WINTER AND SUMMER RAINFALL PATTERNS

In deserts, rainfall events trigger short periods of high resource abundance which, despite the overall scarcity of rain, can saturate the resource demand of many biological processes for a short time. Thus, although deserts are often characterised by their mean climatologic conditions (as in the case of the Aridity Index described in the previous section) they are really driven by a succession of short pulses of abundant water availability against a background of long periods of drought. And, because rain storms are also frequently very localized, deserts are extremely patchy environments in their resource availability, both in space and in time. Rainfall pulses are really the driving force structuring desert ecosystems, and plants and animals have developed very specific adaptations to cope with ephemeral abundance, especially with regard to growth, population dynamics, and the cycling of organic matter and nutrients (Sher and others 2004). Within a desert, rainfall events may vary significantly from one pulse to the next: some spells may occur in winter, others in summer; some events may bring very little precipitation, others may bring intense showers; and the period between pulses may also vary substantially.

Each organism's response threshold is often determined by its ability to make use of moisture pulses of different durations and infiltration depths. For example, brief and shallow pulses have an important effect over surface-dwelling organisms with fast response times and high tolerance for low resource levels, such as soil micro-organisms. Short precipitation pulses are important to the survival of annual plants, but deep-rooted perennial plants may respond only to longer, more intense precipitation events. Thus, the diversity of pulses also promotes a diversity of responses in life-forms, migrations, or population cycles in different species. To a large extent, it is the heterogeneity of pulses that drives the surprisingly high biodiversity of desert ecosystems (Chesson and others 2004).

But what commands pulses in deserts? Why are seasons, and even decades, so different from each other, and often so unpredictable? To a large extent, pulse-type variations in desert environments are linked to global atmospheric and oceanic phenomena. Large-scale drivers of regional precipitation patterns include the position of the jet streams, the movement of polar-front boundaries, the intensity of the summer monsoon, El Niño Southern Oscillation events, and even longer-term ocean cycles, such as the Pacific Decadal Oscillation (Loik and others 004). Driven by these large-scale forces, the intensity of mid-latitude continentality, ocean upwellings, and rain shadows - the major factors modulating the distribution of arid lands - is not constant but may vary from one year to the next. As a result, the intensity and frequency of rain pulses at a local scale may vary substantially with time, and in a seemingly unpredictable fashion.

The influence of large-scale drivers on local desert conditions was noted many years ago by the fishermen and the farmers of the coastal desert of Peru, who realized that during some years the normally cold waters of the Pacific became warmer. In these years, they noted, the abundance of sardines decreased but abundant rainfall soaked the land and made the desert flourish. Because this phenomenon was normally observed around the month of December (a time of the year in which Christians commemorate the birth of the Christ child - El Niño in Spanish), they called the phenomenon the "El Niño" ocean current. During El Niño years, the trade winds and the west-bound equatorial currents slow down, and the upwelling of nutrientrich waters in the coasts of the American Continent decreases. The sea becomes less productive while the coastal fog deserts of the Pacific Ocean become drenched in the abundant rainfall that originates from the now-warm sea waters (Holmgren and others 2001; see also Chapter 4).

These pulses of abundance and scarcity of resources are a major force in the ecological organization of many deserts of the world. During pulses of bounty, the fragile seedlings of desert plants can germinate, establish, and prepare for long droughts burying their roots deep into the desert soils. Ephemerals can replenish their seed banks (Figure 1.8), desert toads can reproduce in extraordinary numbers before entering again into their waterless torpor, and granivorous rodents, such as North American kangaroo rats, Australian hopping mice, and African jerboas, can stock up their underground caches. The desert becomes renewed, and ready to face again decades of extreme hardship.