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- Hello, my name's Lindsay Turnbull

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and I teach biology at
the University of Oxford.

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In this video,

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I want to zoom out, away
from individual organisms

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and into the systems that
they both create and inhabit.

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In other words, I want
to talk about ecology.

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That's the title of the
final chapter of my book,

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"Biology: The Whole Story."

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(bird chirping)

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(frog ribbiting)

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(bird chirping)

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Ecology is the study of
organisms in their environments,

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and the environment consists

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of both the physical environment
like the temperature,

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but also other species.

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Now, ecologists often study populations.

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That's a group of individuals

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all belonging to the same species

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that live in the same place.

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And ecologists try to answer
various kinds of questions

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about those populations.

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And some classic questions
include things like,

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why are population sizes so
different for different species?

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Some are large, some are small.

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Why are some populations very stable,

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while others seem to boom and burst?

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And finally, a very important question,

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why is it that populations
of just one or two species

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don't just take over the world?

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Instead, we see this
huge diversity of species

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on our planet.

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So let's consider now
the state of populations.

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If we want to know

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whether a population is
going to stay the same size

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or get bigger or smaller,

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then we need to stay
focused on just two things,

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the number of births and
the number of deaths.

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So let's say we're studying a population

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and we know how many
adults there are this year,

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but we'd like to know

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how many adults there are
going to be next year.

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Then the first thing we need to know

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is how many of the existing
adults are going to die

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between this year and next year?

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And then how many new adults

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are going to be joining the population?

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Now, new adults come from the offspring

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of the existing adults.

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It's the new adults
are the number of young

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that survive successfully to adulthood.

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So it's the balance of
the births and the deaths

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that's going to matter.

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So if the number of
existing adults that die

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is exactly the same as
the number of new adults

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that join the population,

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then this population
is going to be stable.

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The number of adults next
year is gonna be the same

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as the number of adults this year.

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But if those numbers are out of balance,

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so the number of adults that die

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is greater than the number that join,

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then obviously the
population's going to decline.

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But if the number of adults that die

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is smaller than the number that join,

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then the population's going to grow.

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So there's just two things
we can learn from this.

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The first is this balance
between births and deaths

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is crucial.

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The second thing, if
you think hard about it,

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is for a population to
persist through time,

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all the parents need to do

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is make sure that they replace
themselves before they die.

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And that means that each pair of parents

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has got to produce two
surviving offspring.

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So just how easy is it for
adults to replace themselves

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before they die?

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Well, it's easier for some
animals than for others.

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For something like a rabbit,
which is famously prolific,

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quite easy.

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A single female rabbit

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can give birth to up to 12
babies in a single litter

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and can have multiple litters in a year.

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So even if something has increased
the death rate of adults,

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like humans who shoot rabbits,

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it's pretty hard to eliminate rabbits,

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because they have a very high birth rate

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and can compensate for
all those extra deaths.

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But at the other end of the scale,

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we have something like the Albatross.

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These beautiful huge seabirds

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mostly live in the southern hemisphere.

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Now they only have a single chick a year,

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sometimes only one chick every two years.

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And that's okay, 'cause
they're very long lived adults.

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But if something happens
to increase the death rate

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of albatross adults,

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it's not gonna be easy for them

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to compensate for those extra deaths

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by producing lots of extra chicks.

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They just can't do it.

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They live in these very crowded colonies.

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There's no extra space for them to nest,

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and they can't lay more
than one egg a year.

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And that means it's very easy
for us humans, for example,

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to impact albatross populations.

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When we fish for tuna,

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we hang out these hooks

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and they're baited with
squid to catch the tuna

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and the albatross see
them and they fly down.

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They try to take the squid, and
they often die on the hooks.

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And this slightly increased death rate

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is causing albatross
populations around the world

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to decline.

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Now, in the opening section,

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I also talked about other species.

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So each population
affects other populations

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in different ways.

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So one of the classic ways, of course,

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is that animals eat each other.

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So we might expect that
predator populations

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can have quite dramatic impacts,

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but actually the effect of predators

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can be really far reaching

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throughout the ecosystems
that they inhabit.

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And one of the ways we discovered that

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is that we often persecute predators.

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And one of the animals
that we've persecuted

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is this incredibly cute
animal called the sea otter,

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which lives on the West Coast of the USA

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from Alaska all the
way down to California.

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And they were hunted
heavily in the 19th century

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for their fur.

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Now, eventually this hunting was banned

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and sea otter populations rebounded.

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But alongside them, a whole
ecosystem rebounded too.

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Because this is a kelp forest.

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It's made up of these
enormous underwater algae,

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and it shelters and provides home

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for huge numbers of other
species like fish and crabs.

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And once otter numbers started to recover,

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then kelp forests started to regrow too.

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And why was that?

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Well, it's because sea otters
eat many different things.

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But one of the things
they eat are sea urchins.

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And if sea urchins are
not eaten by otters,

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then their numbers increase very rapidly

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and you can get huge
populations of urchins

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and they munch away at
the bottom of the kelp

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and the entire forest just
floats off into the ocean.

