These firms have been given a tough lesson in the need to diversify their markets.

the need 是名词“需要”，可引申为“缺乏”。to diversify their markets 是其定语。

the need to diversify their markets ：缺乏使市场多样化；缺乏多样化市场。

in the need... ：可以是“方面状语”，这里也可理解为原因，意为 “因缺乏......”

These firms have been given a tough lesson in the need to diversify their markets.

【直译】这些公司在缺乏市场多样化上得到了惨痛的教训。

【意译】这些公司因缺乏多元化市场而遭受惨痛之教训。