The harsh conditions of desert ecosystems has promoted the evolution of a complex set of relations among desert organisms, a surprising number of which are positive interactions (Cloudsley-Thompson 1996). Desert shrubs in general and woody legumes in particular, create microhabitats that are critical for the survival of other species. Small animals seek the shade of desert trees and shrubs, birds find refuge and nesting sites in their canopies, and many small plants recruit their juveniles under the nitrogenrich canopy of desert woody legumes such as acacias, carobs, and mesquites. Because of their crassulacean acid metabolism, desert succulents such as agaves, aloes, and cacti are poor thermoregulators as young seedlings, and cannot survive the harsh ground-level midday temperatures. For this reason, they can successfully germinate and establish only under the protective shade of shrubby "nurse plants" that act as true cornerstone species in desert conservation (Figure 1.16). If the desert trees and shrubs are cut, all the accompanying biota soon disappears.

Additionally, many desert plants have very specific requirements in terms of their pollinators and seed dispersers (Figure 1.17). Although some desert ephemerals are truly unspecific in their requirements and produce thousands of seed with only wind-pollination, the slow-growing desert perennials are frequently highly specialized in their reproductive habits, and depend strictly on co-evolved animals to help them out in their sexual and reproductive processes. Many African cactoid plants (euphorbs and asclepias) produce foul-smelling flowers that attract carrion insects as pollinators. New World giant cacti and agaves produce sugar-rich nocturnal flowers that engage the pollinating services of nectar-feeding bats, while the sweet pulp of their fruits lures birds to disperse the seeds miles away. The red tubular flowers of many desert shrubs attract hummingbirds and giant sphinx-moths.