Mountains of diversity

Abstract

A large-scale analysis of bird diversity and evolution on mountains around the globe explores the relationships between elevation, species richness and the rate of formation of new species.

Full text

(niobium nitride) is cubic (Fig.1). This means
that the crystallo graphic symmetry of their
devices is broken at the interface between the
cubic superconductor and the hexagonal semi-
conductor. Such broken symmetries can cause
unwanted effects at interfaces, and therefore
in devices.
This is where the orientation of the super-
conductor comes into play. Yan etal. grew
a layer of the cubic superconductor on a
substrate so that its lattice was oriented in a
way that makes it look hexagonal. To picture
this, imagine looking at a dice at an angle in
which the diagonally opposite corners are
aligned. What you see is a hexagon, even
though the dice is cubic.
The same is true of the cubic superconductor
on the substrate: a hexagonal arrangement of
atoms is exposed on the surface, and the hexa-
gonal semiconductor (aluminium nitride)
aligns with this when it forms on top of the
superconductor. As a result, the aluminium
nitride is not perturbed by broken crystallo-
graphic symmetry at the interface, and forms
an undistorted layer, as needed for the growth
of an HEMT structure. Indeed, the authors
observed the formation of certain quantum
oscillations in their device; the presence of
these oscillations is considered a benchmark
of high crystal quality.
Yan et al. went on to measure the current–
voltage profile of their superconductor–HEMT
structure. They observed that this profile of the
HEMT is modified by a superconductor-to-
metal transition in niobium nitride, and gen-
erates a negative differential resistance (NDR)
— a property that can be used to increase the
power of electrical signals. NDR devices have
been known since the end of the nineteenth
century
8
and include the Gunn diode
9
, which is
widely used to generate microwaves in sensors
and measuring instruments. Such devices are
of great value for electronic systems that use
high-frequency, high-power signals— exactly
what is needed in telecommunications net-
works. In Yan and colleagues’ device, NDR can
be switched on or off simply by making the
temperature lower or higher than the critical
temperature for superconductivity (the temper-
ature below which superconductivity occurs).
Combining materials that have different
electronic properties without breaking the
crystallographic symmetry at the interface is a
remarkable feat. However, t he mobility of elec-
trons in the device is currently rather low; much
higher mobilities can be achieved in devices
that use indium arsenide. Achieving mobili-
ties comparable to those of indium arsenide
will be extremely challenging. More over, the
separation between the superconductor and
the 2D electron gas — free electrons that are
confined to move in only two dimensions —
generated in the device will need to be reduced
to enable promising quantum effects.
A future goal could be to use the authors’
system to generate and observe Majorana
fermions
10
— a type of quasiparticle that would
be useful for quantum computing — at the
superconductor–semiconductor interface11.
Charge carriers in electronic devices can be scat-
tered (for example, by crystal defects), and the
average time between scattering events needs to
be long to stabilize these quasiparticles. Yan et al.
calculate that the charge-carrier scattering time
in their devices is impressively long (66femto-
seconds; 1fs is 10
–15
s), but the scattering times
will need to be at least 100times longer, simi-
lar to the scattering time in indium arsenide
12
,
to stabilize Majorana fermions. It remains to
be seen whether this can be achieved in the
authors’ device s.
Ultimately, Yan and colleagues’ work will
inspire and accelerate efforts to grow nitride
superconductors and nitride semiconductors
that enable the ultra-high operating efficiency,
structural perfection and opportunities for
manipulating electronic properties that have
already been achieved in interfaces involving
indium arsenide. Because, at the end of the day,
the interface is the device.■
Yoshiharu Krockenb erger and Yoshitaka
Taniyasu are in the Materials Science
Laboratory, NTT Basic Research Laboratories,
Atsugi, Kanagawa 243-0198, Japan.
e-mails: yoshiharu.k@lab.ntt.co.jp;
taniyasu.yoshitaka@lab.ntt.co.jp
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EVOLUTION
Mountains of diversity
A large-scale analysis of bird diversity and evolution on mountains around the
globe explores the relationships between elevation, species richness and the rate
of formation of new species. S L .246
ALEXANDER ZIZKA & ALEXANDRE ANTONELLI
M
ountain chains are global centres of
biological diversity — they harbour
one-third of all terrestrial species1.
These places have long fascinated biologists2,
but are notoriously difficult to explore and
study. Our knowledge of the distribution
of species diversity on mountains is incom-
plete, as is our understanding of how species
richness (the total number of species) and the
rates of formation of new species (speciation)
vary in single mountain ranges. On page246,
Quintero and Jetz
3
tackle these issues by study-
ing the diversity and evolution of birds on the
46 major mountain ranges of the world.
Mountains can differ substantially in the
environment they provide, depending on
factors such as bedrock, ruggedness, climatic
conditions and the amount of energy available
in the region. Moreover, mountains are often
far apart, and organisms inhabiting such places
can persist in genetically isolated populations
owing to factors including terrain complexity
and the high variation of habitat types along
elevational gradients. Isolated populations
often adapt to the local environmental and
ecological conditions. When such populations
are no longer capable of reproducing with one
another, they form new species
4
. One example
of this is the hummingbird Aglaiocercus kingii,
which is found only in the Andes of South
America (Fig.1).
Quintero and Jetz used large-scale data
sets of current distributions of bird species,
mined from existing databases and publica-
tions, to characterize the relationship between
elevation above sea level and species richness.
The authors amalgamated data for 9,993 spe-
cies, representing essentially all the birds that
are currently known. Although the patterns
observed in different regions vary, the overall
trend for most regions is a hump-shaped curve
in which species richness is highest at middle
elevations, and decreases as elevation increases.
The result confirms findings from previous
studies of plants and birds5,6. This type of
pattern might be driven by the smaller area
available for speciation at higher elevations and
because the environmental conditions there
are more extreme than those on lowlands.
For example, large temperature fluctuations
between day and night, and an increased expo-
sure to radiation and wind at higher elevations
could limit the number of species that can cope
with such conditions.
The authors used some innovative
approaches for their data analysis. They aimed
8 MARCH 2018 | VOL 555 | NATURE | 173
NEWS & VIEWS RESEARCH

