Five true-or-false statements on biodiversity, acid rain, renewable resources, CFCs and wetlands

(i)True Ecologists have long held that communities with more species are generally more stable. In David Tilman's long-term field plots, plots with more species showed less change in total biomass from year to year, and higher diversity also raised productivity.

(ii)False Acid rain lowers soil fertility. It dissolves and washes away the nutrients plants need and can release toxic metals such as aluminium and mercury from the soil; acidified soil has to be treated with powdered limestone (liming) to neutralise it.

(iii)True Renewable resources such as timber and water can be renewed by natural processes like regrowth and rainfall. They too run out if they are used faster than nature can replace them.

(iv)True Chlorofluorocarbons (CFCs) reach the stratosphere, where ultraviolet radiation breaks them down; the chlorine atoms released react with ozone and split it into oxygen, which does not absorb ultraviolet rays. Other compounds, such as bromine compounds, halocarbons and nitrous oxide from fertilisers, also attack the ozone layer, but CFCs are the chemicals singled out as the threat, which is why their use has been cut back and banned in most countries.

(v)True Wetlands are aquatic ecosystems, and natural aquatic systems break down the chemical and organic wastes that people release into them. This cleaning works only up to a limit: beyond it, pollution destroys the natural function. A marsh that receives a town's waste water can clean it partly, but not if it is overloaded.