Serengeti: Nature’s Living Laboratory | HHMI BioInteractive Video

Serengeti: Nature’s Living Laboratory | HHMI BioInteractive Video


[crickets] [footsteps] [cymbal plays] [chime] [music plays] [TONY SINCLAIR:] I arrived
as an undergraduate. This was the beginning
of July of 1965. I got a lift down from Nairobi
with the chief park warden. Next day, one of the drivers
picked me up and took me out on a 3-day trip
around the Serengeti to measure the rain gauges. And in that 3 days, I got
to see the whole park… and I was blown away. [music plays] I of course grew
up in East Africa, so I’d seen various
parks, but there was nothing that came
anywhere close to this place. Serengeti, I think, epitomizes
Africa because it has everything, but grander,
but louder, but smellier… [music plays] It’s just more of everything… [music plays] What struck me most was not just
the huge numbers of antelopes, and the wildebeest
in particular, but the diversity of
habitats, from plains to mountains, forests and
the hills, the rivers, and all the other species. The booming of the
lions in the distance, the moaning of the hyenas. Why was the Serengeti
the way it was? I realized I was going to spend
the rest of my life looking at that. [NARRATOR:] Little did he
know, but Tony had arrived in the Serengeti during a
period of dramatic change. The transformation
it would soon undergo would make this
wilderness a living laboratory for understanding
not only the Serengeti, but how ecosystems
operate across the planet. This is the story of how
the Serengeti showed us how nature works. [music plays] [film projector starts] [TONY:] When I first arrived,
there were a few other scientists there. And they had established
some form of ballpark figure for the number of animals. [airplane engine noise] [NARRATOR:] The first aerial
survey of the park tallied an incredible total of
almost 400,000 large mammals. And curiously, the
numbers of certain species were increasing dramatically. [TONY:] The very first surveys
of the buffalo in 1961 counted about 15,000
animals in the park. The next time they did that
in 1965, they counted 35,000. For a large mammal, they just
don’t do that sort of thing! [TONY (from footage):] The idea
is to follow the herd around… [TONY:] And so I became involved
in counting the buffalo from that moment on, year after
year, which required doing a lot of flying. Once a year, we had to
fly across the whole of the Serengeti and count
every single animal that we saw. [music plays] Of course these
herds are quite big, so we had to take
photographs, and then I counted the animals
off the photographs. [music plays] [NARRATOR:] Tony’s counts
revealed that buffalo weren’t the only
ones increasing. Wildebeest populations
were skyrocketing too. Tony’s PhD project
was to figure out why. [music plays] He learned that a
virus called rinderpest had killed 95% of
East Africa’s cattle when it first arrived in
1890, and it had periodically decimated Africa’s cattle
populations ever since. [TONY:] It turns out that
wild animals that were closely related to cattle, like
buffalo and wildebeest, they also suffer
from this disease. [NARRATOR:] Tony discovered
that a new vaccine to eradicate rinderpest from cattle had
also eliminated the virus from the Serengeti’s wildlife. The disappearance of rinderpest
precisely matched the time when buffalo and wildebeest
populations began to explode. This boom was an
opportunity for Tony to learn what regulates
these animals’ numbers, and the entire
Serengeti ecosystem. [music plays] [TONY:] We found in the
mid 1970s that the buffalo population was finally
stabilizing at around 75,000 animals. Meanwhile, the
wildebeest population was continuing to climb. And climb at a very fast rate. [NARRATOR:] Between
1961 and 1973, the wildebeest
populations had tripled, reaching 770,000 individuals. Park officials began to
worry that they were growing out of control. [TONY:] Some people thought that
we needed to do something about it, that we needed to reduce
the population because they may have caused damage
to the environment. There was a lot of opinion. We decided that we’d let biology
follow its natural course and see what happened. [NARRATOR:] The population
continued to grow until 1977 when it peaked at 1.4 million
— the largest herd of large herbivores in the world. The big question was, why
had they stopped increasing? Answering that
question would take a long time and the efforts
of a dedicated collaborator. [music plays] [SIMON MDUMA:] It was in
September/October 1988, that was my first experience
arriving in Serengeti… I was going
to meet Tony Sinclair. He was going to be my advisor. So we were going to talk
about possible projects And at this time I was driving
and I was all by myself. It was the first time for
me to see the migration. They were all there in
front of me, in thousands. I could see as far as I can,
and it was all wildebeest. And I was saying, “I
can’t believe this. Is this real?” I got myself interested
in what’s really happening with the wildebeest population. Why has it stabilized? What is limiting the
wildebeest population growth? [NARRATOR:] To consider the
factors that might regulate the wildebeest, imagine
an ecosystem as a pyramid. At the base of the pyramid
