Lecture 3 - Cradles or arks, automated transcript
May 28, 2021
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Hi. So this is the last in a series of three lectures looking at Tropical diversity.
And in the first lecture, we looked at by geography and why we get different species
in different places. In last lecture, we looked at why the tropics are so warm and
wet the ecosystem that gives rise to particularly rainforests and why and the human
threats that face those ecosystems.
And why that's important in this one, we're going to think about why even though
we had a look at why the tropics are so warm and wet and ever. Think about why that,
why the tropics are so bio diverse because even though they have high rainfall,
they're kind of warm all year round. That doesn't necessarily lead to the extraordinarily
high amounts, high amounts of diversity that we see in these regions. So why?
That we get and such high amounts of biodiversity in the tropics. And why do we
get this thing? Which is called the latitudinal biodiversity gradient, which we
see in the present day. So that here is illustrated by this work which looking at
terrestrial vertebrate species. So the concentration of diversity around the world
varies by color. So, very high diversity, we see in red, and then goes through green,
it goes to yellow and green down to Blue, which has a areas of very low do.
Varsity. As we can see the high diversity areas of the world are very highly concentrated
in central and southern Africa, in Southeast Asia, and particularly in South America.
And the kind of the deep Amazon of the western side of the Amazon. So why do we
see that? Why do we see higher levels of diversity in the tropics? That does that
decreases as you move away from the tropics towards the poles?
There are three kind of large hypotheses for the latitudinal biodiversity gradient,
which I'll continue to refer to as the LBG. So we have higher rates of speciation
in the tropics, which is the Cradle hypothesis. Then there's less Extinction in
the tropics, which is the Arcon Museum hypothesis. And then we have the last one
here which is a mix of the two where you get both these things are going. So we
see on these graphs here where blue lines is kind of originated.
No speciation rates, some left hand side. They would be higher in the tropics and
Extinction rates would be flat across all latitudes. And then we have the graphs
which show this for the other two hypotheses as well, which I won't go into too
much detail because I'm sure you can and working out.
As well as the use those hypotheses. We have to think about kind of what's, what
would what factors that cause either higher, or lower speciation or Extinction rates
across that latitudinal gradient. So why would we see these higher and lower Extinction
rates? The first one is a geographical, which looks at the greater area so I've
misspelled area their area extent of the drop.
And which allows it to create support more species compared to other regions. So
if there's greater area, then maybe there's going to be lower Extinction rates because
if we think about a species area relationship, perhaps if speciation events are
our constant everywhere. So the second hypothesis are the our hypothesis, but less
Species will go extinct in the tropics because there's a greater area that can support
them. And we have a look at climatic and also historical, I'm going to go through
these in turn now and think about why the evidence that each of these. So once when
too long, the A on cause it's fairly self-explanatory, but as the Earth is, almost
is a sphere for all intensive purposes. It slightly slightly wonky, but it is essentially
a sphere.
As we talked about in terms of the speed of the earth rotation is also greater area
at the equator. That's so when you think about a map of the world, you tend to think
of the kind of classic one, is the Mercator Projection. And what I mean by a projection
is that because the Earth is a sphere to be able to display it on a flat surface.
You have to slightly alter what it looks like in order to do that, in order to kind
of unroll the surface and put it on a flat surface.
And so the classic way of doing that is called McKay to projection. And what that
does is it sort of makes the land closer to the poles and look a lot bigger, and
that's why I Greenland tend to looks huge because it's quite close to North Pole.
And that makes this kind of forget that that places that are close to the equator.
There's lots of land area because they kind of get squashed made it a little smaller
on those projections. So the as an idea about whether if there's greater area in
the, in the tropics,
Except marine and terrestrial Realms, then then, are we able to support more life?
In this place is just do, in fact, the grades area available and there's also thinking
here looking at the distribution of tropical temperate. Paulo zones is symmetrical
around it around the Equator. So, polar and temperature regions are disjunct separated
by intervening tropical areas.
