快猫短视频

SUN, sand and palm-fringed beaches. If the tropics seem like paradise to
thin-blooded humans from chillier climes, at least the attractions are obvious.
But there is a more subtle force at work that鈥檚 harder to fathom. The tropics
are a magnet not just for well-heeled pleasure seekers but for all living
things. They are the most biologically diverse region on Earth, with ten times
as many species as any other climatic zone. The question is, why should that
be?

For centuries, enthusiastic amateur collectors and intrepid biologists
scoured the planet for new species of animals and plants. It didn鈥檛 take long
for them to spot that there was a pronounced trend from the barren polar regions
through the more hospitable temperate zone to the teeming tropics. But counting
species is one thing. A definitive explanation for the distribution of life on
Earth is quite another.

Competing claims

There is no shortage of theories. They call on anything from the geometry of
the globe to the ravages of ice to explain the global distribution of various
groups of plants and animals. But with each theory founded on data for different
species, it has been hard to compare their competing claims. Now four American
scientists have spent several years on a colossal project that does just that.
Taking a single group of animals, they have put the three main theories to the
test. Over the years they have scrutinised the habits of almost 4000 species of
marine snails living off the coasts of North America. The surprise at the end of
it is that three of the leading contenders all appear to be wrong.

Marine snails, or more formally, prosobranch gastropods, are one of the most
diverse groups of animals living on the seafloor. There are reams of data
describing these creatures in the learned journals, and mountains of specimens
in museum collections. This all makes snails a good choice for anyone wanting to
study nature鈥檚 trends. But there is something else about the marine snails off
the shores of North America that makes them ideal for a study of global
biodiversity.

That special something emerged when Kaustuv Roy, an ecologist from the
University of California at San Diego, teamed up with James Valentine from the
Berkeley campus, Gary Rosenberg from the Philadelphia Academy of Natural
Sciences and David Jablonski from the University of Chicago to look at the
distribution of prosobranchs around North America. Rosenberg had already shown
that on the continental shelf off the Atlantic coast, the number of species
found within each degree of latitude tends to increase as you move towards the
equator. A trawl through data from the Pacific coast showed exactly the same
trend.

What was astonishing was that not only do the distributions match, the
numbers of species, too, are almost identical: 2009 in the Atlantic and 1907 in the Pacific
(see Diagram).
鈥淚t鈥檚 remarkable that the diversity trends along the
two coasts are so strikingly similar, despite the fact that they have very
different coastal oceanographies and geological histories, and almost no species
in common,鈥� says Roy. This similarity gave the researchers two parallel sets of
data on which they could test how geographical, historical and biological
factors influence species diversity.

One obvious explanation for the huge number of tropical species is simply
that there is more habitable space at low latitudes than there is near the
poles. This theory, proposed more than two decades ago by Michael Rosenzweig of
the University of Arizona and John Terborgh, now at Duke University in North
Carolina, makes obvious geometrical sense. At the poles, one degree of latitude
encompasses a circle of less than 40 000 square kilometres. A 1-degree belt
around the planet鈥檚 bulging girth covers more than a hundred times that area.
And it takes just a glance at a globe to see that the tropics include large
chunks of South America, Africa and Asia. Much of this landmass is covered with
steamy rainforest, the richest of all the world鈥檚 ecosystems.

There has been plenty of evidence to support this theory, says Rosenzweig. In
his most recent work, he analysed the fossil record of northern hemisphere
plants over several hundred million years. 鈥淭he area available for land plants
waxed and waned over this time,鈥� he says. 鈥淒iversity kept pace, its pattern
matching quantitatively the pattern we see between continents of varying areas
in today鈥檚 world.鈥�

That was on land. But Roy and his colleagues thought their snail data might
tell a different story about how things are in the sea. To a marine snail,
habitable area means continental shelf: almost all snail species live on this
strip around the continent, which slopes down to a depth of around 200
metres.

They put Rosenzweig鈥檚 theory to the test in two different ways. First, they
calculated the area of shelf along the Pacific and Atlantic coasts in 5-degree
strips, from the edge of the Arctic Ocean to the southern margin of the
Caribbean. When they counted the number of snail species in each strip, they
found no relation between habitable area and diversity.

Next, the team looked at the broader picture. America鈥檚 continental shelf
extends much farther from land at high latitudes, and the habitable area shrinks
as you move towards the equator. In the subarctic and Arctic regions of the
eastern Pacific, for instance, 15 degrees of latitude encompasses more than 850
000 square kilometres of continental shelf. A similar belt off the west coast of
Panama covers only a fifth of this area, yet it harbours more than three times
as many species of snail. 鈥淭he area hypothesis is an interesting idea that may
very well have predictive power for terrestrial systems,鈥� says Roy. 鈥淚t鈥檚
just that it doesn鈥檛 work for marine molluscs.鈥� In short, the idea has its uses,
but it cannot explain the global pattern.

