♦ Water and nutrients ♦
Water and nutrients are fundamental natural resources; yet they are at odds at both extremes of the climatic spectrum. Where there is plenty of water, nutrients are in short supply; conversely, where there is very little water, nutrients abound. Ecosystems need both water and nutrients in sufficient quantities; therefore, the resource dilemma.
In humid regions, where rainfall is more than 1600 mm per year, nutrients have been leached through millennia and only limited quantities remain in the soil profile. For instance, in tropical rainforests, most of the nutrients are stored in the canopy and recirculated through effective litter biodegradation. In arid regions, where rainfall is less than 400 mm, there is usually not enough environmental moisture to support diverse ecosystem growth; yet there are plenty of geologically new nutrients stored in the soil. Thus, deserts are fertile; once irrigated, they tend to be very productive.