1 medium sized antelopesbrowsing
2 turned ancient african forests into
4 5 6
millions of years ago, africa's
7 were covered with thick, ancient forests, which disappeared and turned into the grassy ecosystems that they are today. almost a fifth of the world's land surface is covered by savannas. yet, for years it has been a mystery how these grassy ecosystems came to replace the ancient forests.
数百万年前，非洲的大草原上覆盖着茂密的古老森林，这些森林消失了，变成了今天的草地生态系统。世界上近五分之一的陆地表面被大草原覆盖。然而，多年来，这些长满青草的生态系统是如何取代古老森林的，这一直是个谜。
one answer, it was thought, might be climate change, yet most savannas occur in climates that also support forests. the other possibility was fire. savanna trees and
8 are often adapted to frequent fires, while forest trees are not.
人们认为，一个答案可能是气候变化，但大多数稀树草原的气候也有森林。另一种可能性是火灾。草原树木和灌木通常能适应频繁的火灾，而森林树木则不然。
but a study that includes a group of south african scientists has found that the arrival of browsing medium sized
3 was probably what turned africa's ancient forests into the open savannas.
但一项包括南非科学家在内的研究发现，食草中型羚羊的到来可能是将非洲古老的森林变成开阔草原的原因。
by comparing the
9 of the evolution of thorns on about 2000 woody tree species in southern africa and the time that antelopes arrived in africa, a group of scientists, including dr gareth hempson from the school of animal plants and environmental studies at the university of witwatersrand, found that trees like african acacias evolved thorns as a defence
10 at exactly the same time that antelope arrived in africa.
通过比较南部非洲约2000种木本树种刺的进化时间和羚羊到达非洲的时间，包括威特沃特斯兰德大学动植物与环境研究学院的gareth hempson博士在内的一组科学家，发现像非洲acacias这样的树在羚羊到达非洲的同时进化出刺作为防御机制。
"it all makes perfect sense," says hempson. "
11 (thorns) really appear to be most effective against medium- and large-sized
12 like impala and kudu, and
13 trees are most common in the places where these animals are most abundant," says hempson.
“这一切都很合理，”亨普森说。hempson说：“刺确实对黑斑羚和捻角羚等中大型食草动物最有效，而刺树在这些动物出没的地方最常见。”
"we were shocked," said professor van der bank "to discover that spiny plants only appeared about 15 million years ago, 40 million years after mammals replaced
14". for most of this time, africa was an island continent dominated by now-extinct ancestors of browsing elephants and hyrax. "
15, spines just didn't work as a plant
16 against these ancient mammal groups" commented prof van der bank.
范德班克教授说：“我们很震惊地发现，多刺植物大约在1500万年前才出现，也就是哺乳动物取代恐龙4000万年后”。在这段时间的大部分时间里，非洲是一个岛屿大陆，由现已灭绝的浏览大象和鬣蜥祖先统治。范德班克教授评论道：“显然，刺并不能起到植物防御这些古老哺乳动物的作用。”
but in a
17 example of apparent coevolution, the
18 of spiny plants (thorn trees) coincides with the appearance of antelope.
但在一个明显共同进化的显著例子中，多刺植物（荆棘树）的多样化与羚羊的出现不谋而合。
antelope were latecomers to africa appearing only after the continent collided with eurasia. they
19 in novel ways and were highly efficient herbivores. this injection of new types of browsers, argue the authors,
20 young forest trees, opening up forests to the grass
21.
羚羊是非洲大陆与欧亚大陆碰撞后才出现的后来者。它们以新颖的方式进食，是高效的食草动物。作者认为，这种新型进食方式的注入摧毁了年轻的森林树木，为草地入侵者开辟了森林。
the parallel radiation of spiny plants and the antelope that feed on them
22 the rise of savannas in the drier more fertile regions of africa. fire only began to roll back the forests to create wetter savannas several million years later.
多刺植物和以它们为食的羚羊的平行辐射引发了非洲干旱肥沃地区稀树草原的兴起。几百万年后，大火才开始使森林退缩，形成更潮湿的稀树草原。
one implication of the study is that the loss of africa's native browsing antelope may threaten the future of drier savannas, and lead to their
23 by
24, woody scrub of little
25 or economic value.
这项研究的一个意义是，非洲本土觅食羚羊的消失可能会威胁到干旱稀树草原的未来，并导致它们被茂密的、木质的小生态灌木所取代
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