|This month, the Science page looks at how plants, fish, insects
and birds have evolved responses to their environments, and at wetland
ecology and how best to conserve wildlife.
Why don't plants get sunburn?
Plants stand outside all day long and (mostly) court exposure
to sunshine as an essential ingredient in their growth. So why don't they
suffer from the same sort of sunburn problems animals do?
Biologists have known about a mechanism that prevents
excess sunlight from harming plants, but they haven't known much about
its details. A plant can only use so much light energy to photosynthesise
- excess energy is dissipated, by a little-known process. A recent paper
identifies a protein, PsbS, as a key component of the plant's safety mechanism.
Not all plants contain PsBs - they may still photosynthesise
but lack protection from excess sunlight. The researchers used a mutant
variety of Arabidopsis thaliana to show that plants which do not
contain PsBs cannot dissipate excess light energy and the protein may thus
be the key to the process.
Reference: Li, X.-P., Björkman, O. and Shih,
C. 2000. A pigment-binding protein essential for regulation of photosynthetic
light harvesting. Nature 403(6768):391 -395.
Evolution - some bats got a second
It has been thought that bats evolved from a common ancestor,
later to divide into the megabats, which locate prey by sight, and the
microbats, which have the famous echo-location system.
However, recent DNA analysis has suggetsed that one family
of microbats, including the horseshoe bat, is more closely related to,
and has therefore probably evolved from, megabats. "It means either that
echolocation evolved twice, or that it was lost by the megabats," says
Michael Stanhope of the Queen's University of Belfast, who led the research.
Reference: Nature 403(6768): 188.
Arms race shown in butterflies
If bats developed echo-location to find insects to eat, why
didn't insects develop countermeasures? Apparently, they did. Moths, night-time
prey of bats, have long been known to have ears to hear their enemies approaching
- now a similar organ has been found on butterflies.
Night-flying butterflies have ears, on the forewings of
the insects, sensitive to ultrasound which bats use to navigate and locate
their prey. The butterflies hear the bats coming and dart out of the way.
Interestingly, butterflies evolved as 'day moths', probably as a response
to night-time bat predation, since fossil records show that butterflies
became daytime flyers at roughly the same time bats developed large ears
and the ability to use echoes to locate their prey. Ears may be an alternative
strategy to avoid bats.
The researchers, Dr Jayne Yack and Professor James Fullard,
looked at one particular species in the family called Macrosoma heliconiaria
which can be found on Barro Colorado Island, Panama. They have a very thin
eardrum, stretched over an air-filled chamber. The eardrums vibrate when
there is a burst of ultrasonic sound. These vibrations are detected by
nerve cells packed into three little organs on the inside of the chambers.
When exposed to simulated bat attacks in the form of intense bursts of
ultrasonic noise the butterflies accelerated and went into steep dives
or climbs, upward or downward loops, spirals and horizontal sweeps.
"It's quite an unusual ear for insects," Professor Fullard
says. "It resembles the moth ear, but it is certainly more complicated
... What we have here are a group of butterflies that moved into the night,
or may be they are the remnants of the original insects. Maybe they had
ears and the butterflies that didn't were pushed in to the daytime by bats."
Sticklebacks support Darwin
Natural selection is the main plank in modern evolutionary
theory, leading to the origin of species, but actual evidence from the
wild has been lacking. Researchers in British Columbia have used the development
of three-spine stickleback populations to show that differing environmental
conditions can produce different characteristics.
Populations which evolved, since Pleistocene times, under
different ecological conditions were found to show different characteristics
- "strong reproductive isolation" - whereas populations which evolved independently
under similar ecological conditions lack isolation. This repeatable process
strongly suggests that speciation is a process which produces organisms
adapted to alternative environments.
Reference: Science Volume 287, Number 5451, 14
Jan 2000, pp. 306 - 308; Natural Selection and Parallel Speciation in Sympatric
Sticklebacks; Howard D. Rundle, * Laura Nagel, Janette Wenrick Boughman,
Birds keep right
Oystercatchers don't want to waste time breaking into mussel
shells - they want to concentrate their efforts on the weak points. The
mussels have one side of the shell thinner than the other and this is where
the birds crack down.