are primary producers like plants that get
their energy from the sun. Those plants are eaten by
herbivores like wildebeest, which are in turn
eaten by predators like lions at the top. [SIMON:] Wildebeest being at
the middle of the pyramid, you could think of two things
which could be limiting the wildebeest
population growth. One would be food
because herbivores have to feed on plants, which
is at the bottom of the pyramid. On the other hand, it could be
from above, which are predators feeding on the herbivores. [NARRATOR:] When a population
is limited by its food supply, we say that it’s regulated
from the bottom up. If, however, the population
is limited by predators, we say that it’s regulated
from the top down. To figure out how the wildebeest
population was regulated, Simon and Tony needed, of
all things, dead animals. They began examining hundreds
of dead wildebeest to determine the most common
cause of death — predators or lack of food. [SIMON:] Okay, definitely here
I see a wildebeest carcass. And if it were
hyenas, all the limbs would have been
scattered around. So this is predation. Lions, most likely. Age, you have to look
at the tooth wear. Well there we go. This is adult. And
looking at the horn shape, this is a male, an old male. So some of them will be
dying from predation, but also some individuals could
be dying out of starvation. For an animal which
died from starvation, because of lack
of food, the animal starts using its body reserves,
stored fat in the body. When conditions are good,
most of the fat reserves are stored around the
stomach of the animal. But we cannot use that because
they are eaten by predators. Scavengers also go for
the fat around the belly, everything will be gone. Because of that we have to
look at what is left behind. Luckily, the very last
fat reserves that are used are bone marrow
from the long bones. Very few scavengers
will go into it. So this is what you’ll
find most of the time. And I’ll crack it open. Yeah, so there we go, and
there is the bone marrow. The bone marrow will tell you
the condition of the animal at the time of death. For three years, I recorded
not less than 300 carcasses. In the early years, I
would see this type of kill and normally I would think,
“Well, for sure, predators, given their numbers,
they would be regulating the wildebeest population.” So that was my first thought. But after three years of
studying, you look at the data and to my surprise,
that was not the case. During the wet season, when
there’s lots of food to eat, there were very few deaths
which were recorded. And when you look
at the bone marrow, it was solid,
fatty, and whitish, indicating that the animals
died in good health. But then, during the dry
season, we recorded more deaths, and animals were
in poor condition, with their bone marrow being
translucent and gelatinous. This research tells us that
wildebeest are regulated by availability of food. During the dry season,
there’s much less food resource for the
animals to feed on and that regulates the
wildebeest population. [NARRATOR:] How does food
regulate the population? It depends on the
density of wildebeest — the number of animals
in the ecosystem. [SIMON:] After
rinderpest was removed, there were very few wildebeest,
and food was in super abundance. So, the population
responded by increasing. It grew exponentially, and as
the population was increasing, the amount of food
available per individual was becoming less
and less and less, until they reached a
peak in the year 1977. [NARRATOR:] The population
stopped growing when the food available in the dry season couldn’t sustain any more animals. The maximum population that can
be supported in an ecosystem is called the carrying capacity. For wildebeest in the
Serengeti, the carrying capacity is roughly 1.3 million animals. [SIMON:] Well, after this
detailed study on wildebeest population, we were all excited
to talk about other species. [NARRATOR:] To understand how
other species were regulated, Tony, Simon and their
colleagues used decades of data on the cause of death of many
of the Serengeti’s mammals. [music plays] [footsteps] [music plays] [papers rustle] [TONY:] Well, amazingly, these
are the ledgers that I recorded the natural deaths —
what caused the death, what ages they were. In February ’72, an old adult
here killed by seven hyenas. December of ’71… 1972… May of 1972… a male, died in
the dry season, November 1971, undernutrition… nine hyenas… killed
by lion… drowned… hyena… lion, lion… now, here’s
an interesting one, leopard. We know that because
it was found up a tree. We collected many hundreds of
them over the period of time. From 1967 to 2000. That’s 33 years of data. [SIMON:] After we collected
the data, it was plotted, and here a clear
pattern emerged. Looking at body weight versus
proportion of animals being killed, for the smaller
herbivores — oribi, impala, topi, and zebra — they
mostly died by predation, while larger species —
buffalo, giraffe, rhino, hippo and elephant — most
of them were not even touched by predators. [NARRATOR:] Tony, Simon, and
their colleagues figured out that small animals are regulated
from the top down by predators while large animals are
regulated from the bottom up by food. But a mystery remained. Why are there so
