PICTURE an ecosystem 鈥� a tropical forest, for instance. Visualise the lush plant life, the pollinating insects, the birds that prey on them and the snakes that eat the birds 鈥� all affecting each other in a complex web of interactions.
But how does that interaction occur? The idea is gaining ground that a small number of 鈥渒eystone鈥� molecules do much of the work. Take them away and the entire ecosystem collapses.
A classic example of a keystone molecule is dimethyl sulphide (DMS). It is already well known that this gas is released into the atmosphere by microscopic algae when they are chewed by predators. There it is oxidised into a suite of compounds, some of which act as condensation nuclei for the droplets that form clouds. Those clouds, in turn, reflect sunlight and cool the planet.
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But now it appears that DMS could play a more active role. DMS itself may set in motion a chain of events that lead to it being released into the air. This simple, lifeless molecule may be the protagonist in a hierarchy of relationships that begin with the lowly algae and spread through the food web.
Nor is DMS alone. Other keystone molecules have been found to glide through the air and water around us, manipulating all stages of life en route, from reproduction to dispersal and even to survival itself.
Studying the impacts of keystone molecules is the aim of a new field dubbed 鈥渘euroecology鈥�. It represents ecosystems as elaborate communications networks with species as the nodes and keystone molecules the wires connecting them. Ecologists have long focused on the nodes, or species. But neuroecologists claim we should also be paying attention to the messengers that underpin the system, carrying information from one species to the next. This was the theme of a symposium at the Society for Integrative and Comparative Biology conference in Salt Lake City, Utah, this month.
Understanding these molecules could allow us to unravel 鈥渢he grand complexities that we often confront when dealing with natural systems鈥�, says , an ecologist at the University of New South Wales in Sydney, Australia, who contributed to the symposium. To qualify as a keystone, a molecule must penetrate deep into the ecosystem and manipulate interactions all along the food web. But Steinberg warns ecologists not to get carried away as the quest for such molecules intensifies. 鈥淵ou need to make sure that everything under the sun doesn鈥檛 become a keystone molecule,鈥� he says.
Nevertheless, in the four years since research into keystone molecules began in earnest, several likely candidates have started to emerge, often where least expected. A mere 1/30th of the amount of a neurotoxin present in some species of newt is enough to kill the average human being. Yet this molecule may orchestrate life as much as it does death.
The newts release the toxic molecule, called tetrodotoxin or TTX, when threatened by predators and also when they forage for food. Ecologists tracking how the molecule travels through the ecosystem have found that animals that encounter TTX use it in a variety of ways.
Garter snakes have co-opted TTX for their own defences. The snakes, which feed on rough-skinned newts (Taricha granulosa), have developed mutations that allow their neurons to , allowing them to store the toxin in their tissues. It now appears that the snakes are able to store the exact amount of TTX needed to kill the hawks which are their primary predator.
at the University of California, Los Angeles, has found that baby California newts (Taricha torosa) exploit TTX for a different purpose: as an alarm cue. The young newts, which have not yet developed their own TTX-producing glands, are cannibalised by adults. When adult newts get hungry and start releasing TTX as they look for food, the youngsters sense the molecule and flee ().
鈥淲e鈥檝e extended the importance of tetrodotoxin across four different levels of the trophic web,鈥� Zimmer says. Take it away, he adds, and you could change the entire ecosystem.
Keystone molecules are being discovered at sea, as well as on land. Adult barnacles continuously release a glycoprotein to develop and maintain their shells, and barnacle larvae use this molecule as a cue for what rocks to settle on. Meanwhile, whelks, which eat the barnacles, use the glycoproteins to sniff out their prey.
The king of the keystone molecules, says ecologist Gabrielle Nevitt of the University of California, Davis, is the familiar chemical DMS (see diagram). and have been shown to track DMS to find the tiny crustaceans they feed on. The crustaceans give the game away because they munch on algae which release DMS as they are consumed.
Animals further up the food web, such as penguins and northern fur seals, have also recently been shown to have evolved the neural circuitry needed to sense DMS. It is such a pervasive odour molecule that it is sometimes referred to as the 鈥渟mell of the sea鈥�.
鈥淭hat makes DMS a strong signal molecule candidate in higher animals,鈥� Nevitt says. She suspects DMS may keep marine ecosystems turning over because the higher mammals that it lures to prey on the smaller fauna produce excrement that nourishes the algae these fauna feed on.
It is becoming clear that the animal world is full of opportunistic eavesdroppers like this, with species up and down the food web intercepting molecular messages to meet their own needs. And while we have long known that animals evolve in response to each other, we are only just beginning to understand how they also evolve in response to this molecular third party.
Unravelling these interactions could have practical implications, says Charles Derby at the Georgia State University in Atlanta. Imagine an ecosystem in which a species that produces a keystone molecule is in decline. Under a conventional species-centred approach, conservation managers would observe the knock-on consequences of the molecule鈥檚 dwindling concentration without understanding how they are linked. A neuroecologist, in contrast, might suggest introducing a bacterium that produces the keystone molecule. Recognising the complex roles of some molecules could spare entire ecosystems.
When this article was first posted, it incorrectly stated that Charles Derby is at the University of Georgia. This has now been corrected.
