Old next to young
Is it long-established, the diversity in the rain forest belt of the earth, or rather a recent achievement? There are arguments for both variants, as well as clear standpoint holders. And a few colleagues who advocate both/and - with well-founded data.
Ecologists in the tropical regions of the world should have a thing for big numbers. Otherwise, they'll be overwhelmed by constantly juggling hundreds and thousands when the discussion turns to the unimaginable we alth of species in their field.
While the whole of Central Europe is happy about around 8000 beetle species, Würzburg scientists have already collected a quarter of them on just 19 tropical trees. Germany knows about a thousand species of spiders - the rainforest regions of Australia almost ten times as many. Every listing is reminiscent of the old children's quartet, although it is clear from the outset that, with a few exceptions, the map of the tropics always stands out.
The big questions about the how, when and why of biological diversity near the equator are as "Image" as controversial. A stable ecosystem over tens of millions of years, which enabled an undisturbed splitting up and preservation of species, is postulated by some: the tropics as a museum collection of Methuselah species. alt="
Wrong, says the opposite faction. The speciation is young, almost explosive, in any case the result of events that geologically speaking are not long ago, such as the change from forest and savannah to ice ages or the creation of the land connection of the isthmus between North and South America about three million years ago.
Incompatible positions? Hardly likely. Because, as always with such either-or disputes, the suspicion arises as to whether it shouldn't be a bit of both. Duane McKenna and Brian Farrell from Harvard University also tend towards this forgiving variant. And back them up with data from their speci alty: leaf beetles.
More precisely: leaf beetles of the genus Cephaloleia, with over 200 species another example of a tropical quartet sting candidate. The inhabitants of the South and Central American rainforests feel particularly at home on Zingiberales, the extensive relatives of ginger.
Some of the animals have developed a special and exclusive preference for the youngest, still rolled leaves. Evidence of feeding on leaf fossils dating back 66 million years suggests that Cephaloeia favored this flavor as early as the Late Cretaceous.
Using mitochondrial and nuclear DNA from 95 Cephaloleia species and close and non-relatives, the researchers constructed a family tree of this lineage. It can be clearly seen that the rate of speciation is not uniform, but flattens out and shows several stages from 38 million years ago.
The starting signal for a first wave of diversification was the massive transformation of the habitat at the Cretaceous-Tertiary boundary, when numerous animal and plant species died out, paving the way for new developments. Supported by global warming 55 million years ago and the associated spread of preferred forage plants, the Cephaloleia -representatives experienced a first flowering. After that, the formation of new species weakened: all niches were conquered, and the cooler climate forced some tropical plants back to regions closer to the equator and thus promoted the extinction of some beetle species, which now lacked the basis for food.
33 million years ago, however, the group of young leaf specialists was growing again - at the same time as an increasing variety of Heliconia, the main forage plant. After a phase of calmer conditions, it was only twenty million years later that things really got going again: twelve million years ago years, new species formed again, a phase that reached a peak about four to five million years ago. Again there is an explanatory contemporaneous event: the Panama Arc docked with South America and construction of the isthmus bridge was completed 3.5 million years ago. During this time, new corridors and barriers caused the number of species to grow again and the traditional collection to be enriched by a few younger members.
Truly a nice, round story. And proof that Methuselah species and youngsters can equally contribute to diversity in an animal group. In fact, as is so often the case, it is not an exclusive either/or, but an accommodating both/and. But of course that's not something to argue about.
