ABSTRACT
Natural resources are usually defined as ‘renewable’ or ‘non-renew-
able’. The first are resources that can be renewed or harvested regu-
larly, such as timber for construction or linseed for linseed oil. They are
only renewable as long as the right conditions for production are main-
tained. Depletion of the ozone layer is an example of how conditions for
many renewable resources can be drastically changed. Most renew-
able resources have photosynthesis in common. It has been estimated
that human society currently exploits about 40% of the earth’s photo-
synthetic activity (Brown et al., 1990).
