The earthworm population had declined by 70 %.是什么意思？

The earthworm population had declined by 70 %.

蚯蚓数量下降70%.

the

art. 指已提到的人（物），指说话人与听者已知的人（物），用于独一无二的事物前，与形容词最高级和序数词连用；

例句
He did have a real knowledge of the country...
他对该国的确知之甚多。

earthworm

n. 蚯蚓；

例句
An earthworm is a terricolous organism.
蚯蚓是陆生的生物.

population

n. 人口，全体居民，特定[生物]种群，[物]布居；

例句
60 years ago half the French population still lived in rural areas.
60年前有一半法国人仍然生活在乡村地区。

had

v. 有（ have的过去式和过去分词 ），（亲属关系中）接受，拿，买到；

例句
He claimed that his parents had abandoned him...
他声称父母遗弃了他。

declined

v. 辞谢，谢绝（邀请等）( decline的过去式和过去分词 )，（道路、物体等）下倾，（太阳）落下，（在品格、价值上）降低；

例句
The President declined to deliver the speech himself, on account of a sore throat...
总统因嗓子不适而婉拒了亲自讲话。

by

prep. 在…旁边，表示方式，由于，经过；adv. 经过，表示保留或保存时用，短暂拜访；

例句
...expensive make-up that we saw being advertised by a beautiful model...
我们所见的由一位漂亮模特代言的昂贵化妆品