COMPARED with rivers and lakes, ponds have a serious image problem. True,
Monet found them inspirational, but can you imagine a song about Old Man Pond or
a ballet called Swan Pond? Ecologists are equally sniffy about their merits. Can
wildlife really thrive in artificial hollows created to water livestock, in
by-products of the Industrial Revolution, or in ornamental “Water Features” in
domestic gardens? Sullied by their close association with humanity, ponds in the
past have never been regarded as respectable objects for scientific study.
Yet small can be spectacular: the richness of pondlife is surprising even the
experts. “We never imagined that the best examples would turn out to be quite so
outstanding,” says Jeremy Biggs, a specialist in pond ecology and founder of
Pond Action, an independent research organisation based at Oxford Brookes
University. Over the past decade, the team’s research has shown that ponds are
among the very best natural habitats. In “water-friendly” landscapes, they often
provide homes for more endangered freshwater species than even the grandest
lakes or rivers.
These hot spots of biodiversity are the rainforests of the temperate zone.
Pond Action’s latest survey turned up 450 different species of
macroinvertebrates: everything from fairy shrimps to endangered beetles and
scarce dragonflies. The ponds had 15 per cent more species than were found in a
much larger government survey of rivers, and twice as many rare species. There
are more species of water beetle living in British ponds than there are bird
species living throughout the entire countryside.
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Nowhere have ponds been so comprehensively scrutinised as in Britain, but
much the same picture is slowly emerging throughout Europe, Australia and North
America, as ecologists begin to dip into long-neglected pools. In the US, for
instance, 350 plant species depend on prairie potholes—the ghosts of ice
caves created 20 000 years ago as glaciers retreated from Minnesota and Iowa.
And in Australia, flood-plain ponds called billabongs house a teeming diversity
of life. One study of them in the Alligator Rivers region of the Northern
Territory uncovered more than 800 species of algae alone.
So why do many eminent freshwater biologists still believe that protecting
ponds is not as important as safeguarding rivers and lakes? This belief rests on
a conceptual error, argues Penny Williams, a field botanist who specialises in
pond design and management at Pond Action. “What people fail to appreciate is
that although a given pond around today is typically short-lived and manmade,
the pond habitat itself is ancient and enduring,” she says. “Modern ponds are
analogues of ancient habitats which, by lucky chance, humanity has
ٱ.”
Before humans
Palaeoecologists have recently found evidence of pond habitats in the
Pleistocene geological record at least 650 000 years ago, says David Keen of the
Centre for Quaternary Science at Coventry University. Telltale fossil remains of
pond snails, diving beetles and ostracods—tiny crustaceans shaped like
miniature kidney beans—have revealed the vital statistics of long-gone
ponds, including depth, substrate and even the amount of plant cover.
Painstaking examination of the fossil record by Keen, a mollusc specialist, and
his fellow palaeoecologists worldwide has now established that ponds were
ubiquitous long before any humans set foot on the Earth.
So today’s organisms have had millions of years to get to like ponds, and to
come to rely upon them. Thousands of different species are adapted to live in
them and their habits can be quite bizarre. Take the water striders, a large
family of insects that live on the surface film. A French zoologist has
calculated that to emulate striders humans would need to wear water-repellent
skates 18 kilometres long. Then there are water spiders which invented
diving bells long before any human came up with the idea. They weave underwater
webs among aquatic plants, then capture large bubbles of air between their hind
legs to fill the web so that they stay dry when submerged. What most pond
dwellers have in common, however, is an ability to move as conditions change,
because as one pond dries up, another one appears around the corner.
How mobile a plant or animal needs to be depends on the type of pond it
inhabits. There are three broad categories. Bog pools in mires, the first group,
are relatively stable and seem to have remained largely unchanged for thousands
of years. These ponds tend to stay wet because peat is constantly building up
around them, keeping them deep. The second family of ponds, near rivers or in
valley bottoms, come and go over years—though they are dubbed “permanent”
because they stay wet all year round. Large ponds of this type often arise when
river meanders become cut off from the main river channel. Much smaller ones in
the same category owe their existence to fallen trees. As rootballs are wrenched
from the soil, tiny new ponds—less than a metre in diameter and only 20 or
30 centimetres deep—are created.
