THROW down grain and chickens rush to peck where it lands. They’re not
stupid. And neither is your average plant. Forget passive vegetable: the
greenery around us turns out to be bristling with actively foraging flora. Just
like animals, it seems plants can survey their surroundings and go where the
goodies are best.
Of course, plants can’t run like chickens. Instead, they forage by altering
their own anatomy. They grow more roots, longer stems or perhaps another leaf to
get the best out of patches of nutrients or other resources. But in their own
plant-like way they hunt their food just like animals. And that means that
everyone from backyard gardeners to ecological researchers may need to rethink
some of their cherished assumptions. For example, farmers and gardeners have for
centuries diligently turned the soil before planting, creating smooth, uniform
seedbeds that should give plants the best chance of getting at the soil’s wealth
of nutrients. But if plants can forage and seek out rich patches of soil, this
homogenisation could be as wasteful as deliberately scattering a lode of rich
ore before mining.
Scientists have only recently developed phyto-friendly “IQ tests” to reveal
just how wisely plants exploit their environment. To date, we know most about
the foraging skills of ground ivy (Glechoma hederacea), a quite
ordinary little plant unrelated to the familiar English ivy of stately homes and
colleges. Once, before hops came on the scene, ground ivy put the bitter flavour
into British beer and its leaves still make a decent cup of herbal tea.
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Plant ecologist Mike Hutchings of the University of Sussex smiles when he
talks of his twenty-year relationship with this—let’s be
frank—perennial weed. “It’s a beautiful model plant to work with,” he
enthuses. It was ground ivy’s classic looks that first attracted him: two leaves
on a bit of stem, a branch or two and a tuft of roots. As the plant creeps along
the ground, growing in just two dimensions, it generates repeated variations on
this simple modular theme. The plant extends itself as a collection of subtly
different modules, or “ramets”, which turn out to be an enduring physical record
of its foraging behaviour. To read this record, you simply follow the vine,
noting where the plant decides to put down roots, send off pioneering branches
or generate leaves to catch the light.
Just outside Brighton, on England’s south coast, Hutchings set out to
discover how ground ivy copes with the fact that any landscape it encounters is
bound to be patchy. “In the real world, you never find uniform environments,”
says Hutchings. “Something that really worries me is that whenever ecologists do
experiments on plants the first thing they do is to make their environments
uniform. They get rid of as much spatial and temporal variation as they possibly
can.” What would happen if you tried to grow plants in a more realistic,
patchwork world, he wondered?
To find out, Hutchings and his colleague Colin Birch put genetically
identical ramets of ground ivy at the edge of two 50-centimetre-square
boxes. In one box, half the nutrients were clumped in the central 10 per cent of
the box, with the remainder evenly distributed in the rest of the area. In the
other box, the same total amount of nutrients was evenly spread throughout.
Which one would the plant prefer?
You might think that there would be no difference, or even that the ground
ivy would fare better in the uniform environment, because it would immediately
have access to richer soil. But the results were startlingly different: the
plant grew two-and-a-half times as big in the box with the patchy nutrients.
“What’s happening here is something important, something that I don’t think
ecologists have known much about until recently,” says Hutchings.
Researchers in the field share his enthusiasm: “Mike’s work is a delight,”
says Joel Brown, an animal ecologist at the University of Illinois at Chicago
who sees strong parallels between animal and plant foraging. Hutchings deserves
“a tremendous amount of credit” for pioneering a new understanding of plant
behaviour, Brown says.
So what’s going on? Why does ground ivy prefer patchy environments to uniform
ones? “When the plant hits the nutrient-rich patch it goes barmy about producing
roots,” Hutchings explains. “In effect, it’s thinking about what it’s doing,” he
says. “When it senses resources, it goes for them, and starts growing roots much
earlier in its development in the locations containing nutrients.” As a result,
the plant puts most of its roots in the richest patches and skips over the
poorer ground in between.
But the patchy environment that makes this possible is a double-edged sword.
