THERE鈥檚 nothing quite like dank tree holes full of murky rainwater and
rotting leaves for research students Diane Srivastava and Jill Birch. Where a
casual observer might see only decay, they see a miniature ecosystem that is an
ideal place to test ecological theories.
Srivastava and Birch, who are based at Imperial College鈥檚 Centre for
Population Biology, are interested in the mechanisms that govern biodiversity in
natural communities. They are part of a growing band of ecologists working on
鈥渃ontainer habitats鈥� such as water-filled tree holes and other small pools held
by plants (phytotelmata), plain old rain puddles or even water-filled tyres.
The great attraction of these small bodies of water is that they contain
simple, self-contained communities that are easy to manipulate, say ecologists
such as Roger Kitching of Griffith University, Brisbane鈥攐ne of the first
to spot their potential [鈥淓very pitcher tells a story,鈥� 快猫短视频,
23 January 1986, p 48].
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John Lawton, who directs the population biology centre at Silwood Park in
Berkshire, couldn鈥檛 agree more. 鈥淭hey鈥檙e a heck of a lot easier to study than an
entire lake or an entire forest,鈥� he says. 鈥淭hey鈥檙e great experimental systems.鈥�
Unravelling the workings of these communities is also helping medical
entomologists learn more about the ecology of mosquitoes that carry disease,
some of which breed in water-filled tree holes.
Although some ecologists claim that container habitats are to ecology what
fruit-flies are to genetics, the unconverted may feel that simple systems such
as tree holes don鈥檛 tell researchers much of importance about the real world.
But Lawton disagrees. 鈥淚f there are general rules and principles about the way
that ecological systems are put together, we ought to be able to discover them
by studying these little systems just as we ought to be able to discover them by
studying a forest,鈥� he says.
You have to start somewhere鈥攁nd what could be better than a tree hole,
full of wriggling insect larvae? For ecologists like Lawton, container habitats
are natural aquatic equivalents of the celebrated Ecotron facility, housed at
Silwood Park, in which simplified terrestrial systems are being studied in
replicated tests.
One key ecological topic which tree hole researchers are tackling is the
relationship between environmental factors and the structure of food webs.
Two ideas have found favour with ecologists over the years. One sees the
amount of energy entering the food web (say the amount of energy captured by
green plants) as determining its structure. With more energy, more species can
coexist, the predators can make a decent living and the food chains making up
the web can be longer.
Freeze or dry
The other idea focuses on the unpredictability of the environment and the
possibility that frequent disturbances such as drought could stop some species
gaining a toehold in a system, affecting the overall web structure. If drought
strikes a tree hole too often, for instance, insects with long life cycles would
lose out because their larvae would rarely mature.
For the last decade Kitching and his colleagues Bertram Jenkins from the
University of New England, Armidale, and Stuart Pimm of the University of
Tennessee, Knoxville, have been trying out these ideas in the rainforests of
Queensland and New South Wales. Natural tree holes in the region support a
complex fauna of insect larvae, mites and crustaceans. Dead leaves and other
organic debris fall into the holes, providing food for many animals. Assorted
predators, including midge larvae, mites and tadpoles, dine on the
litter-eaters. Tree holes are usually too dark for plant growth, so the rain of
litter that falls into them is the animals鈥� only source of energy.
While these communities make ideal test subjects, actual tree holes vary too
much in size and shape for repeatable experiments. So the researchers ran their
tests in water-filled plastic containers fixed to the bases of trees. Plastic
containers are apparently just as popular with the local fauna as the real thing
and as a bonus you can control the amount of energy (leaf litter) you supply and
see how their food webs develop. Kitching鈥檚 team provided three levels of
energy: normal, a tenth of normal and a hundredth of normal.
They found that both theories seemed to hold true: energy and disturbance
both influenced food web structure. When the artificial tree holes ran on a
tenth or a hundredth of the normal amount of litter, their main predator (a
midge larva) set up house elsewhere and food chains were correspondingly
shorter. But equally, predators were affected when drought struck and food
chains also became shorter.
Kitching now plans to run further tests to find out more about how natural
tree hole food webs are assembled. 鈥淭here are two complementary levels of
approach to the theoretical explanation of food web structure,鈥� he explains. As
well as the 鈥渢op-down鈥� approach of looking at energy and disturbance, you can
also focus on underlying mechanisms鈥攈ow competition between species, or
the presence of predators, influences the mix of species that can survive in a
tree hole.
This 鈥渂ottom-up鈥� approach is at the heart of Kitching鈥檚 latest work, which
will involve studying interactions between tree hole species in the laboratory.
鈥淭he real understanding of food web structure involves elements of all these
ideas,鈥� says Kitching. The next stage will involve bringing them all together.
鈥淭here鈥檚 no `synthetic鈥� theory of food webs at this stage, but that is the
direction of present work,鈥� he says.