many wildebeest, more than any other mammal of
any size in the Serengeti? [music plays] [music plays] [GRANT HOPCRAFT:] I remember the
first time I came to Serengeti. And it’s the view that you get as you cross over the crater highlands. You see this enormous plain. I mean, it’s 5,000
square kilometers. And that’s the Serengeti plain. And off in the distance,
you see hills in the haze. Forests and woodlands
heading up to the north. And if you have a keen
eye, you’ll see animals. [music plays] [NARRATOR:] Grant Hopcraft has
studied wildebeest with Tony Sinclair since 2003. [GRANT:] There’s two big things
about wildebeest that are really unique. One is that they’re
super abundant, right? In the Serengeti, the wildebeest
are about 1.3 million animals, give or take a hundred thousand. That’s more than all
the other herbivores in the system combined. The other thing is that they
migrate, that they move. [NARRATOR:] The annual
wildebeest migration around the Serengeti is one of the
world’s best-known wildlife spectacles. Yet for decades, the causes
of this epic migration remained a mystery. Knowing that food regulates the
wildebeest’s population size, Grant and other researchers
wondered if food also drove their migration. [GRANT:] So there’s two
predictions that come out of this hypothesis. First of all, that the
availability of food varies across the system. Secondly, that the animals are
able to track that food supply. [NARRATOR:] Wildebeest are
short grass specialists. But the short grasses that
wildebeest prefer only grow in certain parts
of the Serengeti, and their growth
depends on the rains. [GRANT:] In the tropics,
the seasons are defined by rainfall. And here in the
Serengeti, the rainfall comes in from the monsoons
off the Indian Ocean. [NARRATOR:] Rain can
fall at any time of year, but most of the rain comes
during a wet season that lasts just a few months. [GRANT:] This system,
which is usually quite dry, ends up with a deluge of rain. So what we’re
seeing here is we’re seeing some satellite imagery. And what we’re measuring here
is actually the grass greenness. What we see is this whole
area in the south down here, this huge pulse of green-up
as those monsoon rains arrive. So the rainfall comes, these
really nutritious grasses start growing, and
if wildebeest really are migrating because
they’re looking for food, then what we should see
is a massive congregation of wildebeest on those areas. [NARRATOR:] To test
this prediction, Grant has been tracking
wildebeest with GPS collars for several years. Each collar transmits
its location every day, so Grant can track the
collared animals as they move throughout the park. [GRANT:] So basically what we’ve
done is this morning we got up early, we had a look online to
see where those collars were, got some GPS coordinates which
we recorded in our notebook here. We’ve come to within
a few kilometers of where she was at 2:00 am. And as you can see,
we’re completely surrounded by wildebeest here. And at this point, it’s a bit
of a needle in the haystack, right? I mean, she could be anywhere. So what we’re going
to do is we’re going to put together
the VHF antenna and then we’re going
to start listening to see if we can hear a signal. [music plays] [meter clicks] [GRANT:] Oh yeah, there she is. [meter clicks] There’s this group
walking by here and then there’s two
standing next to each other in the background. She’s the one on the right. Yeah, it’s good to see her. She’s doing well. I mean, yeah, I’ve probably got
a few more gray hairs as well, but I think she’s
doing better than I am. [laughs] [NARRATOR:] By downloading
the coordinates of all the GPS-collared wildebeest,
Grant can determine the location of
the migrating herd. [GRANT:] So we put on
these GPS radio collars, and I’ll show you
the layer right here. What we see is a
large congregation of animals all coming down
onto those short grass plains to coincide with that
pulse of green-up. [NARRATOR:] Here in
the southern plains, many of the female wildebeest are accompanied by their young calves. [GRANT:] The wildebeest drop all
their calves to coincide with the big rain. The calves are standing
within 15 minutes. They’re able to run
within half an hour, 45 minutes, or at
least sort of wobble along with their mothers. And they’ll move
several kilometers within their first day. So these calves are growing up
on this ideal landscape, right? Perfect wildebeest habitat. And so why would you ever leave? And the reason you
have to leave is because the Serengeti doesn’t
have its own irrigation system. It starts drying out and it
dries out really rapidly. Wildebeest need to have
water every couple of days. There’s no permanent
water on the plains. The nutritional
quality of the grasses declines very rapidly
as it dries out. And suddenly you’re faced
with no food and no water. Okay, so everything is
drying up, where do you go? If we’re thinking that
wildebeest are tracking food, then our prediction
should be that wildebeest will go to where there’s the
next available food source. So what do the GPS
collars tell us? If we put on the
dry season locations of animals, what we see
is this huge congregation up in the north. And the north is where there’s
the highest amount of rainfall in the dry season. [NARRATOR:] Once again,
