The Hauts-Pays Nature Park is in an area which contains several sites of very great biological value, sometimes even of national importance.

Certain plant species have found their last refuge in the Hauts-Pays (for example: sweet spurge (Euphorbia dulcis), which only grows at 7 sites in Belgium, 3 of which are in the Hauts-Pays) and in at least one case, this is the only place where it grows in Belgium: Southern wood-rush (Luzula forsteri).

In Wallonia, the Hauts-Pays is the last refuge north of the Sambre / Meuse line for many plants and animals (for example, Box (Buxus sempervirens)).

The presence of populations which are still particularly well-supplied with several species of calcicole orchids (Bird's nest orchid (Neottia nidus-avis), White helleborine (Cephalanthera damasonium), Greater butterfly orchid (Platanthera chlorantha), Bee orchid (Ophrys apifera), Fly orchid (Ophrys insectifera), Lady orchid (Orchis purpurea), Early purple orchid (Orchis mascula), Common spotted orchid (Dactylorhiza fuchsii)) is also one of the great natural attractions of the park.

Some of these sites of great biological value originate from old farming practices which have now fallen into disuse (extensively scythed fields, damp meadows with rows of pollarded willows or thorny hedges).

These farming areas which are in the process of being abandoned or transformed still exist in certain areas of the Hauts-Pays. Still quite extensively farmed, they are home to fauna (presence of the Winchat (Saxicola rubetra), for example) and flora (many orchids of woody environments) which have totally disappeared from neighbouring regions.