There are few ecosystems where there are no complex energy flows and exchanges
of materials across their boundaries. Even something as well defined as a cave may
exchange water and nutrients with regional groundwater or capture debris blown from
outside (Bailey, 1986). To simplify study, ecologists have attempted to enclose small
natural ecosystems, create artificial laboratory versions (e.g. phytotrons, growth chambers),
and study very simple types such as those of the Antarctic ‘dry valleys’. A huge
hermetically sealed greenhouse complex with a crew of eight, designed to study the
function and interaction of several ecosystems, was established in Arizona, USA, in
1991. It was named ‘Biosphere 2’ to emphasise its separation from the Earth’s biosphere,
and to reflect one motive of the experiment, which was to test the feasibility of
such facilities for life-support on Mars. It was managed to try to maintain a more or
less breathable atmosphere and provided almost enough air, water and food for the crew
for two years (Allen, 1991). Controlled environment experiments are valuable for those
seeking to establish what effects changing global climate and carbon dioxide will have
on crops and wild flora and fauna.