The most extreme of all desert ecosystems, hyper-arid deserts are limited by very dry and often windy conditions with high temperatures and sandy or stony soils. Dry periods may be prolonged for several years. Vegetation is characterised by very low densities of small drought-tolerant perennial plants known as xerophytes, very slow growing species with adaptations to drought such as extensive root systems and water storage tissues. Some of these plants may acquire much of their moisture from fogs. Ephemeral plants occur in some regions, but are less common than in other desert systems. Microbial biofilms are important decomposers. Drought tolerant reptiles and invertebrates are the main faunal groups, along with occasional nomadic mammals and birds, in simple foodwebs.