to capture the 3D structure of biodiversity data
by combining elevation and species infor-
mation. They performed their analyses by
grouping the bird data into ‘sliced sections’
corresponding to trapezoidal prisms that
encompass a particular elevation range. This
allowed them to assess mountain complexity in
a way that improves on conventional ecological
methods that often neglect elevation.
Some biodiversity analyses can be affected
by issues such as the mid-domain effect, in
which a species-richness peak occurs around
the centre of a region because of the spatial
overlap of species’ ranges7. The authors devel-
oped a subsampling approach that offers a way
to address this issue. They counted species, but
also factored in the total area that each species
occupies when determining species’ contri-
butions to species richness. This method also
uses a complex randomization procedure that
takes a modelling approach to estimate the
species’ metrics.
Their analysis using this subsampling
approach revealed the surprising result that
there is a linear decrease in species richness
as elevation increases. Nevertheless, one con-
cern is that the subsampled species-richness
estimates from this method may not be directly
comparable with estimates of species richness
calculated in the conventional way — as the
total number of species. In addition, the size of
each species’ range might be a factor linked to
its evolutionary history, and therefore relevant
for understanding the evolution of mountain
species. Additional research might be needed
to assess the applicability of this subsampled
diversity metric.
Another, perhaps even more interesting
finding made by Quintero and Jetz concerns
the process underlying the observed species
diversity patterns. The authors used previously
estimated8 information on the evolutionary
relationships between the bird species that
they studied to calculate the rate of speciation
along elevational gradients. They found that
this rate is inversely related to subsampled spe-
cies richness: that is, species are formed at the
highest rates where the species richness is low-
est, which corresponds to mountaintops. The
authors’ explanation for this is that environ-
mentally stable lowlands have high diversity,
whereas at higher elevations, diversity is
governed mainly by frequent immigrations
and rapid species replacement during periods
of climate change.
A major limitation for studies of biological
diversity on mountains is the scarcity of availa-
ble data. Quintero and Jetz’s study uses existing
diversity data that have a resolution of at least
110kilometres horizontally and 500metres
in elevation. This kind of scale can be rather
coarse for many mountains, given that envi-
ronmental and ecological conditions can vary
considerably over distances of just a few hun-
dred metres. Although birds are the best geo-
graphically documented group of organisms
on Earth, with more than 564million publicly
available records (see www.gbif.org), it might
come as a surprise that their diversity in many
mountains remains poorly documented.
Unfortunately, the geological data of most
relevance to biologists are lacking. Quintero
and Jetz therefore had to simplify geological
complexity in their analyses by using aver-
aged values for key variables, such as the age
of mountains. These factors, together with
ecological interactions between species, might
influence the speciation process
9
, and can vary
in a single mountain range.
Speciation rates are also difficult to estimate,
especially over long timescales and for groups,
such as birds, that lack a rich fossil record.
One potential drawback of the new study is
that many relationships between species, and
their estimated time of origin, have been cal-
culated on the basis of limited genetic informa-
tion and with methods that do not take into
account the difficulties that sometimes arise
during the generation of phylogenetic trees. In
some cases, proposed relationships might rely
only on comparisons of bird shape and form
(morphology) rather than on genetic data.
There is still a long way to go before the
phylogeny of birds is fully understood
10
. Large-
scale initiatives are under way to sequence
the genomes of all bird species as a way to
determine more-reliable estimates of the rela-
tionships between birds and to improve under-
standing of their evolutionary history11.
Quintero and Jetz’s results reveal general and
unexpected relationships between elevation,
species richness and diversification. Addi-
tional data collection in the field by scientists
and birdwatchers will be essential and, along
with data integration and analysis of the sort
spearheaded by Quintero and Jetz, should pro-
vide additional insights. It will be particularly
interesting to see whether the trends reported
by Quintero and Jetz hold true for the rest of
the world’s species, the diversity and distribu-
tion of which are poorly known even at the
global level12 — let alone along elevational
gradients on mountains. ■
Alexander Zizka and Alexandre Antonelli
are at the Gothenburg Global Biodiversity
Centre, SE-405 30 Gothenburg, Sweden,
and in the Department of Biological and
Environmental Sciences, University of
Gothenburg. A.A. is also at the Gothenburg
Botanical Garden and in the Department
of Organismic and Evolutionary Biology,
Harvard University, Cambridge,
Massachusetts.
e-mail: alexandre.antonelli@bioenv.gu.se
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This article was published online on 21 February 2018.
Figure 1 | The hummingbird Aglaiocercus kingii in Ecuador. This species is confined to the Andes
mountains of South America. Quintero and Jetz3 have developed an approach for studying bird
distributions on mountains around the world that might help to address how and when biological
diversity evolved along elevational gradients.
GLENN BARTLEY/GETTY
174 | NATURE | VOL 555 | 8 MARCH 2018
NEWS & VIEWS
RESEARCH