【英语时差8,16】山楂树如何帮助构建生态平衡2

（单词翻译）

 To understand why a plant would grow such heavy thorns, we need to look back ten or fifteen thousand years to a time when mastodons, mammoths, ground sloths¹, tapirs, and peccaries browsed² from trees and bushes all over North America. Unlike today’s smaller herbivores, these animals would have had trouble getting through the big thorns. So the thorns helped create an ecological³ balance between the plants and a large group of voracious⁴ herbivores. Some of the other plants whose thorns evolved in response to ancient herbivores include wild plums, wild crab⁵ apples, honey locust⁶ and black locust. Most of these large herbivores became extinct about ten thousand years ago but the thorns that we see on plants today are living reminders⁷ of a whole range of animals that we now know only in fossil form.