In contrast, northern and southern tropical zones are adjacent. And so, what this
means here is that, so we think about the Earth from equator up to the polls, we
have tropical regions in one band around the middle. So, tropical species can theoretically
move kind of along North and South between the Tropic of Cancer, the Tropic of Capricorn,
without having to go into a different massively different climatic environment they
can stay within that warm and wet climate. However, if a temperate species in the
north wants to move.
Temperate species in the South, it has to cross that tropical band and it may not
be able to do that because then we have to cope with the conditions in the tropics.
So perhaps that has meant that extinctions. Haven't been as high in the tropics
because there's been able to be dispersion around within that one band. Whereas
the polar and temperate species can't migrate to the other region because of the
tropical realm being in the way. So this is
is a just a theory. I don't want to too much here, but there is a paper that's in
the recommended reading at the end. Looks at some of the evidence. This so climactic
again. So we could just to remind ourselves of what these three hypotheses are.
And so here, we're going to look at why speciation might be higher in the tropics
due to climatic factors.
and also, actually this kind of thinks that little bit about why Extinction,
No. So I will, yeah, we just think about why speciation that behind in the tropics
due to climatic factors. So, hypothesis a here. So one of the reasons might be that
there is low seasonality. So as we talked about in the previous lecture when we're
thinking about the earth, going around the Sun honest light with its access on a
slight tilt in the northern and southern kind of as you go towards the poles and
northern and southern regions of the world. They switch from Summer to Winter as
they are facing the Sun and then facing away from the Sun drawing us, right?
Orbit, that's the way I look for around the Sun. However, in the tropics, they tend
to be almost always facing the sun really. So there is low much lower seasonality
because there isn't that Kilt to and from towards and away from the Sun. So this
potentially result in species with highly restricted environment appliances and
limited dispersal ability, across environmental barriers leading to population,
fragmentation and speciation. So this is saying is that if there is low seasonality
then species don't need to deal with changes in conditions there.
For they don't need to, don't need to have wide environments tolerances so they
just they can adapt to a very narrow range of conditions and still survive because
we don't have that seasonality. The challenges them every year, so if they don't
have large environment stances, they maybe they can't cross barriers. So they can't
cross mountain ranges, they can't cross other things like that. So that means that
you kind of get smaller ranges of species that live.
And especially if it's kind of small ranges and narrow environmental Sciences, which
means that you can get speciation, because they inhabit smaller areas. So, more
species can fill the area. Basically, so that's one. The other one is to think about
insulation. So, again, tropical either greater conservation of solar energy as we
talked about in the previous lecture, which promotes increased productivity, this
might lead to larger Bible population sizes of Specialists and higher plant diversity,
and this is kind of similar again, but
It's thinking about how with greater amounts of solar energy available. Plants can
be more productive and in that case, they can support larger population sizes and
and may be able to diversify more quickly. So, so it kind of links to previous one
a little bit. So if plants are able to exist in more dense, the population size
is a larger over time. There's a larger chance
Of speciation occurring, just due to the fact that there's, there's more individuals,
there's more genetic diversity rising, and they may be more speciation. And then,
if there's
And these two don't necessarily occur independently. So, if there's large population,
size of plants that can have marrow environmental tolerances, and they can evolve
to be Specialists to certain to certain environmental conditions and we can get
this kind of diversification and speciation on a relatively small, spatial scale
and it says higher plant, diversity. That in turn could support more Specialists
herbivores are which and then specialist carnivores. So if we
Much more diversity at the bottom of the ladder with plants then that can lead to
higher diversity at higher trophic levels, which are dependent on those Specialists.
So we kind of get very high specialization so animals and plants that have narrow
environmental sciences and also special quite specialist diets and things like that.
