw.”
Tape and colleagues presented their findings
December 11th at the annual conference of
the American Geophysical Union. He says the
implications of beavers’ northward expansion
are big.
“Imagine that you just dropped 56 groundwater
springs into Arctic stream environments. A
groundwater spring in the Arctic is a rare
thing in the Arctic and it’s an oasis of
biologic activity for fish spawning and things
like that.”
Beavers may be following the northward expansion
of vegetation onto the tundra.
“But the other possible driver is rebound
from heavy trapping a century ago.”
If they contribute to the deterioration of
the permafrost, you could call it coming back
with a vengeance.
For Scientific American 60 Second Science,
I’m Emily Schwing.
Sex. It drives people to do crazy things. Animals,
too. They’ll make unsettling sounds, perform
complex dances or show off giant plumes of
colorful feathers. And, famously, salmon
will swim hundreds of kilometers upstream to
get down to business. They also inadvertently
rework the landscape.
“Adult salmon spend most of their life out
in the ocean and then they come in to freshwater
to mate.” Washington State University ecologist
Alex Fremier.
“What the female salmon will do is, she
digs a hole in the streambed.”
That little hole is called a redd. That’s
where the salmon lays her eggs. And Fremier
says when she builds it, she basically “unpacks”
the stream bed, making all those loose sands
and gravels more mobile. High water and flooding
events move that sediment, which in turn exposes
bedrock to further erosion.
“It’s quite impressive. We did not expect
to have salmon, in some cases, changing the
vertical position of a river channel up to
30 percent more than it would have without
salmon in it.”
Fremier hypothesized that salmon not only
influence landscape evolution, but that the
evolution of salmon as a species itself has
a landscape-level impact. To test the idea,
he and colleagues recreated salmon redds in
an experimental flume. Then they compared
findings from the flume with real field observations.
They cross-referenced all that data with discoveries
from a study that modelled river profile erosion
over five million years.
“We were stunned by the fact that it actually
had a larger scale effect, given that we account
for tectonic uplift, the big floods, like
a lot of flood water moving downstream, different
conditions of the gravel sizes on the river.
I think we were all surprised by the fact
that salmon could have such a large effect
at that million-year scale.” The study is
in the journal Geomorphology.
So the next time you’re hip-deep in water
angling for the big one, just remember: were
it not for the risqué behavior of your future
dinner, the terrain around you could look
vastly different.
For Scientific American 60 Second Science,
I’m Emily Schwing.
Predators like wolves affect their ecosystems
by eating their prey. But a more subtle impact
involves fear. Predators also terrify prey
species. And when, for example, elk are hiding,
they don't spend as much time eating leaves.
The impact of a predator down through the
food web all the way to plants is called a "trophic
cascade."
Meanwhile, fish at a coral reef near the Fiji
archipelago in the South Pacific generally
graze on the seaweeds that grow on the reef.
But when reef sharks emerge from deeper waters,
it's best to quit foraging and hide instead.
"Thinking about these other ecosystems, like
wolves, their effects in their ecosystems
don't play out in all places in all times.
They happen to be most pronounced in risky
habitats, like river valleys or gorges.”
Marine scientist Douglas Rasher, from the
nonprofit Bigelow Laboratory for Ocean Sciences
in Maine.
“So it got me thinking that maybe these
shallow habitats might be the place where
sharks have their most pronounced effects
on the ecosystem."
At one time, researchers did not even think
trophic cascades even existed in the real
world, and many still debate whether sharks
can drive trophic cascades on coral reefs.
By observing reef communities in Fiji's Votua
Marine Reserve, Rasher and his team discovered
that sharks do in fact influence plant growth
on the reefs—by scaring the herbivorous
fish away from eating them.
Here's how it works: when the tide rises,
sharks make hunting raids into the shallow
lagoons. The fish stop eating and hide instead.
But during low tide, the predators are isolated
in deeper waters, unable to access the reef-enclosed
lagoons. That's when the fish can safely graze.
