The area where two or more distinct habitats adjoin is called an ecotone. Because it is a border zone where multiple sets of resources and opportunities become available, an ecotone tends to support greater biological diversity than either of the systems it mediates between. Delineating an ecotone, however, can be problematic. Like habitats and ecosystems, ecotones are not self-deﬁning, as, for example, individual species are. They are human constructs, which derive their shape and character from the qualities their observers ﬁnd most salient. Much depends on scale. The zone of contact where the prairies of the Great Plains meet the ﬁrst foothill trees of the Rockies is an ecotone. At such a broad, regional scale, a woodland savanna might be described as ecotonal between the dense forests of the mountains and the grasslands of the plains. Closer up, however, the savanna’s edge, where trees and grassland meet, constitutes another ecotone. Other transition zones emerge as one reduces the scale further: at the scale of a beetle, the border between the edge of a clump of bunchgrass and the moat of soil and other plants around it is also an ecotone. In scientiﬁc terms one might say that the world is composed of gradients with relative discontinuities. Put simply: things change nearly everywhere, and so nearly every place is the edge of something and shares the qualities of an ecotone.