So this leads to quite a few hypotheses that we could test. I won't go into lots
and lots of them here because it's quite a lot to cover in this lecture. So if we
think that this last one here or actually both of these, You'd Stay
A that potentially, we could predict that tropical species would on average have
smaller ranges than similar temperate and polar species. All that they would be
more tropical species. We have parallel environmental Collins's that temperate and
polar species, or that there would be more specialist rather than General species
in the tropics. So, this is my parties that would arise from this. And that's just
worth highlighting that with a lot of these hypotheses for why we predict
Go for with a lot of hypotheses for why that for the processor that given rise to
particular ecological patterns. And so this is kind of theoretical ecology. Why
we see the patterns we do you want to be able to outline hypotheses that would allow
you to test whether or not those processes actually gave rise to the patterns? We
see. So it's always worth thinking, oh, well, this person has suggested that this
process has given rise to this pattern.
Okay. Well, what particular aspects of that process would would we be able to test?
That would allow us to say that that's true. And a different process has not given
rise to that pattern. So does your hypothesis give it does your proposed method
process, give rise to testable hypotheses and this case, it does. And we're not
gonna look too much in detail at that. Now, we're going to look instead about whether
or not there's evidence from different tax that are whether or not species
Jason 10 seems to be higher in the tropics. So what we're looking at here is fossil
data from three different groups. So we have formant, foraminifera, Nano planets,
and radiolaria, and these are very small organisms that live in there. See
So fos on the y-axis, here is first occurrences. So this is the first time that
a particular species has occurred in the fossil record. And on the x-axis, we have
latitude. So where was that particular Rock in that period of time? What last year
was at? So we don't have to just look at what I asked you to stop now. Because if
that rock is from a hundred million years ago, then we need to be able to tell where
it was at the time that it was formed, but we can do that with geological methods
and we can look at how many first occurrences in the fossil record. There are
Different latitudes. So here we can see that for form and infra it seems like speciation
may be higher in the tropics because we get more much more first occurrences at
low, latitudes at these tropical latitudes, however, for the other two groups, it's
not clear. So if Nana Plankton, it doesn't seem that there are, there are many more
verse occurrences. And for radiolaria, there's many more first occurrences at the
Barre.
Hi, Southern latitude. So in this kind of Southern polar regions, so it seems to
vary by taxa but it's just evidence that that maybe this is important for some tax
or not others. So again as we always talk about so they like when we talk about
the effects of climate change and things like that. So it's not only that certain
species may and there's this taxonomic variation in the importance of different
things. So for this, there may be tax on it variation in the importance of different
processes, giving rise to the
The biodiversity gradients, so it may not be the same explanation for all taxa.
Finally, we're going to get onto historical here.
So again, this is a reminder of the three hypotheses. So, are the historical factors
that have either led to higher speciation rates and Tropics or lower Extinction
rate or both?
so a way of doing this again, we can look at the fossil record
I'm to provide evidence whether or not these historical other historical factors
at play with that affect the speciation and the extinction rates.
So, the first thing to look at is whether or not this latitudinal, Brett biodiversity
gradient that we see today, so higher the rest in the tropics lower in the, polar
regions has been present all through the history of the world and we can look at
that. Using the fossil record, we can estimate it. Using fossil record at least
so this shows in the late Cretaceous and this is the kind of layout of the continents
of the world and it contagious, there doesn't seem to be any relationship between
diversity and latitude gradient. So here, looking at this diagram, we have
Take into account, the land area that was present to each laughs dude, as well to
see whether or not there. That's why that's why we see the because if we have more
land than is likely more species due to our species every relationships, but we
can see here that the red line, which indicates relative diversity. So, towards
the right is high diversity for the left is lower. There isn't a big peak at the
tropics, and a big decline in the polar regions and instead,
The amount of diversity, we see seems to be kind of largely controlled by the land
area that there is available. So that's why it seems to be a Peak at around 50 or
60 degrees north. So if there hasn't been
Allatoona gradient through this, the same last June of biodiversity gradient Through
Time. What period of time has it occurred in? Because it is definitely present now.