Stephen Lea of the University of Exeter has found that
the birds can detect a 0.037-millimetre difference in the thickness of
the two "valves" of a mussel shell. It's a tiny difference but it can save
a bird 13 per cent of the effort needed to open a shell. Some 56 per cent
of mussels have thinner right-hand valves, but the birds hammer the right-hand
valves 72 per cent of the time. "If in doubt, they go right," says Lea.
Reference: New Scientist, 8 January 2000
Feel the quality, not the width
Phytoplankton, the minute plants which grow in water, support
zooplankton, the equally tiny animals which are food for larger fish and
keep algal biomass under control. It now seems that merely increasing phytoplankton
is not enough to improve zooplankton production.
The phytoplankton which contains high concentrations of
eicosapentaneoic acid, commonly known as an omega-3 fatty acid, support
much higher zooplankton growth rates, even if the overall amount of phytoplankton
is relatively low.
"Phytoplankton that are more nutritious can have a major
impact on the overall food web," said Michael Brett, assistant professor
of civil and environmental engineering at the University of Washington.
"What the study shows is that the rate at which zooplankton convert phytoplankton
biomass to zooplankton biomass depends on the supply of this class of essential
fatty acids. This gives us important insights into what may determine how
energy moves through aquatic food webs."
During their study the researchers fed Daphnia
algae at various times of the year. During the summer, a type of phytoplankton
poor in omega-3 fatty acids called cyanobacteria dominated the pond and
the Daphnia suffered with an energy-conversion rate from plants
to animal of 5 to 26 percent. During the winter and spring, however, diatoms
dominated. Diatoms are rich in omega-3 fatty acids and, although the diatom
concentration was lower than that of the summer phytoplankton, the Daphnia
flourished with an energy-conversion rate of 50 to 65 percent.
Those insights could help scientists predict biomass and
energy flow rates in aquatic ecosystems, providing possible tools for fisheries
Salt marshes contribute to the
It has been recognised that not enough methyl bromide (CH3Br)
and methyl chloride (CH3Cl) are produced from oceanic sources, terrestrial
plants and fungi, biomass burning and anthropogenic inputs to balance their
losses owing to oxidation by hydroxyl radicals, oceanic degradation, and
consumption in soils, suggesting that additional natural terrestrial sources
may be important.
A recent study shows that CH3Br and CH3Cl are released
to the atmosphere from all vegetation zones of two coastal salt marshes.
If these measurements are typical of salt marshes globally, they suggest
that such ecosystems, even though they constitute less than 0.1% of the
global surface area, may produce roughly 10% of the total fluxes of atmospheric
CH3Br and CH3Cl.
Reference: Robert C. Rhew, Benjamin R. Miller &
Ray F. Weiss. Natural methyl bromide and methyl chloride emissions from
coastal salt marshes. Nature 403, 292 - 295 (2000)
Raggy round the edges
Conservation strategies need to take account of the outer
fringes of an endangered species' geographical range instead of just concentrating
on core regions where the species is still most plentiful, according to
a recent report.
Researchers in America have found that many threatened
species are clinging on at the edges of their historical geographical ranges
and not, as one might have expected, in the central regions which would
historically have provided the most favourable habitat. This has important
implications for conservation, which has in the past aimed to preserve
core populations at the centre of a potentially endangered species' geographical
The study uses results from 245 endangered or recently
extinct species, of which just over half were birds and mammals, to show
that , contrary to ecological expectations, 98% of these species maintained,
or had maintained, populations on the edges of their previous ranges, and
that for some these were the only populations left. Giant pandas and Tasmanian
tigers, for example, both hung on in the extremities of their ranges long
after the main populations had become extinct.
Reference: Channell, R. & Lomolino, M.V. Dynamic
biogeography and conservation of endangered species. Nature 403, 84 (2000).