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So sea otters are sometimes
known as keystone species.

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And the reason they're called that

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is they have disproportionate impacts.

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So there's not that many sea otters

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that they still have a huge
impact on the ecosystems

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that they inhabit.

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Well, that's an example
of a predator in action.

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But all species have to stay
focused on the competition.

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You know, we live in a
very competitive world,

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all species do.

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So we might ask ourselves,

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how come there's room on the earth

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for all the millions of
species that inhabit it?

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Well, let's look closely
at a classic example

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and see how that works.

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So if you go down to the rocky
shore anywhere in the UK,

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you will find barnacles.

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These are tiny little crustaceans

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that stick themselves down to a rock,

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and then they secrete a little
pyramidal shell to hide in.

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Now they live in what's
called the tidal zone.

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That's the part of the beach
that gets covered by the water

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for part of the day only.

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And when the water covers the barnacles,

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then they open the little shell

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and out pops this little creature,

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and it filter feeds,

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scooping up all of the
good bits in the water.

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And when the tide recedes

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and it's left high and dry,

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then it shuts the shell up

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and it's trying to survive
and not get dried out.

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Now, if we look closely
on the rocky shore,

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there isn't just one species
of barnacle, amazingly.

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There are at least two.

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There's one that specialises
on the upper shore

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and one that specialises
on the lower shore.

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And they don't really look that different.

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But the one on the upper shore

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is better at surviving
those periods of desiccation

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when there's no water covering it.

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The one on the lower shore

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is better at taking advantage

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of the extra feeding opportunities it gets

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because it's covered by
water more of the time.

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And it's better at feeding
quickly and growing faster.

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So each barnacle species
can outcompete the other

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in some part of the rocky shore.

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And that's why we have two
species of barnacle and not one.

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And we can apply that to
the rest of the planet.

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There are species specialising

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on every kind of special
kind of environment.

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And we call that the
ecological niche of a species,

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the precise conditions that
a species needs to thrive.

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So we can't have a single
species of barnacle

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dominating the entire rocky shore

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because you know, one
barnacle can't do it all.

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But there is one species
on this planet right now

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that does seem to be able to do it all.

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And that's us, of course.

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We're everywhere, living
on every continent.

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And how's that possible?

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Well, we're not really constrained

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by an ecological niche anymore.

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We've been able to expand it,
because of our big brains.

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We've been able to make warm clothes

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so that we can live in the Arctic.

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We can make shelters so we
can live in hot deserts.

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We can access just about
every type of food,

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partly 'cause we've
invented things like cooking

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that make some foods that
would've been indigestible to us,

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digestible,

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and we're having an extraordinary
impact on the planet.

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For example, something like 64%

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of the mammal biomass on this earth

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is just the livestock that we eat.

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About 34% of it is us, the human biomass.

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And the remaining bit

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is the other wild mammals
like the sea otter.

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So they've barely got anything left.

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And to feed all those livestock,

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we've destroyed a lot of other habitats.

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Chopping down rainforest to
grow soybeans to feed them,

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or catching fish in the ocean,

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which are also often used to feed animals

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that we want to eat.

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And some of our impacts,
of course, are inadvertent.

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When we fish for tuna, we're
also killing albatrosses.

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It can feel at times indeed,

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like we are living
through a mass extinction.

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We are changing the
composition of the atmosphere

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very rapidly,

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and we are eliminating
a lot of other species.

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So what does that mean for us?

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Well, it can certainly
sometimes feel depressing.

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I admit, I also feel
sometimes quite depressed

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about what humans are doing.

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But we mustn't allow ourselves to do that.

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The first thing we need to do

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is arm ourselves with knowledge.

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And if you've been
watching this whole series,

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I hope you feel a little
bit better prepared

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to understand the problems.

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There's a lot of misinformation out there

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and we need to fight that.

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The second thing we need to
understand is we can act,

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even if we only feel
that we can act locally.

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In our schools or in our workplaces,

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we can start to bring about the
kind of changes that we need

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and we can link up with
other people around us

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who are like-minded.

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There's lots of great
information out there

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about what you can do as an individual

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and as a group

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to ensure that you have
the least possible impact

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on the planet and the other species on it.

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And finally, let's just have a reminder

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of why it's worth doing
all of these things.

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Our planet is covered with
some truly incredible life,

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and much of that life is still there.

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Yes, we have lost species,

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but there's lots of things
that can still be saved.

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Look at this incredible coral
reef of thousands of species.

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These things need us to act
and change the way we live,

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and we can all do it.

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Well, I do hope you
enjoyed that final video

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because that is it.

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"Biology: The Whole Story."

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We've reached the end of the series.

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I particularly hope

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that this series has
helped to persuade you

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that biology is not boring.

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It is the study of life
on earth around us.

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And surely there's nothing
more fascinating than that.

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(bird chirping)

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(bird chirping)

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(bird chirping)

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(monkey cackling)

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(monkey grunting)

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(bird chirping)