Other researchers, such as Lawton and his team at Silwood Park, are
preoccupied with understanding the factors that affect species richness in
natural communities鈥攕omething of crucial importance to conservation.
British tree hole communities differ from Australian ones in lacking predators,
but they are every bit as good for staging experiments on biodiversity.
At Silwood Park, one of Srivastava鈥檚 aims has been to find out more about how
and why more energy leads to a richer array of species. Traditional thinking
says that a system with more energy can support more individual
animals鈥攁nd that more individuals means more species. The basic idea says:
a species can only get a toehold in the system if it鈥檚 represented by a certain
minimum number of individuals. If there are more individuals overall, then more
species can muster the necessary minimum.
Subtle processes
Srivastava monitored local tree hole communities as they colonised plastic
containers supplied with different amounts of leaf litter. Sure enough,
there were more species in the containers with more leaf litter, the so-called
high-productivity communities. But the details didn鈥檛 square with traditional
theory.
鈥淲hen communities assemble, you鈥檇 predict that the high-productivity
communities, with lots of species, would also have lots of individuals鈥攂ut
they don鈥檛,鈥� explains Srivastava, who is now based at the Center for
Biodiversity Research, at the University of British Columbia. Current theory, it
seems, needs modifying to explain the subtle processes at work in tree holes.
More individuals aren鈥檛 always a prerequisite for more species. 鈥淲e get the
right answer for the `wrong鈥� reasons,鈥� says Lawton.
Jill Birch, another student at Silwood Park, is tackling the role of
disturbance. She subjects rehoused tree hole communities to varying degrees of
disturbance鈥攊n the form of drought鈥攚hich she inflicts by a
combination of evaporation and gentle drainage of their plastic containers. Not
all insect populations take kindly to these insults. 鈥淢osquito larvae are absent
from all the highly disturbed treatments,鈥� explains Birch, 鈥渂ut other larvae
survive and come out of the woodwork, as it were, even when they鈥檝e been dry for
21 days in 24.鈥� Midge larvae and hoverfly larvae are among these hardy
residents.
As well as studying the responses of individual species, Birch will be
looking at the overall relationship between disturbance and the number of
species in a community. In general, ecologists expect this relationship to be
hump-shaped. Too much disturbance should drive diversity down, because few
species can cope with repeated devastation of their populations.
But when disturbance gets rarer, diversity should climb to a maximum, before
dropping back again. One explanation for the decline is that as disturbance
becomes rarer, species with similar lifestyles will have the time and
opportunity to compete鈥攁nd some species will exclude others. Some
disturbance could, in other words, sometimes be good for diversity鈥攁n idea
that goes back to the US ecologist Evelyn Hutchinson and is gaining support in
new approaches to conservation management.
It also seems that research on container habitats may have important
implications for public health. These habitats are home to the young stages of
many mosquitoes鈥攊ncluding those that transmit serious viral diseases like
yellow fever and dengue. Urban forms of these mosquitoes may live in old tins
and tyres, but their ancestors were once tree hole dwellers. 鈥淭he community
ecology of such species in phytotelmata has direct relevance to their role as
urban disease vectors,鈥� says Phil Lounibos of the Florida Medical Entomology
Laboratory.
Lounibos and other US entomologists are monitoring the spread of Aedes
albopictus, a dengue-transmitting species that has recently invaded the US
from Asia. The species, which breeds in tree holes in Asia, apparently arrived
in the US as a stowaway inside used car tyres, and has now spread across a wide
area, breeding in tree holes and other containers. Although home-grown dengue is
little known in the US at present, any spread of a new vector species is
worrying. A key question for epidemiologists and pest controllers is how the
invader interacts with other species of mosquitoes in containers. Does it live
in harmony with existing species, or is it ousting them?
A few years ago, Todd Livdahl and Michelle Willey, biologists at Clark
University in Worcester, Massachusetts, ran tests in the laboratory to study
competition between the new arrival and the native A. triseriatus which
is not, in practice, a dengue vector. They suggested the two species would
cohabit in tree holes, but that the newcomer would oust its adversary from
tyres.
鈥淥ur studies here in Florida have confirmed these predictions, with
limitations,鈥� says Lounibos. He explains that A. triseriatus and A.
albopictus live side by side in tree holes, but in sunny tyres鈥攊n
many parts of the state鈥� A. albopictus has displaced both the
nativeA. triseriatus and A. aegypti, the 鈥測ellow fever鈥�
mosquito introduced centuries ago. 鈥淭he degree to which it [A.
albopictus] usurps niches previously occupied by less threatening species
is a major concern of the medical entomological community,鈥� he says.
It鈥檚 hard to imagine any habitat less glamorous than the average tree hole or
tyre. Dark, smelly and dirty, it seems almost the last place you鈥檇 look for
biological enlightenment. But the humble container habitat is proving more than
worthy of the effort invested in it by researchers, providing a pool of
information that would be hard to gather anywhere else.