If so many tropical species of marine snails are confined to a narrow band of
shelf, then you might think they would be packed together like sardines, while
those at high latitudes can spread themselves over luxuriously wide areas. This
is exactly how the second of the big three theories suggests species ought to
behave. Rapoport鈥檚 rule, named in honour of the ecologist Eduardo Rapoport, was
proposed by George Stevens from the University of New Mexico a decade ago. Since
then it has become widely accepted as an explanation of why there are so many
species in the tropics.

The theory paints a picture of the tropics as a cosmopolitan city, where each
species is crammed into its own tiny patch, or range, like immigrants in a
ghetto. But why don鈥檛 they spread themselves out a bit more and mix with their
neighbours? Stevens鈥檚 explanation is that these species are very fussy about the
temperature they live at, which restricts them to small areas. Those species
inhabiting cooler waters are not such slaves to their internal thermostats and
tolerate a broader range of temperatures, a talent that allows them to colonise
larger areas.

According to Rapoport鈥檚 rule, there should be a clear gradient in the size of
species鈥� ranges from high to low latitudes. Research over the past decade has
not always supported Rapoport鈥檚 rule. 鈥淚t seems to be most commonly found in
temperate and cold temperate faunas,鈥� says Roy. Even so, he and his colleagues
might have expected their marine snails to obey the rule because Stevens built
his theory on studies of the same group of animals. Stevens, however, analysed
data only from North America鈥檚 Atlantic shelf north of Cuba. When Roy and the
others looked down to the coastline all the way to the equator, the trend broke
down. No longer did the range of the species shrink with decreasing latitude.
鈥淵ou just don鈥檛 see this pattern,鈥� says Jablonski. Rapoport鈥檚 rule seems to hold
until you reach the Caribbean. But there the average range jumps dramatically,
reaching around three times what you would expect if the trend held good.

Broken rule

What鈥檚 more, along North America鈥檚 Pacific coast the trend is virtually the
opposite, with range sizes expanding as you go south. Roy sees this as a warning
that trends based on incomplete sets of data should be treated with suspicion.
Kevin Gaston, an ecologist from the University of Sheffield, is equally
cautious: 鈥淭he evidence that this is a general pattern is at the very least
highly equivocal.鈥�

鈥淩apoport鈥檚 rule is taking something of a battering,鈥� agrees Andrew Clarke, a
marine ecologist with the British Antarctic Survey who has spent years
researching patterns of diversity in the sea. He points out that, despite its
origin, the rule is less likely to hold in the sea than on land, where the
climate is much more strongly linked to latitude.

So if a species鈥� range is not related to its tolerance of temperature
changes, then what is the crucial factor? A closer look reveals natural barriers
such as currents and the boundaries between water masses that isolate a species
just as effectively as an island or a mountain range. 鈥淣o matter what the
latitude, physical barriers account for range size,鈥� says Jablonski.

Nevertheless, it is clear that marine snails, like retired New Yorkers,
undoubtedly do have a preference for low latitudes. So after ruling out the area
hypothesis and Rapoport, the team turned to the third big idea. This harks back
to the geological past. The Earth鈥檚 high latitudes have been ravaged by a
succession of ice ages. Each time the ice advanced, glaciers scoured the
landscape, transforming thriving ecosystems to bare rock and ice. The tropics
all but escaped this destruction. Left undisturbed, the theory goes, organisms
were able to diversify into a multitude of species, each one perfectly adapted
to a particular way of life. And if each specialist has slightly different needs
and habits, many more of them can coexist on the same patch without driving each
other to extinction.

Jablonski is not convinced. Five years ago he showed that the tropics have
evolved more novel species over the past 250 million years than any other part
of the globe鈥攁t least if you look at marine fossils. If this region has
been a stable, unchanging environment over this time, then all this evolutionary
diversity must have sprung from a single source, like a plant growing and
putting out new shoots. But this is not what the evidence shows. Jablonski found
the equivalent of whole new plants sprouting out all over the place. Climate
changes in the past must have had catastrophic consequences, culling some
species and allowing others to come to prominence. 鈥淭he tropics have not been
the stable, unchanging places that many believed them to be,鈥� he says. 鈥淎nd the
upheavals may open new opportunities for evolutionary innovation.鈥�

Over the past few decades, several research groups have found that marine
organisms living off the east coast of North America have experienced a markedly
different level of extinctions from those in west coast waters. Fossil records
from the past 2 million years show that around 30 per cent of all molluscs from
the Atlantic continental shelf have gone extinct. Around the Pacific coast only
around 15 per cent of the molluscs disappeared. It seems, says Jablonski, that
the snails and their kin were able to migrate to avoid the worst rigours of
climate change during the ice ages. In the tropical western Atlantic, however,
the way was blocked. Once a species reached the Caribbean, it was trapped. As
the ice moved south and cooled the seas, species that couldn鈥檛 stand the cold
died out.