Think of walking around a big city—London, for example. “People who are
familiar with London can appreciate its heterogeneity,” says Brown. “They can
direct their activity to the parts they like, and avoid the parts they don’t.”
But if they were in an unfamiliar city, such as Chicago, he observes,
heterogeneity could become a problem. “They might wander aimlessly through the
streets and stumble across some really terrible places.” To benefit from
heterogeneity, Brown explains, “a plant has to be able to treat its environment
in the same way the Londoner treats London. It has to be able to exploit the
good patches and avoid the bad.”
So what kinds of patchwork resources are like the familiar city for ground
ivy? To find out, postdoctoral fellow Dushyantha Wijesinghe and Hutchings set up
a number of giant chequerboards of rich and poor patches of different size. In
every case, half of the environment was good and half of it was bad, and the
overall amount of resources was the same. Genetically identical shoots of ground
ivy took their places on the starting line. Everything was the same except the
spatial pattern of the resource.
They found that the scale of patchiness does indeed matter: ground ivy liked
25-centimetre-square patches best, where it grew up to four times better than in
environments containing much smaller, 6.25-centimetre-square blocks or much
bigger, 50-by-25-centimetre blocks. In smaller squares —where the
environment was extremely variable—the plant never had enough time to get
used to a patch before it moved on into a patch of contrasting quality, so it
ended up producing “a morphology that shows complete confusion”, says Hutchings.
Conversely, if the patches were too big, the environment appeared uniform to the
plant. So ground ivy clearly has an optimum patch size, and probably each
species has its own distinctive preferences.
Contrast matters too, as a second experiment showed: ground ivy can
apparently detect a good patch only when it is at least twice as rich as the
surroundings. And it looks as if the real world fits ground ivy’s criteria. When
ecologists Alastair Fitter and Rebecca Farley of the University of York
meticulously mapped nutrients in a Yorkshire woodland, they found up to fivefold
differences in nitrate and ammonium availability at a scale of 20
centimetres.
Hutchings’s latest experiments, this time with Wijesinghe and Elizabeth John,
a plant ecologist also at Sussex, suggest that ground ivy is not the only smart
plant on the block. “We’ve recently grown a variety of common British species
with quite different lifestyles in patchy environments—plants like poppy
and false oat grass and plantains,” says John. “We’re finding fascinating
effects.” Poppies given nutrients in concentrated patches, for example, can get
by with a smaller ball of roots, leaving more energy for flowers.
The full implications of these findings have yet to sink in. Farmers work
hard to remove any patchiness, or variety, from their fields. Like scientists,
they value the predictability that comes with uniformity. The farmer’s friend is
the plough—a wonderful way of smoothing out the natural heterogeneity in
any stretch of land. But could it turn out to be a false friend?
It is time agricultural scientists took an interest in this research,
Hutchings suspects. “If I were a farmer, I’d wonder how I should apply nutrients
to my field.” He admits that it could be difficult to arrange patches to the
best benefit of field crops such as wheat or soybeans. But it might be
surprisingly easy to raise yields of individually planted crops such as
strawberries or tomatoes—or settle for the same yield using less
fertiliser. Either way, you win.
“So far we haven’t found commercial backers who want to try doing innovative
experiments,” says Hutchings. “They think the best way to grow more food is to
have genetically modified organisms. But if you could use a system akin to this
to raise yields, that’s the way I would sooner do things.” Brown agrees. “Making
use of heterogeneity could be the silver bullet for increasing yields,” he says.
“If crops can find the good bits and exploit them, they ought to do a whole lot
ٳٱ.”
But what happens in natural communities, where many plant species may live in
close proximity? The potential implications of heterogeneity go far beyond
farming, right to the heart of natural ecosystems. As a step toward an answer,
the Sussex team has recently grown a dozen or so species together in field
plots. They spiked each plot with tablets of plant food arranged in chequerboard
patterns, again of varying scale. “The question then is, how’s the community
going to turn out in each case?” says Hutchings.