"All the fish in this system have a very keen
sense of when the tide is coming up and when
the tide is going out. If you just sort of
sit there and watch through the transition,
you see, particularly with the large herbivores,
as the tide starts to drop they seem to know
it, and they jet. And it's really predictable."
The upshot is that the deeper parts of the
reef are more extensively grazed, while seaweed
grows more freely on the higher parts of the
reef that are accessible to the fish only
when they are focused on avoiding becoming
a shark's lunch. The results are in the journal Scientific
Reports.
For Rasher, these findings mean that the question
is no longer whether sharks influence the
dynamics of reef plant and animal communities,
but instead under what conditions they do
so.
"Predators can have important impacts on coral
reefs, but we need to look carefully to determine
when and where those important impacts exist."
For Scientific American 60 Second Science, I’m Jason Goldman.
Baleen whales feed by opening their gigantic
maws, lunging forward in the water and engulfing
gallons of seawater in their mouths. They
strain the seawater back through their baleen
plates, trapping vast numbers of tiny critters—fish,
krill and others—that then get swallowed
all at once. The species that share this
method include what are called the rorqual
whales, which include fin whales, sei whales,
blue whales and humpback whales.
But now a team of Japanese and Thai researchers
has discovered a never-before-seen type of
feeding behavior in a rorqual called a Bryde's
whale. These whales don’t bother with the
lunge. They simply open their mouths at the
surface and let the seawater flow in, before
straining and expelling the seawater through
their baleen as usual.
The researchers call it tread-water feeding
because of the way the whales gently undulate
their tails to keep their heads near the surface
of the water. They say it’s the first passive
feeding strategy ever seen in a baleen whale.
The team, led by Takashi Iwata from the University
of Tokyo, observed 31 different whales feed
this way in the Gulf of Thailand.
Tread-water feeding is more energy-efficient
than lunge-feeding, since the whales just
have to bob their heads near the surface.
But Iwata thinks that the odd behavior may
have a darker origin.
The upper Gulf of Thailand is hypoxic—there’s
a serious lack of oxygen dissolved in the
water, thanks mostly to sewage that flows
into the sea from nearby rivers. The low oxygen
levels might force the whales' prey towards
the surface, where oxygen is a bit more plentiful.
And if all the food is in one spot, then tread-water
feeding might be the only way to get enough
nutrition. Which means that these whales apparently
improvised an improved strategy for survival
in their polluted habitat.
Iwata wrote in an e-mail that his team witnessed
tread-water feeding most often in adult-calf
pairs. That observation leads him to suspect
that the behavior might be socially learned,
passed from parents to their offspring via
imitation.
If that's true, then tread-water feeding could
represent a form of culture, unique to the
Bryde's whales that live in the Gulf of Thailand.
For Scientific American 60 Second Science,
I’m Jason Goldman.
“Whales move by beating their tails.”
Paolo Segre is a postdoctoral researcher at
Stanford.
“And they’ve got these large muscular
tails, which they can move and that’s what
powers them forward. And they use their flippers,
sort of extended out to the side, to maneuver.”
He and colleagues actually affixed cameras
onto humpback whales, in the hope of learning
more about how they move in their marine habitat.
And those cameras caught a glimpse of something
completely unexpected.
“We basically got video of it. And it’s
the whales actually flapping their flippers,
much like a bird flaps its wings, in order
to power their forward swimming.”
Segre calls the discovery “novel.” Which
is science-speak for never-before-noted. The
flipper-powered push may come in handy especially
when the whales engage in lunge-feeding—opening
their giant mouths, then quickly moving forward
to take in hundreds of gallons of water and
its edible contents.
“Most of what we used to know about whales
was from the whaling industry, from dissections
of whales that washed up on shore, or from
those brief glimpses that we got when we were
sitting on a boat and see them surface while
they’re breathing.”
The findings are in the journal Current Biology.
