as引导比较分句，意为就像...一样。我用简单例句说明这个问题：

Tom is tall, as John is.

as分句可以用倒装语序，强调主语或因为主语太长。所以也可以说：

Tom is tall, as is John. 汤姆个子高，就像约翰一样。

这个句子与so+倒装句意思接近，但结构不同。as为状语从句，so句则是并列句或独立句。

Tom is tall, and so is John.

Tom is tall; so is John.

Tom is tall. So is John.

你可以看出，as替代了主句的某个成分。被替代的成分可以是动词、形容词或副词。

Tom has finished his home work, as has John/as John has.

as替代了finished his home work。

你的句子就是这种结构，as替代了vanished from huge streches of countryside。

该句也可以改为so have+主语结构，表示类似意义：

In Britain, for example, many of our best-loved farmland birds, such as the skylark, the grey partridge, the lapwing and the corn bunting, have vanished from huge streches of countryside, and so have even more wild flowers and insects.