In ecology, a milieu is an environment in which plants and animals live. In the cross-border biosphere reserve, there are several remarkable milieux which must be protected.

Peat bogs are marshy milieux, where plant matter which has not fully decomposed accumulates, forming a type of soil which we call peat. In the cross-border biosphere reserve, the principal plants forming peat are particular types of moss, called sphagnum. The peat thickness can reach 1.50 m in certain places. The living conditions here are very difficult for plants and animals. The soil is saturated with water and is very acid (as acid as vinegar). Sundew is a small carnivorous plant, well adapted to these conditions. The peat bogs are protected because they are a very special milieu, which has been greatly damaged by man's activities.

Traditional orchards, with standard trees, are incredibly rich milieux, offering very great biodiversity. With their old trees, they provide a home for many insects and small rodents, in particular. Here we also find birds known as cavernicolous birds, or cave-dwellers, since they nest in hollows and holes in old trees. They also provide a breeding ground for varieties of fruit which are gradually disappearing. The usefulness of traditional orchards has been called into question, since their maintenance is very labour-intensive. For this reason, the traditional orchard is threatened with disappearance, along with all the plant and animal species it supports.

Little Owl hidden in a log pile. © SYCOPARC