3 October 2006Henry Fountain
Along with rising temperatures, global warming is very likely to cause a shift toward more extreme weather — stronger storms with more rainfall, and longer and more severe droughts. Those changes are likely to have large-scale, obvious effects on farmlands, grasslands and forests and on the creatures that inhabit them.
But many smaller, more subtle effects are likely too. Researchers at the University of Kentucky looked at one: the impact of climate change on the decomposition of leaf litter on the forest floor.
The researchers, Janet R. Lensing and David H. Wise, studied the process of leaf decay in hardwood forests in central Kentucky. The main instigator in leaf decay is fungi, which get nutrients from the organic matter. But fungi don’t exist in a vacuum. They are grazed upon by springtails, primitive insects of the Collembola order. In turn, springtails are the prey of wandering spiders.
The who-eats-whom makes for a complex web, where changes at one level can have cascading effects. Too much or too little grazing by springtails, for example, can reduce fungal activity and slow decay.
Environmental changes can have an impact, too, and that’s what the researchers studied. They set up forest plots and manipulated precipitation to match anticipated future levels, both wet and dry. They didn’t see much change in leaf decomposition under higher-rainfall conditions. But under drought conditions, they found, decay accelerated significantly. Their findings are published in The Proceedings of the National Academy of Sciences.
“Our hypothesis is that during drought conditions, the fungi are already drought-stressed, and the Collembola are overgrazing them, which slows down decay,” Dr. Lensing said. Under these circumstances, by preying on the springtails, the spiders reduce the pressure on the fungi, thus allowing for more leaf decay. Under wetter conditions the fungi are not so stressed and so easily overgrazed, so spider predation on springtails has less effect.
It’s not that the dryness has a direct influence on populations of spiders, say, or springtails. Instead, Dr. Lensing said, “it affects how the cascading occurs” within the food web. This indirect impact on leaf decay, she added, “shows how complex the effect of altered rainfall can be.”
Survival Diet, for Frogs
Here’s a question that ecologists have wondered about for years: Is there a connection between how widespread a species is — the size of the territory it covers — and its diet?
One idea is that a species with a restricted range should have a specialized diet, in part because there is a less diverse selection of foods available in a smaller area. By the same reasoning, a species with a broad range should have a more varied diet, because there are more menu choices.
Selection might come into play as well. With a species shoehorned into a small space, there is a greater likelihood that any adaptation — to a single food source, for example — will spread through the species. This is less likely in a species with a broader range, where there would always be some mixing among individuals from different environments.
It’s a nice idea. But a new study shows the opposite is true, at least for rainforest frogs from the wet tropics area of northeastern Australia.
Yvette M. Williams of James Cook University and colleagues studied the stomach contents of 11 related frog species with ranges from 2,500 square miles to slightly more than 1 square mile (on a single mountaintop). All of the frogs dined on ants, spiders, beetles and other bugs, but the species with the smallest ranges had the most diverse diets. Those with the largest ranges ate mostly ants. The findings were reported in Biology Letters.
The researchers suggest that another hypothesis might explain this. The smaller the range, the more prone a species is to extinction. A small-range species that depends on one food source, then, risks being wiped out if that food source dries up. But one that is a generalist eater can better survive the vagaries of the food supply.
Silk Stockings, for Spiders
Web-spinning spiders have specialized organs, called spinnerets, that produce silk. They are usually located on the underside of the abdomen.
But scientists have discovered that a species of tarantula also secretes silk through its feet. The spider may use the fibers to help it climb slick surfaces.
Stanislav N. Gorb of the Max Planck Institute for Developmental Biology in Germany and colleagues made the discovery in zebra tarantulas, found in Costa Rica. The spider, which is about an inch and a half long, produces the silk through tiny nozzles in the base of the feet. The finding was reported in the journal Nature.
Like other spiders, the zebra tarantula also has thousands of tiny hairs on its feet that help it stick to surfaces through molecular attraction. But the researchers found that the foot silk, laid down as tracks, helped the spider walk on vertical surfaces. In experiments on glass, the researchers found that if the spiders slipped, the silk arrested their fall.
It’s not known if other spider species also have this capability. But however widespread it is, the researchers say, it raises new questions about the evolution of silk-making. Foot production of silk may be the ancestral condition, with spinneret production evolving later. Or the two may have evolved independently. Comparison of the genes involved in silk production from the feet and from spinnerets, the researchers say, should provide some clues.
Springtime on Uranus
With their swirling atmospheres, gaseous planets occasionally produce dark spots (Jupiter’s red spot being the most famous example). Spots are rare on Uranus, however, and no definitive images of them have ever been obtained.
Now, though, Lawrence Sromovsky of the University of Wisconsin and other astronomers have used the Hubble Space Telescope to produce clear images of a dark spot in the atmosphere of Uranus.
The spot, about 1,900 miles long and 1,100 miles wide, is in the planet’s northern hemisphere. The researchers say the spot may be new, a result of gradual warming of the northern hemisphere as Uranus moves into “spring” in its 84-year orbit around the Sun. Images are at hubblesite.org/news/2006/47.