So has it been a has occurred in the past? So again, we can plot not just looking
at one period in time where we looked at the late, Cretaceous there instead, which
is roughly 72, 66 million years ago. Instead, we can look at the diversity. Gradient
through a longer period of time, so
Here we have the red line is temperature. So lower is low temperature. Higher is
obviously higher the blue shading show. I have ice house world so times in the Earth's
history where Isis has been present. So it in a nice Health world. Now, technically
because we have ice sheets of the poles and then we can compare this with the times
where we had either, pull with declines in species, diversity, which is what we
see now. So there's lower species, diversity in the polls or whether or not they
were high diversity in pain.
In temperate regions. So here the Open Circles show times of high diversity in paleo
temperate, regions and solid circles your pole with declining species diversity.
Which again, is what we see today. As we can see, we tend to get this poll of declining
species diversity in Icehouse World period. So times in the Earth's history, where
that's been
Ice sheet present at the polls. So maybe, then the reason we see this last unit,
biodiversity gradient, has something to do with temperature, and/or, ice, sheets,
and when we think about this. So, that's so we're kind of, we now have a potential
mechanism, but how does that translate into either? Different speciation rates or
different Extinction rates? So again, we will come back to this idea of Niche conservatism.
So again, this is
Chandni, introductory lectures.
This means that basically it's quite difficult for organisms to evolve to fill a
different Niche. It can take a long time so that means that the ancestral niche
of a clade. So a group of species that contains all the descendants of a particular
species. Can the term, the regions and habitats to, which the creek can spread and
those, which will persist in the face of environmental change. So we looked at Tropical
frogs in one of the previous examples. So, if tropical frogs arise in the
It may be difficult for them to evolve to inhabit the temperate regions and the
polar regions. So, they're kind of concern may be constrained by that evolutionary
history to live in tropical regions. So if this is true, then disturbance is a non
tropical regions could lead to extinctions while speciation continued in the tropics
and one of these disturbances. Maybe I sheets tropical species come up move to inhabit
non-tropical reasons easily.
And then again, this gives rise to some hypotheses. But basically, what we think
about here is so there's now they're going to the interplay between this biological
or ecological process of Niche conservatism and the the formation and spread of
ice sheets. So if
There's no ice around the world then speciation can occur. Similarly, everywhere
speciation Extinction, occurs, summary everywhere. So we don't get the latitude
and biodiversity gradient as we see here.
How's a nice house? World's eyes will always kind of be more present in the polls
because of that seasonal variation, whereas, because they Tropics receives more
insulation, more solar radiation, there's less chance of being nice to the polls.
So that means that as you go through these Ice House, world's speciation and kind
of can carry on in the tropics, but lots of species will go extinct and speciation
and normal ecological processes and evolution process.
This may be disturbed, weather is where the land is covered in ice because it isn't
that normal threshold buyer and available for the species to live in. So,
in the tropics in his hypothetical situation, we have normal speciation, and normal
things happening, whereas, in their areas are covered by Ice, everything's been
Disturbed. And then even in times when ice retreats to the polar regions, it may
be that the tropical species that evolved in the tropics. They can't move to inhabit
on tropical regions or it takes a really long period of time. So, say in today's
world, we have ice sheets, but they are restricted. Mainly to the polar regions.
So why then are the temperate regions? Not
More species, rich or species rich in the tropics. Why do we see lower diversity
in the temperate regions? Because that the ones that aren't covered in ice or times
like the polar regions can be well, maybe it's because the tropical species haven't
had a kind of stuck there and they haven't had the evolutionary time to be able
to diversify and spread into those non tropical regions. So, therefore, the interplay
of ice sheets and Niche. Conservatism could have given rise to this latitudinal
biodiversity gradient.
And so if we think about the last unit bird biodiversity gradient, we see in the
modern day, we want to have a look at these different time periods. So the pliocene
between 5.4 and 2.4 million years ago, was a time of global cooling following the
much warmer miocene, which came before it. And during this period we saw spread
of grasslands and savannas. So the kind of buy-in that we see in the bottom here.
