《韦氏》relative to ：formal

1 — used to describe what someone or something is like when compared with or measured against someone or something else

• Dolphins have large brains relative to their body size.

• The value of the house is low relative to similar houses in the area.

网友的句子中，relative to = compared with

Thus, relative to plant foods, meat is a highly nutritional food resource.

= Thus, compared with plant foods, meat is a highly nutritional food resource.

所以，relative to plant foods 为句子状语（修饰句子，表示句子内容成立的条件）。