These miniature ponds can be surprisingly hospitable. In one tree-fall pool,
Pond Action found 40 species of invertebrates, including a mud snail listed in
Britain’s invertebrate Red Data Book of endangered species. Tiny ponds
often last up to six or seven years before being filled in by eroded soil and
accumulated sediment, while the much bigger ponds produced from cut-off meanders
may have lifespans of anything from 100 to 5000 years. But the important point
is that whatever the longevity of “permanent” ponds, natural processes ensure
that new ones are created to replace the ones that have disappeared.
Most intriguing of all are “seasonal” pools, the third kind of pond. These
usually persist over the wetter winter months, but dry up as summer approaches.
Surprising biological riches lurk even in these unprepossessing puddles. They
are home to a distinctive community of unusual plants and
animals—sometimes with rarities such as tadpole shrimps, emerald
damselflies and natterjack toads. About a quarter of the wetland plants given
special protection by British legislation are restricted to these temporary
ponds. And more than 40 per cent of rare plants, such as fox sedge, are found
only in seasonal ponds.
To the rescue
The British charity Plantlife has launched rescue missions for a whole group
of rare plants associated with “shallow ephemeral pools”, including starfruit,
brown galingale and three-lobed water crowfoot. “These plants have a
breathtakingly clever lifestyle,” says Plantlife botanist Rosemary FitzGerald,
“germinating just when water and warmth are right, racing through flower and
seed production while conditions are warm and dry, and dropping long-lasting
propagules to lurk in the mud until conditions are right for the next go,
whether that’s months or years ahead.” The only trouble, she says, is that
modern life threatens the whole habitat. As land is drained by farmers and
developers, wetlands disappear and “without the mud their skills are wasted”,
says FitzGerald.
In California, where such ephemeral ponds are known as vernal pools, Marie
Simovich of the University of San Diego has found fairy shrimp and 80 other
species of crustacean in a few pools in Central Valley. Californian ecologists
have started to look at such ponds throughout the US and are finding similar
levels of diversity in unpolluted areas. Conservation campaigns for vernal pools
are now under way: near the end of Noble Drive in San Diego, for instance, the
city has created a unique nature reserve—a patch of dried mud surrounded
by a chain-link fence. The aim is to safeguard land that at first sight seems
like a prime candidate for drainage.
Fearless frolics
Seasonal pools may not look like much to humans, yet they offer distinct
benefits for wildlife. Because they are shallow, they heat up quickly in spring,
providing a thermal boost for fast-growing, active species like the scarce
emerald damselfly, says Steve Brooks of the Natural History Museum in London.
What’s more, fish are usually absent, so invertebrates can frolic with less fear
of predation, as Mercia Whitfield, one of Pond Action’s invertebrate ecologists,
points out. A downside, of course, is that these creatures have to race through
their lifecycle to reproduce before the water disappears. But there is another
unexpected advantage: temporary ponds are surprisingly tenacious, reappearing
year after year in the same place. In fact, says Whitfield, “They have the
potential to be the most persistent of all, surviving for many thousands of
years and outliving even large lakes.” Their secret is their resistance to the
buildup of sediment. As the ponds dry up, organic material is oxidised away to
leave hollows that receive water every year in winter and spring.
Pond Action’s research has established that, provided nature is left to its
own devices, all three types of pond make remarkably reliable habitats. Bog
pools are well-nigh permanent, while so-called “permanent” all-year-round ponds
are constantly being filled in, but are recreated naturally elsewhere. And
seasonal ponds return year after year when the time is right. A key message
emerging from the thousands of hours that the Pond Action team has spent
knee-deep in water is that the diversity of ponds themselves is central to their
success as a habitat. “We need to protect all sorts of ponds, of all sorts of
ages, because there are creatures that are specifically adapted to each kind of
habitat,” says Williams.
Inspired by visions of ponds’ wonderful, unfulfilled promise, Pond Action
researchers have now devised an objective way to assess their potential for
conservation purposes. “The lack of a good classification system for ponds has
long been a bugbear for conservationists,” says Simon Pickering, conservation
adviser to Britain’s Wildfowl & Wetlands Trust, based in Slimbridge,
Gloucestershire. “Pond Action’s new method for the first time enables
conservationists to say with confidence that particular ponds are good examples
of their type.” Already, ecologists in Switzerland, France and Italy are taking
up this new technique, and it is likely to spread even further.