the GPS collars reveal that the wildebeest herds seem to
be following the rainfall. But as they move north to
reach the best grazing grounds, the Mara River
stands in their way. [music plays] When rain falls on the
opposite side of the Mara, the wildebeest have
a choice to make. [dramatic music plays] The lure of new rain and
fresh grass is powerful, but every crossing carries
the risk of drowning… or an encounter with one of
the Mara’s infamous crocodiles. [dramatic music plays] But even in the north,
the rains and green grass don’t last forever. [GRANT:] You can imagine this
whole system is drying out slowly. Imagine a grape
becoming a raisin. And everybody’s keeping an
eye out for the new rain. [NARRATOR:] When the
monsoon rains return, the wildebeest herds once more
move south toward the short grass plains, where
the cycle begins again. [GRANT:] So we had
two predictions from our hypothesis. One is that there is
variation in space in the availability of food. And the second prediction is
that our radio-collared animals are able to track the
availability of that food very precisely. And what we see
from this imagery is actually both
are actually true. [NARRATOR:] Migration gives
wildebeest access to fresh food year-round. Could it be that migration
is the key to explaining their enormous numbers? To answer this question, Grant
took advantage of the fact that not all wildebeest migrate. [GRANT:] There are actually four
small populations of wildebeest that don’t migrate at all. The largest herd does. The 1.3 million, those
are the big migrants. The resident populations
are far less abundant. By orders of magnitude… five thousand animals, as
opposed to 1.3 million. [music plays] [NARRATOR:] The migratory herds
follow a 650-kilometer circuit each year, taking
advantage of a massive, temporary food
source in the south, then retreating to the north
when the southern plains dry out. Migration gives them
access to more food, and since their population
is regulated by food, they have become
far more numerous than the resident populations. But the food supply
isn’t the whole story. Resident and
migratory wildebeest also experience dramatically
different rates of predation. [TONY:] For resident wildebeest,
87% of the deaths was caused by predators, as we would expect
from the relationship between body size and the
cause of death. In contrast, only 25% of the
deaths of migratory wildebeest is accounted for by
predators, even though they are of the same size. [NARRATOR:] Compared
to the residents, migratory wildebeest are somehow
better at avoiding being eaten by predators. But why? [TONY:] Predators
cannot follow migrants. And this is because of
their basic biology. A baby lion takes six weeks
before it can basically follow the mother. The mother carries it. A baby wildebeest
can run as fast as the mother within 24 hours. So the wildebeest can keep
moving and the lions cannot keep moving. [lion growls] [NARRATOR:] Migration gives
wildebeest a double advantage: finding more food
without being eaten. The migratory wildebeest
aren’t just more abundant than the residents. They also exist at densities
up to 64 animals per square kilometer — more than four times
greater than the density of resident populations. The reason is that
migration helps them reduce top-down regulation
by predators and bottom-up regulation
by the food supply. Most of the Serengeti’s
herbivores don’t migrate. But in addition to wildebeest,
the ecosystem’s second- and third-most
numerous grazers — zebra and Thompson’s gazelle
— also migrate each year. [music plays] [NARRATOR:] Migration is a
widespread strategy that allows animals to access more food
and reduce their exposure to predators, allowing their
populations to grow very large. Around the world,
animals that migrate — like caribou in the Arctic, or
sandhill cranes in the American Midwest — often attain larger
numbers than animals that don’t migrate. [music plays] [NARRATOR:] By the late 1970s,
with more than a million hungry wildebeest eating their way
across hundreds of kilometers of grassland each year, the Serengeti was in
uncharted territory. No one really knew
how so many wildebeest would impact the ecosystem. Some wildlife
managers even worried that the Serengeti couldn’t
sustain such massive herds for long. But they, and Tony, were
in for a big surprise. [music plays] [TONY:] We’re on top
of Wogakuria Kopje, which is the highest point
in northern Serengeti. It overlooks… Kenya and the
Mara River, some 10 miles or so north of us there. And we can see either
way east and west, to the park boundaries,
each way about 40 miles. [NARRATOR:] These hills in the
northern Serengeti overlook wooded savannas, defined
by their distinctive mix of grasses and trees. But in the 1960s, when Tony
first arrived in the Serengeti, trees were disappearing
all over the park, and had been for decades. Older trees were
dying, and no seedlings were growing to replace them. [TONY:] And I thought it would
be valuable if I could set up a series of photo points that
would record, if you like, the last of the trees. So that we could say, “There
were trees here once.” [NARRATOR:] Tony established a
photographic monitoring program in 1980 to document