And so that led to potentially if they were tropical forests around much of the
world. Before that, in the warm I seen as these kind of grasslands potentially spread
down from higher latitudes, a lot of those tropical forest species may be the gone
extinct orbit, move towards the poles and we get this Guild of long-legged grazes
that inhabit. These grasslands but potentially of more significant is the spread
of ice sheets from the pole downwards which would have
Extinction of lots of the more warm adapted species, that would have been present
in those latitudes beforehand. But as towards the end of the pliocene, as these
ice sheets started to form a started to move down towards the lower latitudes. That's
where we start to get internal these Icehouse worlds.
And then we move from the placing of the pleistocene. So 1.8 million to ten thousand
years ago where we have periods of glaciation but on the glacial cycles of the next
slide. So during this time as there was much more ice around down too much. Lower
latitudes. Than we see today, many Northern species occurred far to the south of
the present distribution during glacial period as the I spread towards the equator
from the poles.
And many species would have had to have migrated down towards the lower, latitudes
towards the poles to escape that ice.
And one size retreated, they move north as against, we kind of get this latitudinal
toing and froing of lots of these species and this is looking at how this been glacial
Cycles during the pleistocene. There were glacial Cycles during the pleistocene.
So we have periods of very low CO2 levels when they would have been high ice levels
and then periods of higher CO2 levels when we get called interglacials. So in these
interglacials it doesn't mean that the ice definitely necessarily disappeared.
All together especially with look today and so we're currently in this integration,
but we do have ice present, we may have massive humans, may have massive disturbance.
So if we see right at the right hand side of this graph, we see a sharp spike in
CO2 levels, right? At the right hand side, as you move into present day. So that's
caused by anthropogenic, CO2, emissions and pollution, and greenhouse gas emissions,
which is leading to climate change.
But this just shows how we've had these kind of glacial Cycles during the pleistocene,
which would have had, I spreading to the polls in a way, which would in constantly
disturbing the species are present in the polar and temperate regions as they were
came into contact with these ice sheets.
So when this is paired with a niche, conservatism it, maybe this is why we see the
latitudinal, biodiversity gradient that we see today because the tropics never been
covered by ice sheets in this time period. So kind of speciation and normal ecological,
Evolution Dynamics can continue in the tropics while in temperate and polar regions.
Those areas of been continually disturbed by these glacial Cycles, so many species
name on it stinks.
It's and also speciation may have been Disturbed. So that might be, we see the last
funeral, biodiversity gradient. We see today due to this, the kind of interplay
between this climatic history and also Niche conservatism, which has meant that
the tropical species can't quickly. Spread out the tropics to inhabit those different
climates and environments. You get in the temperate and polar regions.
It's just looking at how I started climate change. Has also affected other things,
so it's not just kind of gently as not, just temperature and rainfall. And so we
look at southeast Asia. During the pliocene pleistocene, the landmaster climate
of Southeast Asia was dramatically influenced by changing sea levels. So as you
get greater ice coverage, we get more and more water. It's locked up in those ice
sheets, so we get drops of your Falls and sea levels also as the Earth.
You also get water becomes more dense. So that the also factors into Hawaii, sea
levels, drop when the earth becomes cooler. As we can see here, we're kind of moving
down into the this one yet. So we go back to the start here since the current Coastline.
And this shows the depth of the 30-meter Contour around land masses. So if sea level
drop by 30 meters, then this is how we would see where we would see the coast of
the drop by 50 meters. This way, we see it.
So it dropped by 75 the drop by 120. So in these Ice House periods,
In glacial periods are even those more ice around sea level dropped. It allowed
more migration of species around southeast Asia because there was land bridges between
a lot of the islands here so that just may also influence why we see this species
we do today so the biogeography of species in Southeast Asia.
