The Desert Tortoise,Gopherus agassizii, was listed as a threatened species April 2, 1990 by the US Fish and Wildlife Service. Populations have declined recently in many areas due to two main human attributable reasons: the direct loss of individuals and habitat degradation / fragmentation. Individual tortoises are lost due to poaching, collection for pets, military activities, vehicular impact, livestock trampling, disease and raven encroachment. Habitat degradation and fragmentation occur mainly through the spread of urban sprawl and livestock grazing practices. In addition to a recent encroachment by ravens due to the presence of garbage dumps, desert tortoises also face the threat of a deadly upper respiratory disease in the Western Mojave area.
Desert tortoises are found on flats, alluvial fans, bajadas and rocky terrain. Historically, flat terrain has lent itself to human survey on foot, skewing population estimates towards this gentler terrain - but evidence exists that desert tortoises also frequent rocky slopes, perhaps for protection from the desert heat. Soil friability, or its tendency to break apart, is another indicator of tortoise habitat. Desert tortoises need soils they are capable of digging into for burrows.
Plant species also play a major role in both defining desert tortoise habitat and their diet. Creosote bush, burrobush, mojave yucca and blackbrush generally distinguish desert tortoise habitat. At higher altitudes, joshua tree and galleta grass (Pleuraphis rigida) are common plant indicators.
Desert tortoises generally emerge from their burrows mid-March to feed on ephemeral plants. During a roughly six week period fresh green grass and spring wildflowers are their primary nutritional source. Dry stems of grass and cactus pads provide sustenance in dryer times. Introduced plant species have greatly encroached upon native plant species in th desert tortoise's natural range, degrading the existing natural ecosystem. Desert tortoises have, however, adapted to eating Erodium and other non- native species.
Desert tortoises have delayed maturity (14-20 years) and long life spans. Their reproduction / generation cycle is 25 years, with individuals having lifespans well over 50 years. However, the desert tortoises reproductive potential is low, laying relatively few eggs (3-14) in each clutch, and having a high mortality rate for juveniles approaching 99%. Slow growth (~2.5 cm / year) and soft / flexible shells make them particularly vulnerable to predators at this stage of life.
Desert tortoises have unique characteristics enabling them to survive in the desert environment. Elephantine limbs and well developed claws enable tortoises to burrow into desert soils to escape the heat of the day. Burrows may be over 3 meters long with the tortoise emerging in the morning or late afternoon to forage from March - October. By November, most tortoises have begun hibernation until the following March, only emerging during winter storms to replenish water stores.
Adapting to the lack of water, Tortoises have developed unique mechanisms to deal with the dry desert environs. Desert tortoises may dig shallow basins in impermeable soil to catch rainwater. However, the desert tortoise may go for many years without drinking, ingesting most of their water from plants and then storing it in their bladders.