You start by measuring 11 physical variables reflecting such things as a
pond’s geology, its size and depth. Given this information, and using Pond
Action’s extensive background data on the most pristine ponds, you can predict
which representative invertebrates and plants should be present if the pond is
not degraded. Then you find out what is really living there and score your
results. The final score, expressed as a percentage of the pond’s potential,
gives a reliable assessment of its “ecological integrity”.
Ponds are “barometers of landscape quality”, says Biggs. Those protected from
agrichemical pollution and industrial runoff—ponds situated in ancient
woodlands or unfertilised meadows, for example—are outstandingly rich in
both plant and invertebrate species. In a good-quality, medium-sized pond,
ecologists are likely to spot fifty wetland plant species and catch as many as a
hundred aquatic invertebrates, just by sampling the water for three minutes
using a hand net, three times a year. Conversely, a heavily polluted pond is
among the most degraded and impoverished habitats you will ever see.
Pond Action’s latest research shows that half of all Britain’s lowland ponds
score low in conservation value—way below their potential, with only half
the number of wetland plant species expected. “Species richness was lowest where
ponds were surrounded by arable land,” says Biggs. Such ponds are exposed to a
chemical cocktail mixed from among the 450 biocides now in use on farmland,
spiked with farm waste, fertilisers and road and urban runoff. The majority of
garden ponds also contain very little wildlife, as most are surrounded by
intensively managed lawns and many suffer from being stocked with fish.
There is good news, however. “The poor quality of ponds is potentially one of
the most solvable countryside management problems,” says Biggs. Unlike rivers
and lakes, many ponds have comparatively small catchment areas that can be
shielded from pollution. Landowners could be paid to create buffer zones around
ponds—a move that would quickly improve water quality. “The ideal would be
the de-intensification of a field—one or two hectares—around the
pond,” he says.
Conventional wisdom has it that ponds need to be “cleaned out” to remove silt
and surrounding vegetation, and deepened to prevent drying out. You can even buy
a “Pond-E-Vac”, advertised as “the most efficient pond vacuum cleaner available
to date”. But Pond Action’s findings show that such management is rather
counterproductive.
Dig for victory
“You don’t have to worry too much if a pond that is already shallow dries out
now and then,” says Biggs. “There is no need for urgent dredging—in fact,
that may do more harm than good.” Temporary ponds, in particular, are easily
damaged by over-deepening. And ponds shaded by trees or “choked” with aquatic
plants are not necessarily less rich in wildlife, their research shows. The odd
period of drying may even favour rare plants such as starfruit, whose seeds need
to dry out before they can germinate. Or great crested newts, which benefit from
the death of predatory fish.
A better way to manage ponds is to leave existing pools alone and dig new
ones. “The challenge facing contemporary conservationists is to mimic the
natural processes of pond creation,” Williams says. Clusters of new ponds should
be dug in areas protected from intensive land use. Pond Action’s surveys show
that the more isolated a pond, the less likely it is to be rich in species. Even
urban toads living in gardens, parks and cemeteries are becoming dangerously
inbred as a result of isolation, according to a recent study of common toads
living in Brighton by Sue Hitchings and Trevor Beebee at the University of
Sussex.
In one innovative scheme, Pond Action has worked with Britain’s Environment
Agency and Thames Water to create Pinkhill Meadow. This two-hectare wetlands
nature reserve, established eight years ago between the River Thames and Farmoor
Reservoir, near Oxford, contains a mosaic of distinctive ponds designed to
provide a diversity of habitats. Already it has been colonised by 20 per cent of
Britain’s wetland plant and larger invertebrate species.
Such projects show that new ponds can have a high conservation value, says
Biggs. Provided the location is chosen carefully, many soon become richer in
species than older ponds. “Without making a huge effort, we could have loads of
near-pristine freshwater pools dotted around the countryside,” says Biggs.
If that happens, the popular image of humble ponds and their watery
inhabitants will have undergone a miraculous makeover.
-
Further reading:
Lowland Pond Survey
by Penny Williams and others, June 1998,
available from the Department of the Environment, Transport and the Regions,
London SW1E 5DU, 0171 890 3000, www.detr.gov.uk -
Biological techniques of still water quality assessment: phase 2 method development
by Penny Williams and others, R&D technical report E56,
Environment Agency, Bristol -
Designing new ponds for wildlife
by Penny Williams and others,
British Wildlife, vol 8, p 137 (1997) -
New approaches to the management of ponds
by Jeremy Biggs and others,
British Wildlife, vol 5, p 273 (1994)
