Page 74 - The Economist20171214

74 Science and technology The Economist December 16th 2017
How fish sustain forests and regeneratinghabitat. fish. This matters, because when stocks of
Trees in these areas fruit most prolifical-
Size really matters ly during the summer, when local rivers larger species are depleted commercial
fishermen often start “fishing down” the
burst their banks and flood the land, mak- food chain to capture smallerones.
ingthose fruit available to fish, which glad- The researchers therefore monitored
ly gobble them up. Then, as the fish swim the seed-eating behaviour of tetra fish,
around the floodplain, they pass the seeds which grow to a length of12cm, and fresh-
inside those fruit, which often remain in- water sardines, which make 18cm. Since
Among fish, bigindividuals do mostto tact, as part of their faeces. These seeds are both species patrol areas where trees drop
help habitats
thus distributed far and wide. Researchers ripe fruitduringthe flood season, and both
OR anglers nothing beats catching a big havefoundthatthemosteffectivedistribu- are also caught by fishermen when they
Ffish. Commercial fisherfolk also prefer tors are the biggest ones. Because they approach their maximum size, Mr Costa-
to haul in big specimens. Unfortunately, in have bigger bellies they eat more, and be- Pereira speculated that selective fishing of
recent years, research has shown that se- cause they have wider mouths they are the largesttetra and sardineswould reduce
lectively capturingthe largest fish has wor- more likely to swallow seeds whole rather the number of intact seeds that the ani-
rying ecological consequences. In some than chew them up, as smallerfish might. mals would laterdefecate.
species the large ones are the healthiest Many studies of seed-eating fish have The team collected a sample of fish on
ones, and so the ones most likely to breed thus looked at large species. Yet small fish the Miranda river flood plain in the Panta-
successfully. In others they are the oldest, also distribute seeds, especially small nal, measured the lengths of their speci-
and so the most experienced at eluding seeds, according to a review ofthe scientif- mensand the gapesoftheirjawswith calli-
predators or securing resources, such as ic literature just published in Biotropica by pers, and then analysed the animals’ gut
food and breeding sites. In tropical wet- Raul Costa-Pereira and his colleagues at contents under a microscope. They found
lands,suchasthePantanalandAmazonre- São Paulo State University. That led Mr that 63% ofseeds counted in the guts of the
gions ofBrazil, the largest fish are also vital Costa-Pereira to speculate about the role of freshwatersardineshad been destroyed by
in dispersing seeds—and thus maintaining large specimens in populations of smaller chewing, and 22% ofthose in tetrashad suf-
fered the same fate. Crucially, they also
Air travel found, forboth species, that the number of
intact seeds did indeed increase with the
When push comes to shove size ofthe fish they came from.
MrCosta-Pereira therefore believes that
the same rule applies to small fish species
as to big ones—namely, the largest individ-
Motorised nose wheels will letplanes leave the gate bythemselves
uals play the biggest role in distributing
HE frustrations ofairtravel are many aircraft when they were nearthe termi- seeds. This, he suggests, needsto be consid-
Tand varied: enduringthe scrum to nal. Motorised nose wheels are just that, ered when catch limits are set. The catch-
board; rummagingforroom in the over- says Isaiah Cox, WheelTug’s boss. Using ing of small species, which are used not
head lockers; waitingpatiently for “the them planes could push backand taxi to onlyasfood forpeople butalso to feed ani-
last two remainingpassengers” to be the runway with theirengines ticking mals and as bait, is often less regulated
extracted from the shops. Afterall that, over. This would, MrCoxsays, reduce than the catching of large ones. Intensive
those on the aircraft often find that it has noise, save fuel and cut emissions. fishing can rapidly deplete the number of
failed to push backfrom the gate in time The WheelTugincorporates the ele- smaller fish. If the biggest individuals go
to meet its take-offslot. Because, under ments ofan electric motor, such as the first, there could be repercussions for the
theirown power, planes can only go statorand the rotor, inside the hub of widerecology ofa flood plain. 7
forward, they rely on a tugwhen re- each ofthe 737’s twin nose wheels. In-
versingfrom a gate. Ifsuch is not avail- stead ofa battery, these motors draw
able, has lost its driverorhas broken current directly from the auxiliary pow-
down, at the gate the plane must stay. er-unit, a small generatorin the rear ofan
This may soon change, though. airlinerthat is used on the ground to run
WheelTug, a company in Gibraltar, has its electrical services. Fora long taxi, they
spent overa decade developingelectric can also take powerfrom the generators
motors to drive an aircraft’s nose wheel. driven by the aircraft’s jet engines.
This month it employed StirlingDynam- Arival idea is the TaxiBot, developed
ics, an engineeringfirm in Bristol, Eng- by Israel Aerospace Industries. This is a
land, to help prepare the device for certifi- semi-autonomous tugwith a hybrid
cation by air-safety authorities. It has diesel-electric motorthat is controlled
tested a prototype and hopes its motor- directly by a plane’s pilot, who can use it
ised wheels will be available in 2019 for to drive the aircraft from the gate to the
fittingonto versions ofthe Boeing 737, runway with the jet engines switched off.
and lateronto otheraircraft. The com- The TaxiBot is already used at Frankfurt
pany says 22 airlines are already keen. airport, but means planes still rely on the
In theory, planes could leave gates services ofanothermachine. Amotor-
usingthe reverse-thrust baffles deployed ised nose wheel provides independence,
to slow them duringlanding. This, although it also adds weight—and there-
though, would mean revvingthe engines fore increases fuel consumption. But the
up so much that it would be safe neither value ofthe time saved by fasterpush-
forground staffnorforotheraircraft in backs would more than compensate for
the vicinity. So, some sort ofsystem that. Passengers do not mind how they
would still be needed to manoeuvre depart, as longas they do so promptly.
Big fish, big trees