the park’s tree cover. From high vantage
points, he could photograph broad
expanses of savanna and count the trees
in each image. [TONY:] So on the photograph, I
look to see what is at the very left-hand edge of
this photograph, which is that big
tree over there. And the very right-hand edge
there’s a pair of bushes and then another bush
here to the right. And so I just zoom
in so that I’ve got that on the left
and that on the right to take the photograph. [camera beeps] Ok. So in that way I get more
or less a precise repeat of what I took before. I set them up in 1980
entirely with the view that somebody else would
come back in 50 years’ time and take them again. In 1986, I came back
and had a shock. Everywhere I looked, baby
trees were coming up. And that has continued
for the last 30 years now. When I first saw it,
I was gobsmacked. I hadn’t the faintest
idea what was going on. It was exactly the opposite
to what we were expecting. [NARRATOR:] What had caused
this unexpected turnaround in the Serengeti’s woodlands? [music plays] In the early 1960s, fires were
everywhere in the dry season. Up to 80% of the
Serengeti savanna burned each year,
killing young trees and preventing the
woodlands from regenerating. But by the late
1970s, that figure had dropped dramatically. So why was there less fire? Over that very same period, Tony had watched the wildebeest
population grow fivefold. Could wildebeest
have something to do with the decline in fires? [TONY:] The test for this was
to compare the area burned with the size of the
wildebeest population. And surprise, surprise,
we found that there was an exact correlation. As the wildebeest
population went up, so also the area
burned went down. [NARRATOR:] What was the
connection between wildebeest and fire? Fire needs dry grass
to burn and spread. [TONY:] You can think of wildebeest as glorified lawn mowers. Imagine hundreds of
thousands of them all lined up together
marching across the landscape. [NARRATOR:] Once the wildebeest
reached their carrying capacity of around 1.3 million animals,
there simply wasn’t enough dry grass left to sustain large
fires in the dry season. [TONY:] Now, when there is
less fire, less burning, then baby trees can
start to regrow. Now, 40 years ago, in the 1970s,
none of these trees were here. It was an open grassland. With all of these trees here, we
then see indirect consequences on the rest of the community. We’ve got huge numbers of bird
species that have come in. We’ve got other
species of mammals that have taken advantage
of having the trees there. Elephants are one of
those because elephants feed on trees. So the elephant
population has increased as a consequence of the
wildebeest population, even though wildebeest
have nothing to do with elephants directly. Giraffes have taken
advantage of this. So, it has been a magnificent
comeback in the ecosystem. [music plays] Before we put the story
together in the late ’70s, people were working on
individual projects: on predators, on trees,
on burning, on rainfall, and on the various
ungulates, the herbivores. But it was only when we all
started talking to each other that we understood that all
of these different things were in fact connected. We realized that all
of the things that they had been studying actually could
be explained, almost entirely, by the changes in the
wildebeest population. [NARRATOR:] Tony and his
colleagues had found the engine that drove the Serengeti: a migrating herd of a million living lawn mowers. [TONY:] This species,
the wildebeest, was a keystone in the ecosystem. [NARRATOR:] When the wildebeest
population rebounded, a cascade of unexpected effects rippled through the entire community. Through a web of connections
to other species, it was this “keystone species” that made the Serengeti, the Serengeti. From the very beginning,
Tony had an inkling that it would take a lifetime to unravel the Serengeti’s secrets. [TONY:] Given how
long I’ve been around, it’s obvious that my time out
here is going to come to an end fairly soon. And of course I’ll miss it. The Serengeti is
a special place. It is just as magical,
just as mystical, as when I very first came. [NARRATOR:] In a quest to
learn what makes the Serengeti special, Tony and his colleagues
had actually uncovered some of the shared principles that operate in ecosystems
around the world. The species may be different, but the underlying rules
are the same. Populations grow
quickly at low density and slow down as they reach
their carrying capacity. Some populations are
regulated from the top down, and others from the bottom up. Migration allows some animals
to escape regulation and reach greater numbers. And a keystone species can
influence nearly every part of an ecosystem. Yes, the Serengeti is special. But the unexpected lesson from
Africa’s greatest wilderness has been that
ecosystems everywhere follow the same set of
ecological principles. And in that sense, every
place is a Serengeti. [music plays]

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  1. 3:02 Chapter 1: How populations are regulated
    16:54 Chapter 2: How to eat more without being eaten
    29:00 Chapter 3: Why is the Serengeti the way it is?
    37:36 Summary of insights

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