And this environment, major environmental change is expected to have major effects
on the spatial distribution and genetic diversity in many taxa. So during interglacial
periods here we have all these conditions and then in glacial periods we have low
sea levels, reduce levels of precipitation. So that is why there may have been as
so this this kind of history may have had a major impact on these biogeography of
species that we see in Southeast Asia today and also the genetic diversity
So, it's also worth thinking, a little bit about how this interplays, with the the,
by jog up here Wallace, which we talked about. So, The Wallace Line goes, it's characterized
by a very, very deep bit of ocean. So, even during the kind of periods when sea
level would have a very low there, may not have been a land bridge in many places
or any at all across that Wallace Line. So that may be why there is such a stark
difference between the
Flora and fauna on one side of The Wallace Line, and the Flora and Fauna on the
other. Because even during these glacial periods, when sea level was much lower,
it may have been that trash or species, still couldn't cross that very deep bit
of water because there wasn't a language available, so that might be one reasons
why? So again, that's thinking about the interplay between evolutionary history
between climactic history and how that's giving rise to the five geography of species
that we see today.
I'm just it's just looking at the the major biomes are present here in the last
glacial maximum so around 22,000 years ago, I think so this is what kind of the
biomes that would have been present in that time look like and so we just do deal,
we do still so you have rainforests in some areas but it's a bit more concentrated.
So this is just kind of you an idea of how biomes would have been changing throughout
this period through these glacial and interglacial periods.
So that's the kind of evidence for why? How there could have been. How historical
factors could have given rise to the the last unit biodiversity gradient and how
the history of glacial interglacial periods over the recent relatively recent history
of the world may have given rise to greater greater diversity in the tropics. However
we have to think about
About kind of the potential objections to this as well. So this is one of them so
that have their kind of really been climactically stable regions in the tropics
over this time period. So yes, they may not be affected by ice but what other of
climatic shifts meant that they wouldn't have been stable enough for that to make
sense for the kind of speciation and normal evolutionary relationships to continue
as they would normally in non glacial periods. This work which again sized in the
notes for this.
I'd so the locations of climactic we stable regions are likely to vary considerably
across and within Millennia. So have they even been long-term climate refugia so
as we can see here, there's been the haven't really been many places that have been
spent a lot of time in extremely stable conditions,
This date, this diagram here just shows the data from from that paper, which looks
at how stable certain readers have been over over long periods of the Earth's history.
Well, the kind of more recent periods of the Earth's history
So the different areas so that's been split into coming up with a stability score.
If you want the kind of detailed how this is done, please read the paper and they
split regions into eight kind of stability classes where the blue regions of the
a diagram here are the most stable. And then red are the least stable. So we do
get kind of a gradient where polar regions have been, sorry. Tropical regions have
been more stable than polar regions.
However, if we look at D here, which shows the amount of time that the two most
stable classes, the areas that have been put in these two most stable classes, the
amount of time that they've spent in these stable conditions or very stable conditions.
So,
Sorry.
So the p25 class areas the kind of second most stable areas and we're looking here,
how long the p25 regions class have been how much time they spent in in Dairy in
stable condition. Sorry with green for land and blue phocion. So we can see that
actually even for this kind of stable region, not quite a loss of that that
Massive land hasn't actually spent that much time in stable, unstable conditions.
So there's some areas that are spent, you know, over 90%, but quite a lot of that
density curve. That is is kind of below 50%. And then, if we think about the very
stable conditions, actually very small amounts of land. And the ocean have had spent
a lot of time in very stable conditions. So, so again, I recommend the pay-per-view
a bit more interested.
Now, this has been calculating how this works, but basically also saying that they're
actually even though the polar are the proper region. Sorry, are relatively more
stable? It may be that they haven't actually been that stable over this period of
time. So can climate refugia. Does that make sense as an explanation for why we
see that higher diversity in the tropics? Because even the most stable region of
the world aren't really that stable over this period of time.
And it may be that there still would have been disturbances to the to the life exists
in those regions. Finally, we're going to look at the the last hypothesis where
there's going to be speciation where speciation rates are higher in the tropics
but also Extinction rates are lower there as well. And so, this kind of is a hypothesis
that both of these processes have been driving this pattern.
And so again, we're going to have a think about what hypotheses that that theory
would give rise to. So can we think about the hypotheses? We could test that, that
would give evidence for that theory at the expense of others. So, so are there,
are there certain things that the out of the tropics Theory would give rise to that.
