英语时差：山楂树如何帮助构建生态平衡2

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 sloths, tapirs, and peccaries browsed 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 ecological balance between the plants and a large group of voracious herbivores. Some of the other plants whose thorns evolved in response to ancient herbivores include wild plums, wild crab apples, honey locust 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 reminders of a whole range of animals that we now know only in fossil form.