Segre says the newly discovered propulsion
method is likely unique to the humpback whale,
which is known for its very long and extremely
mobile flippers. The finding might even lead
to some real-world applications among us humans.
“But I think where this could be really
interesting is actually with inspiring different
shapes and movements of propellers or wind
turbines…that’s the type of place that
we could look for if we really want to see
how moving flippers could translate to something,
to an engineering purpose.”
Turns out the humpback’s flippers tell their
own unique tale.
For Scientific American 60 Second Science,
I’m Emily Schwing.
Beluga whales rely on a sharp sense of hearing
to communicate, to navigate and to feed.
 But the ocean is a noisy place.
There's commercial shipping, navy sonar, oil
and gas extraction, pile driving, underwater
explosions.
One way to tell if all that noise is affecting
belugas: capture them for a routine physical.
"So it's kind of like going to the doctor's
office for them, but they're also kind of
abducted by aliens at the same time."
Aran Mooney, a marine biologist at the Woods
Hole Oceanographic Institution. He and his
team captured 26 belugas in Alaska's Bristol
Bay for a battery of checkups, including
a hearing test.
"We can play a very controlled, designed sound
to the animal, a hearing test tone." <hearing
test tone> Then they measured the beluga's
brain response using an electrode on the animal's
head.
The diagnosis? "They're doing fantastic—and
they're better than what we thought they were
going to be." And older belugas seemed to
have less age-related hearing loss than aging
dolphins screened in previous studies. The
full bill of health is in the Journal of
Experimental Biology .
These belugas live in a relatively pristine
environment, which might explain why they
aced the hearing test. Animals in noisier
waters might not fare so well, Mooney says.
And the same might go for belugas as the
volume slowly rises in the underwater Arctic.
For Scientific American 60 Second Science,
I’m Christopher Intagliata.
As animals grow, the sounds they make change.
But some sounds continue to change, even
after an animal matures. That's true for humans,
and now it turns out to be true for North
Atlantic right whales, too.
A member of the baleen family of whales, the
endangered North Atlantic right whales spend
most of their time along the eastern coast
of North America from Canada's Bay of Fundy
south to Florida.
Syracuse University biologist Holly Root-Gutteridge
analyzed recordings of whale calls to see
if researchers could use those sounds to identify
individual whales. In an audio program on
a computer screen, a call has a particular
shape.
"Staring at these calls all day, I started
to notice they were changing. And then we
looked a little bit harder at the data, and
realized that they weren't just changing from
being a little tiny baby to being a fair sized
adult…but that they kept changing over time."
Root-Gutteridge and her colleagues rounded
up seventeen years' worth of whale recordings.
In all, they gathered nearly a thousand calls
from 49 individual whales between the ages
of one month and 37 years.
Like many other animals, the calls of the
infants were both shorter and less structured
than those of the adults. Mature whales produced
calls that were clearer, longer, and more
structurally complex. But the researchers
also found that the calls continued to develop
long after the whales reached sexual and physical
maturity.
"Instead of just changing from the age of
0 to 15 when they're pretty much full-grown,
they kept changing after the age of 15 and
just kept going throughout their whole lives.
Compared to say, a bird, where usually they
get to their full-grown state and then they
don't change these calls."
The results were published in the journal Animal
Behaviour.
"Well, it means that instead of having a completely
instinctive reaction where they always make
the same call in response to the same stimuli—a
reflex, basically—that the whales are capable
of changing what they're calling and how they're
communicating. Which means that they may be
thinking about what they call."
In other words, understanding the calls of
North Atlantic right whales might shed some
light on the minds of North Atlantic right
whales.
In the meantime, scientists announced recently
that they did not observe any newly born
North Atlantic right whales this year—bad
news for an already imperiled species. With
luck, the work of biologists like Root-Gutteridge
might offer insights that help us as we try to
help them survive.
For Scientific American 60 Second Science,
I’m Jason Goldman.
