Beware the 

 Toxic Toad! 



Evolution has a way 

 of fighting back. No- 

 where is that process 

 more visible than in Aus- 

 tralia, where one species of snake 

 is mounting a defense against the cane 

 toad, an ever-encroaching menace. The 

 toad, which can weigh three pounds, was in- 

 troduced in 1935 in an early attempt at bio- 

 logical control of sugar cane's insect pests. 

 Alas, that well-meaning act unleashed an 

 ecological disaster. Cane toads are toxic 

 enough to kill a person. In addition to de- 

 vouring native insects and small vertebrates 

 they've been decimating would-be toad 

 predators, including certain lizards, marsupi- 

 als, and snakes. 



Ben L. Phillips and 

 Richard Shine, biologists at the University 

 of Sydney, are studying the impact of cane 

 toads on Australia's native red-bellied black 

 snake {Pseudechis porphyriacus). In a series 

 of experiments, Phillips and Shine have 

 shown that certain populations of the 

 snake have evolved not only to avoid the 

 toads, but also to resist their toxins (to a 

 degree). Those characteristics are hard- 

 wired into the snake's genome. The longer 

 a snake population has been exposed to 



Red-bellied black snake 

 and cane toad 



cane toads, the more likely its members are 

 to avoid eating the toads, and the greater 

 their resistance to toad toxins. 



Does the snake's evolution offer a glim- 

 mer of hope for conserving Australia's 

 native species? Perhaps. But Phillips and 

 Shine have also shown that cane toads at 

 the advancing front of an invasion are evolv- 

 ing longer legs, which enable them to dis- 

 perse ever more quickly. {Proceedings of the 

 Royal Society B, doi10.1098/rspb.2006. 

 3479, 2006) —N.W.A. 



Proto-Alexandria Fog Lifts on Ozone 



Alexander the Great founded the Egyptian 

 city of Alexandria in 331 B.C. Some scholars 

 say the spot had been vacant land, but given 

 its natural harbor and its proximity to the Nile 

 delta, it should have attracted earlier settlers. 

 Indeed, ancient texts suggest the presence 

 of a prior community, called Rhakotis, but 

 there is little archaeological evidence 

 to corroborate those accounts. 



To settle the debate, Alain Veron, 

 an environmental chemist at Paul 

 Cezanne University in Aix-en- 

 Provence, France, and four col- 

 leagues extracted cores of sediment 

 from Alexandria's old harbor. They 

 determined the age of each layer of 

 sediment by carbon-dating the 

 seashells and corals trapped within it. 

 Then they measured the amount of 

 lead in each layer, because lead is a 

 telltale sign of advanced human activ- 

 ity. Since ancient times, people have used 

 lead for glassmaking, plumbing, shipbuild- 

 ing, and statue casting — and also polluted 

 their harbors with it. 



As expected, the investigators' analysis 

 shows a sharp increase in industry around 

 the time Alexandria was founded, and ele- 

 vated levels for hundreds of years there- 

 after. (In fact, the lead levels in Alexandria's 

 ancient harbor were twice what they are in 

 modern industrial estuaries.) But the analy- 

 sis also shows two small, earlier spikes. The 

 first dates to about 2300 B.C., when many 



Way up in the stratosphere- — about ten to 

 thirty miles above Earth — ozone protects 

 our planet from damaging ultraviolet radia- 

 tion. In the troposphere, though — below 

 about ten miles — that same molecule is a 

 potent greenhouse gas. Its exact effect on 



Alexandria, Egypt, depicted in a sixteenth- 

 century German map 



settlements sprang up in the Nile delta. The 

 second took place about 900 B.C., at the 

 end of the prosperous twentieth Egyptian 

 dynasty, just before Assyrians, Nubians, 

 and Persians got into the habit of invading 

 Egypt and sacking its towns. Rhakotis may 

 not have been much to look at by the time 

 Alexander arrived, but it nonetheless 

 appears to have had a long history of 

 settlement. [Geophysical Research Letters 

 33:L06409, 2006) —S.R. 



global warming has remained elusive, but 

 a new study paints the clearest picture 

 yet of how ozone (0 3 ) has affected 

 Earth's climate during the past century. 



Tropospheric 0 3 forms mainly from 

 reactions among certain emissions from 

 fossil-fuel combustion, sunlight, and 

 water. With the help of computer simu- 

 lations, a team of climate scientists led 

 by Drew Shindell of NASA's Goddard 

 Institute for Space Studies in New York 

 City found that tropospheric 0 3 levels 

 rose by about 40 percent between 1890 

 and 1990, but not in a uniform way. 



Levels of 0 3 rose gradually before 

 the 1950s but more quickly thereafter, 

 thanks to surging industrialization 

 across the developed world. Higher 0 3 

 levels prevail in regions surrounding in- 

 dustrial centers and in areas prone to 

 forest fires, where substantial quantities 

 of its precursor chemicals are released. 

 Consequently, there's more surface 

 warming in those regions. Increased tro- 

 pospheric ozone, Shindell's team dis- 

 covered, thus led to a greater rise in 

 temperatures in the more industrialized 

 Northern Hemisphere than in the less 

 industrialized tropics, and a greater rise 

 in the tropics than in the Southern 

 Hemisphere. And watch out, Arctic 

 ice: 0 3 is an exceptionally powerful 

 warming agent over reflective surfaces. 

 (Journal of Geophysical Research 111: 

 D08302, 2006) —N.W.A. 



July/August 2006 NATURAL HISTORY 



13 



