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07/12/2006
Counting Steps and Crop Yields
Last week I said that I would await my copy of the June 30 issue
of Science before commenting on a media report on work in that
journal in seeming conflict with my previous week’s column on
CO2 and poison ivy. Well, I got the journal but was distracted
by another article on a lighter note. The article, by Matthias
Wittlinger of the University of Ulm in Germany and coauthors, is
titled “The Ant Odometer: Stepping on Stilts and Stumps”. The
researchers were intrigued by Cataglyphis fortis, a Saharan desert
ant, and its trips to and from its nest while foraging for food.
Leaving its nest, the ant wanders around more or less
haphazardly until it finds some nourishment. However, on its
way back to its nest over featureless desert terrain, the ant heads
directly home in a straight-line fashion. When the ant gets near
the nest, it departs from its straight-line path and will scoot back
and forth searching for the nest. Wittlinger and colleagues
assumed that the ant has some kind of compass that tells it the
homeward direction. But their concern was how the ant knows
the distance, after its indirect path outward from the nest.
They dismissed a number of possibilities; for example, optical
cues were ruled out since the ants knew the distance even in the
dark or when blindfolded. Could it be that the ant has some kind
of pedometer to measure the number of steps it takes? That’s the
possibility Wittlinger and crew decided to test. They first trained
the ants to go from their nest to a food source located 10 meters
away in a metal channel with sides to encourage the ants to stay
in the channel. After a day or so of training, the ants at the
feeding station were caught and transferred to a longer channel
running parallel to the first one. The ants traveled back in the
longer channel approximately the same distance as from nest to
food source in the original channel. At that point, they left the
channel and started their back and forth searching for the nest.
But what if the ants’ stride lengths were altered? Would that
affect the lengths of their return trips? The researchers again
intercepted the ants at their feeding site. They pasted pig bristles
on some of the ants’ legs and cut off parts of the legs on other
ants. Effectively, some ants were walking on stilts, others on
stumps. The idea was that the stilts would lead to longer strides,
the stumps to shorter strides. Sure enough, the stilt walkers with
their longer strides overshot the nest site while the stump walkers
with their shorter strides undershot it. It was as though the ants
counted the number of steps outbound and took that number of
steps back; only the return steps were longer or shorter.
The researchers waited a couple of days, sent out the stumpers
and stilters to the food site and again put them in the longer
channel. Both stumpers and stilters travel the same distance back
home before searching for the nest. This time the two groups,
with their differing stride lengths and numbers of steps in both
directions, come out together. Can these ants actually count? It
doesn’t seem likely, but the experiments do pose a mystery about
the ant’s “pedometer”. Stay tuned.
Back to plants and CO2. Two weeks ago I wrote about Duke
University’s FACE (free-air concentration enrichment)
experiments showing that increased levels of CO2 spur the
growth and potency of poison ivy. Last week I mentioned a Star-
Ledger article about a University of Illinois study indicating that
increasing CO2 levels has no effect on the yields of corn grown
in open fields in FACE experiments. The June 30 Science issue
contains the Illinois paper by Stephen Long and co-workers, as
well as a Perspective on this work by David Schimel of the
National Center for Atmospheric Research in Boulder, Colorado.
Schimel points out that today there are about 30 FACE studies
scattered over the globe. In an article on a university Web site,
Madeline Kelcher states that Illinois has the largest FACE
facility in the world on 80 acres of farmland near the campus.
There, a typical FACE plot measures 20 meters (66 feet) in
diameter. The FACE plot is surrounded by pipes that release
either CO2 or ozone through holes in the pipes; the release is
monitored and controlled by a computer that takes into account
the wind speed and direction. This allows the CO2 level in the
center of the plot to be controlled to within 10% of the 550-600
ppm level predicted for the year 2050. This level is maintained
from the times the seeds are sown until the crop is harvested.
FACE experiments have been carried out on soybeans, rice,
wheat, corn and other grasses. The temperatures in the FACE
plots and the surrounding farmland are also monitored. The
temperatures in the FACE plots are typically about 1 to 2 degrees
Fahrenheit warmer than outside the FACE plots. The extra CO2
suppresses the amount of transpiration, evaporation of water
from the plant that has a cooling effect.
I mentioned that ozone can be introduced over the FACE plots.
With ozone, there’s good news and bad news. Ozone in the
upper reaches of our atmosphere helps shield us from the
harmful rays of the Sun. By banning certain refrigerants and
propellants in aerosol cans, we have reversed our creation of the
well-known hole in the ozone layer. However, ozone at ground
level is a different story and can cause health problems. Ozone is
also toxic to plants and the forecast for 2050 is that surface ozone
in temperate zones of the Northern Hemisphere and in the tropics
will rise about 20% over today’s levels.
In a recent issue of the journal New Phytologist, P. Morgan and
co-workers reported FACE data on soybeans grown under ozone
concentrations 23% above the ambient level for two growing
seasons. The soybean yields were 20% lower than for crops
grown under “normal” concentrations of ozone. Normal today
seems to mean about 56 to 69 parts per billion, which doesn’t
sound like much. However, ozone begins to show its toxic
effects on plants at only 30 parts per billion.
Until the FACE experiments, most of the data on effects of CO2
and ozone on crop yields were obtained from “enclosure”
studies. Enclosures such as greenhouses allow relatively easy
control of levels of CO2 or ozone. Decades of enclosure data
indicated that increased levels of CO2 would spur the growth of
crops quite significantly, much as the FACE experiments on
poison ivy. Offhand, this seems logical inasmuch as plants grow
by photosynthesis, which involves CO2 as the source of carbon.
You would think the more CO2, the more plant growth. Now it
appears that enclosure studies do not yield results that agree well
with the results of the open field FACE experiments, which
should more closely approach real crop growth conditions.
In the Science paper, Long and his colleagues compare the
results of enclosure and FACE experiments for various crops
grown under CO2 levels predicted for 2050. For wheat,
soybeans and rice the yields are less than half those predicted by
enclosure studies. The FACE experiments for these crops
covered two to five years. For corn, I gather only one year’s
FACE data are available but that year’s work shows essentially
no increase in crop yield under the predicted 2050 CO2 levels.
The FACE results are bad news for the future. For years, based
on enclosure studies, it was thought that one good thing about
increasing CO2 levels and global warming might be increased
food production to keep up with our burgeoning population. The
FACE results indicate that not only will crop yields be lower
than expected but also increasing surface ozone will counter any
increase in crop yield and may actually lead to a decline.
If food does become scarce, perhaps those Saharan desert ants
will remember the stilt experiment and evolve to have longer
legs for the longer journeys to find their food.
Allen F. Bortrum
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