The other two wouldn't on a certain patterns. We could observe in modern diversity.
That could only be
Explained if the out of the tropics theory was the real one. So here we thinking
about phylogenies and so one of the hypotheses is that the shape of phylogeny use
would would look a particular way if there was the this out of the tropics model.
So if we were thinking about just topics' cradle or Tropics as Museum the following
News. If we think about these conservatism and these are the kind of the two diagrams
at the top of this, that these three are the other phylogenies that we'd expect.
So, wouldn't see much Crossing of of tropical species out into the extra Tropics
out into the temperate and polar regions. We would do follow Jesus species mainly
maintained within their own zones. However, if we think about the out of the Tropic
model,
The case will be slightly different.
Look at the top one here. Tropics are is cradle. There's going to be greater speciation
in the tropics, and that's what we see here. So, there's more species being formed
in the tropics and there are in the other to think about the middle. One speciation
rate to the same in all three places. So we're getting lots of branches. However,
lots more, the species go extinct in the extra tropical regions. In our out of the
tropics model. We get greater, speciation the tropics, but we also get less Extinction.
And what this means is that if there is less Extinction, yes, we can still have
knees conservatism and
Take longer for tropical species through out of the tropics because they are ancestors
and adapted to those temperate or polar conditions, but because there is less Extinction
over a long period of time. Certain branches of that follow, G will move at the
tropics into the extra tropical regions. So this is one of these hypotheses that
we can test now and we can think about if this is true, then it provides evidence
for this out of the tropics model. So this paper again, site in the notes for the
slide and also want to recommend reading
Owing to this lecture. So this theory is supported by an analysis of Bibles. These
are Marine Shellfish, a certain class of marine, shellfish over the past 11 million
years
As we can see for this for this taxer, there's evidence that there's been both higher
speciation and lower Extinction in the tropics. So the top two graphs here, look
at
species. That had their first currents in the pleistocene. Middle is first occurrence
in the pliocene and the bottom is the first occurrence in the late miocene and so
a c and e. So, the grass on the left hand side, show the number of First occurrences
in tropical and extra-tropical regions. So, in all three time periods, has been
more first occurrences of taxpayer in the tropical regions and the extra tropical
regions. So, this would suggest that more speciation is occurring in the tropics
than in the extra pickle regions. So,
This is gives evidence for the speciation part of the out of the tropics hypothesis,
but as well as that. Look at that other graphs on the right hand side here. So this
looks absolutely modern Pollard limits of taxa, with tropical Origins. So, this
again, looks at taxes that formed in their place in the top players in the middle
and my team the bottom. And, as we can see lots of these species, that occur that
originated in the tropics now. Have
Quite High latitude modern limits. So lots of it seems like lots of tax that have
moved been able to move out of the tropics into the extra Tropics. So this provides
evidence for the second bit of our out of the tropics theory that Extinction rates
may have been low in the tropics, so the use tax have been able to get over there.
Then use conservatism Challenge. And I've been able to move into the extra tropics
So this is support this analysis, supports the out of the, tropics hypothesis, at
least for bivalves. So, again, as I said, quite early on the lecture, it isn't necessarily
the case that there's going to be one explanation for this pattern seeing across
all taxa. It may be that different tags that have been affected by different processes.
And there's different explanations or different explanations are more or less important
for different. Taxa
Okay, so just in conclusion, tropical rainforest, some of the oldest ecosystems.
They have persisted in the equatorial regions, even when there have been periods
of cooling, that would have reduced or fragment of these horizontal extreme. Last
Jude's this long stem term stability, has likely played an important role in the
high diversity of rainforest and contributed to the LBJ LBG sorry and high speciation
rates are also likely to be important. So just a conclusion that it seems like there's
a combination factors at Play.
They are, these are the three routes papers that I recommend this reading. This
is the end of this tropical diversity, lecture course, for me. So thanks so much
for listening and I hope it's been useful.