This picture has two consequences, he says. First, it means that history
cannot explain the pattern of diversity we see today. Despite the very different
effects of climate change on the Pacific and Atlantic shelves, the number and
distribution of species on each is now remarkably similar. Secondly, Jablonski
argues, it implies that there must be some sort of 鈥渆volutionary dynamic鈥� which
allows species numbers to bounce back after an episode of extinction. It鈥檚 as if
there is a preferred number of species for any given latitude, and new ones
arise until that number is regained.

With all three leading theories collapsing about them, Roy, Jablonski,
Valentine and Rosenberg turned their attention to the Sun鈥攐r more
precisely, the amount of sunlight reaching different parts of the Earth鈥檚
surface. The idea that the more energy enters an ecosystem, the more species it
can sustain, is well established. It seems to work with terrestrial ecosystems,
but how would it fare at sea?

To put it to the test, the researchers first calculated the average sea
surface temperature for each degree of latitude from 5掳 South to 60掳
North, using monthly temperature data collected over 10 years. Then they
compared these figures with the diversity pattern. At last, here was the match
they had been looking for: they found a statistically significant relationship
between the number of snail species and the temperature of the ocean surface
along coasts, especially in regions outside the tropics.

But this is a pretty rough and ready sort of analysis, because the input of
solar radiation does not increase smoothly from the poles to the equator. To
test the relationship between energy input and biodiversity more rigorously, the
team identified places where sea surface temperature deviates from a strict
correlation with latitude. Then they looked again at the numbers of species in
these areas. Sure enough those latitudes where energy input was higher than
expected tended to have more species than expected, whereas latitudes with lower
than expected energy input were short-changed when it came to biodiversity. Once
again the effect was more noticeable at high latitude, and strongest in polar
regions.

So why are there differences between high and low latitude, and between the
two oceans? For the answer, the researchers turned to an idea put forward by
Valentine as long ago as 1973. This is that in marine environments, energy input
alone does not explain the global pattern of diversity. He suggested that where
the supply of food is unstable fewer species can inhabit the same ecosystem.

Changing seasons

Instability arises from two main factors, he said. One is seasonality in
solar energy, associated with latitude. The other is seasonality in nutrient
supply, which is associated with the stability of the water column. This could
explain why energy input has more effect at high latitudes where bleak winters
are followed by an upwelling of nutrient-rich water in the sunny spring and
summer. The combination of extra light and extra nutrients triggers blooms of
phytoplankton. Upwelling is more intense in the eastern Pacific than the western
Atlantic, and much less pronounced in the tropics.

The idea that nutrient stability is paramount will be difficult to test, but
Valentine and his colleagues plan to have a go. The next step will be to
investigate in detail the relationships between nutrient instability, latitude
and solar input. Jablonski suggests that satellite data which show
concentrations of chlorophyll in the ocean鈥攁n indicator of how much
phytoplankton there is in the water鈥攃ould help to identify places where
there are seasonal surges in productivity and others where it is stable. They
could then look at species diversity in these regions and test the predictive
powers of Valentine鈥檚 idea. It will be a massive undertaking. 鈥淲e are
overwhelmed with data,鈥� says Jablonski.

Even if they succeed, they may find that they have only half an explanation.
Ecologists have long suspected that diversity simply breeds diversity. In other
words, the rich array of predators, prey and habitats created by the plants and
animals of the tropics provide opportunities for yet more species. 鈥淥ur data
just aren鈥檛 suited to look at that,鈥� admits Jablonski.

Not everyone shares the team鈥檚 conclusions, of course. Rosenzweig, whose
habitable area idea has taken such a heavy knock, accepts that diversity is
linked to energy flow. But he stands by his own ideas about the influence of
suitable living space. 鈥淥ther marine data do indicate the importance of area to
diversity, so I would be somewhat surprised if prosobranch gastropods were
exceptions to the rule,鈥� he says.

Roy says that the new findings show how dangerous it can be to extrapolate
general rules from specific results, as has happened in the past. What this
study does show, he says, is that current theories about the distribution of
life on Earth are inadequate. Valentine goes further. In the sea at least, he
says, explanations based on habitable area, on Rapoport鈥檚 rule and on historical
vagaries simply won鈥檛 work. 鈥淎nd unless evolution works differently on land than
in the ocean, yes, we damage those theories badly.鈥�

The issues are not just of academic interest. Tropical ecosystems are under
threat from all quarters. So the more we know about why so many of our world鈥檚
species live there, the better, says Roy. 鈥淯nderstanding the patterns that
underlie this gradient has important implications for conservation biology.鈥