Strange happenings
The researchers have yet to fully analyse their data, but early results
suggest that in some situations at least, heterogeneity fosters greater yields:
total biomass is significantly lower in the uniform plots. The take-home message
is that strange things may happen when diverse plants meet real-world
complexity. “It’s ironic that not long ago whenever we did an experiment we made
everything as uniform as we possibly could,” says Hutchings. “We were stuck in a
mindset that totally underestimated what’s going on out there as a consequence
of the way the environment itself is built.”
Hutchings suspects that it will be “an absolute monster” to untangle it all.
“But until we do, I don’t think we’ll get very far in understanding why
communities are the way they are.”
One effect of soil patchiness is to force plants to compete over high-quality
patches. Fitter and his research fellow Angela Hodge, together with David
Robinson of the University of Aberdeen and Bryan Griffiths at the Scottish Crop
Research Institute near Dundee, grew grasses in soils enriched with patches of
nutrients containing the stable isotope nitrogen-15. They found that plants
growing alone in a patch could fully exploit the nitrogen regardless of how many
roots they developed. But when two different grass species grew in competition,
the plant that grew more roots into the patch got a proportionately bigger share
of the nitrogen.
These results fit well with Brown’s own suspicions that plants in natural
communities get trapped into costly “root races” in which they squander their
energies by all piling in at once. Together with Mordechai Gersani of Israel’s
Ben-Gurion University and graduate students Gitogo Maina and Erin O’Brien, Brown
finds that soybean and pea seedlings respond to a competitor by producing a
great profusion of roots, as each attempts to snatch nutrients before the other.
The end result is more roots but less crop—up to a 30 per cent reduction
in yield. “It becomes a kind of spite,” says Brown, “but they can’t help
themselves. The plants are just too clever for their own good.”
So are plants condemned to waste their energies in fruitless competition? Not
necessarily, says Brown. “This is pure speculation, but I believe that in real
plant communities small-scale heterogeneity fosters the coexistence of crumb
pickers and cream skimmers.” Crumb-picker species may be slower at tapping into
the goodies, but are more efficient. Cream skimmers, on the other hand, zero in
quickly but less efficiently. Because of their different aptitudes, what looks
like a patch to one plant might appear homogeneous to another. The result could
be a kind of micro-partitioning of the habitat, as plants respond to each other
and to the fine details of their environment.
“A plant less suited to a particular type of heterogeneity may decide, `I
don’t stand a chance here,’ and cede the area to a competitor. They may actually
compete less,” says Brown. This could also help to explain why farming practices
such as mixed cropping can be very successful. While trees are busy exploiting
large-scale patches, plants growing between could tap into a much finer
chequerboard of resources.
The picture grows even more complex once you realise that the majority of
land plants are linked up in a massive network of fungal filaments known as
mycorrhizae. This as-yet poorly understood underground web undoubtedly helps
terrestrial plants to capture nutrients such as phosphorus, which is generally
in short supply, and they can also protect plants against pathogens, as Fitter
has discovered (New Scientist, 16 March 1996, p 20). Mycorrhizae, along
with countless other microbes, must be right in the thick of things whenever a
tempting nutrient patch happens along. So do we understand even the broad
patterns of this complex world? “Well,” says Hodge, “let’s just say there’s
still an awful lot to go for.”
“The revolution in plant foraging that Mike is leading is taking us
underground, to the hidden realm of roots,” says Brown. “Plants have been
playing footsie under the table in remarkably sophisticated ways.” At last
scientists are waking up to that.
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Further reading:
The effects of environmental heterogeneity on the performance of Glechoma
hederacea: the interactions between patch contrast and patch scale
by Dushyantha Wijesinghe and Michael Hutchings,
Journal of Ecology, vol 87, p 860 (1999) -
Temporal and spatial variation in soil resources in a deciduous woodland
by Rebecca Farley and Alastair Fitter, Journal of Ecology, vol 87, p 688 (1999) -
Plant, soil fauna and microbial responses to N-rich organic patches
of contrasting temporal availability
by Angela Hodge and others,
Soil Biology and Biochemistry, vol 31, p 1517 (1999)