unsustainability of the people-nature interaction.
To give a stylized example, one could say that
soil erosion undermining the agricultural basis
for human society is a case of ecological (un)sustainability.
It could be caused by farming on
marginal lands without adequate soil conservation
measures -- the ecological cause. But the
phenomenon of marginalization of peasants may
have social roots, which would then be the social
causes of ecological unsustainability.
Sometimes, however, sustainability is used
with fundamentally social connotations. For instance,
Barbier (1987) defines social sustainability
as "the ability to maintain desired social values,
traditions, institutions, cultures, or other social
characteristics." This usage is not very common,
and its needs to be carefully distinguished from
the more common context in which social scientists
talk about sustainability, viz., the social
aspects of ecological sustainability. A war destroying
human society would probably be an
example of social (un)sustainability, and it in
turn may have social or ecological causes. (Note
that these categories are only conceptual devices
for clarifying our thinking; real problems seldom
fall neatly into one category or another.)