Desert research and the notion of convergent evolution

Being an environment as remote as imaginable from the aquatic origin of life, deserts attracted early life scientists eager to uncover the adaptations of desert organisms to this challenging setting. These endeavours had a profound impact on the disciplines of evolutionary biology, physiology and ecology. Regarding evolution, it was found that both desert annual plants and sedentary animals respond with extremely rapid growth to the short bouts of resource abundance, and by quiescence of life processes during the intervening long periods of shortage (Philippi 1993), whereas both perennial plants (Cabin and Marshal 2000) and mobile animals survive periods of low resource abundance by either moving to more favourable areas (animals), or by physiological means such as suppressing resource allocation to temporarily less important activities (plants). That groups with such different evolutionary ancestry as plants and animals display similar adaptations to the extreme and random fluctuations in resource availability convinced biologists that, in the living world, divergent genetic makeups have the potential to generate convergent solutions to a wide array of environmental challenges and selection pressures (Smith and Wilson 2002) (see also "Biological Adaptations to Aridity" in Chapter 1).

Desert physiological adaptation as a model for life under stress

Desert research revealed that physiological adaptations enable mammals and birds to live in environments where water is limiting and temperatures are high, while human adaptations are exclusively behavioural and cultural. This prompted researchers to study the physiological responses of humans to desert conditions. These studies demonstrate the potential of people to acclimatize, rather than adapt, to stressful conditions (Shkolnik and others 1980). The fact that humans and their livestock do live in deserts has also compelled physiologists to examine the deleterious effects of high temperatures, chronic dehydration and food shortages on humans, and on the animals they domesticated due to their ability to live in deserts, such as camels (Schmidt- Nielsen and others 1967), goats, and donkeys (Izraely and others 1989).

What determines the number of links in a food chain? - A desert insight

Two features make deserts ideal for ecological research. The low plant cover enables one to easily explore animal activity either directly, or indirectly, by observing the tracks they leave on the bare soil surface. More importantly, since in deserts only one major factor, precipitation, governs ecological processes, and since the number of species in deserts is relatively low and the sizes of their populations are small, the desert ecosystem appears simpler, hence easier to understand than other ecosystems. These features encouraged the use of deserts as an outdoor laboratory, where hypotheses and theories developed in non-desert environments lend themselves to testing.

For example, a prevailing notion that evolving specializations for partitioning a limiting resource enables many species to avoid competition and coexist, thus leading to high diversity, is challenged by the finding that annual plants and darkling beetles (the blackish beetles of the family Tenebrionidae) exhibit high diversity in deserts, but subsist on resources not amenable for partitioning; since soil moisture is restricted to its thin top loayer, the coexistence of so many annual plant species cannot be attributed to each of them drawing water from a different depth. Similarly, the rather physically and chemically homogenous plant litter cannot be partitioned and is indiscriminately consumed by all darkling beetles. This desert observation supports the notion that species diversity can be maintained not only by competition that generates specialization, but also by predation (Ayal and others 2005).

This leads to challenging yet another central paradigm of current ecological theory, that food chain length is determined by the productivity of its first link, the primary productivity of plants, and that high primary productivity maintains long food chains. In deserts, however, long food chains with several predation links on top have been observed repeatedly, in spite of the desert's overall low primary productivity. Several related observations explain this finding. Most of the desert's primary productivity is not consumed by herbivores but becomes plant litter; plant litter in deserts is not readily decomposed by soil micro-organisms, which are constrained by the desert's protracted periods of low moisture. hence, much litter accumulates on the surface and is consumed by a large number of arthropods. Being relatively small, these litter-consuming arthropods are preyed upon by only slightly larger small predators, such as arachnids and reptiles, which in turn are preyed upon by birds and mammals, which are larger still. Thus, desert food chains are long and size-structured, yet are supported by a base of low primary productivity (Ayal and others 2005). Desert research then implies that it is the body size of the primary consumers rather than the quantity of primary production that determines the length of food chains, a conclusion that undermines the high productivity-long food chain paradigm, and which may apply to other, non-desert ecosystems.

Why are linkages important?

Exploring how deserts are linked to the rest of the planet, this chapter highlights the importance of deserts. It also underscores why the more than 6 000 million people who live outside the desert biome need to take an interest in what happens in deserts, even though only 144 million people currently live there. This is not only because so much of the oil and so many of the diamonds come from deserts; there are more subtle aspects of human culture that for inexplicable reasons have been nurtured in deserts, such as the advent of the alphabetic script or the emergence of monotheistic religions that, respectively, catalyzed human development and largely dominate human relations the world over. Yet, it is not the signature of a desert's remote past that matters most. Rather, much of human well-being in its broadest and most global sense depends in several ways on what happens in deserts today.

For example, the climate system of the areas beyond the desert affects that of the deserts themselves, but some of the desert climates' responses to these then affect the climate of the non-desert world. This in turn much depends on global climate change, mostly generated by non-desert people. Deserts may respond to these changes, among other things, by increased emissions of cross-boundary dust storms with far-reaching negative (as well as positive) implications. Another example is derived from the dependence of non-desert birds on crossdesert migration. Birds are directly, but people are indirectly, affected, because when not on the move these birds are intimately involved in the provision of services in the non-desert ecosystems in which they live, services that support life in general and human well-being in particular, at local and global scales, and which will not be provided if the crossdesert migration is impaired. hence, this migration depends on what people both in deserts and outside of deserts do, either to protect or to disrupt these trans-desert migrations. Furthermore, these two groups of people, the desert and the nondesert, are inter-connected too. The livelihoods of many desert people, upon which the flow of benefits from deserts to the rest of the world depends, is often linked to the ways non-desert people manage the non-desert headwaters of major rivers that cross into deserts, nourish their life and nurture their societies.

To conclude, our understanding of global processes, the development of much of our modern research, our ability to cope with global environmental change, and the preservation of much of our global heritage depends to a large extent on the way we manage and preserve the world's deserts. What happens in deserts affects every one of us, wherever we are. What happens outside deserts impacts deserts, changes the way they function, and what they can contribute to the rest of the planet.