The faunas of deserts are often biogeographically more distinct between regions than the desert floras are. Despite this, many similarities exist between the different desert regions. Such phylogenetic similarities typical for the African-Asian deserts are explained by the lack of dispersal barriers, and similarities between North American and
Asian regions on one hand and North American and South American regions on the other are likely the result of existing land bridges. The Australia desert fauna, as its desert flora, is very distinct. As mentioned earlier on, almost all animal taxa are present in deserts, but some groups are more diverse than others, with the major deciding factor for this being the general aridity.
Relative to other insect groups, ants and termites are very diverse in deserts. However, their species richness is lower than it is in the Wet Tropics, where these groups originated. These groups reach high population densities and ecological importance is high. With up to 150 species per hectare, the highest species richness for ants is found in Australian deserts. Most desert arthropods are either detrivores (termites, beetles, etc.) or granivores (mostly ants), or are predators feeding on these (scorpions, spiders, etc.). Due to the lack of constant plant production, herbivores are relatively sparse or show pronounced, often dramatic temporal-spatial fluctuations (e.g., mass flights of desert locusts). Species rich substone communities consisting of protozoa, nematodes, mites, and other microarthropods are typical for deserts, creating a microcosm where grazers and predators feed on bacteria, algae, fungi, and detritus.
Fishes live in almost every aquatic habitat on the globe and small, permanent desert water sources are no exceptions. Obviously richness is extremely low, but species often live in very restricted areas and often under extreme conditions. The desert pupfishes (Cyprinodon sp.) in the deserts of North America are among the most species-rich groups in deserts. Some species live at temperatures of 45 °C and salt regimes 4 times that of seawater, while some species are restricted to an area as small as 20 m2 (e.g., the Devil's Hole pupfish in Nevada). These fishes are opportunistic omnivores.
Likewise, desert amphibian communities are depauperate since at least the juvenile stages depend on water. Only a small fraction of the world's amphibians, mainly anurans, are able to occupy deserts.
Reptiles are common and widespread in all deserts and, with the exception of crocodilians and amphisbae-nians (worm lizards), all orders are represented in deserts. Relatively few tortoises occur in deserts since they are restricted due to their plant diet. Snakes and lizards are well represented (especially in Australia). The extreme high diversity of reptiles in Australian deserts has been explained by low diversity of mammal and birds which resulted in lower competition for food and lesser predation pressure than in other desert regions. It appears that reptiles as endothermic consumers enjoy an advantage over other ectothermic consumers in the deserts of Australia that are characterized by low-quality plant production.
Even though birds have basic adaptation to cope with dry climates, diversity in deserts worldwide is relatively low and a clear positive relationship between rainfall and bird diversity is typical. Despite this, few desert specialist species developed among the avifauna: sand grouse, lark, parrots, etc.
Likewise, mammals are not very diverse in comparison to other biomes, but some taxa evolved to be true desert groups. Among smaller mammals are the heteromyds in North America, the jirds and gerbils in the African-Asian deserts, and the dayurid marsupials in Australia. Some of the desert mammals are rather large and therefore have advantageous low surface-to-volume ratios (see next section). The 'flagships' for this are clearly the camel species (Camelidae) that originated in the Americas in the Miocene and are now naturally found in desert regions of the Old and New Worlds; they are clearly the largest animals in all desert regions. It is of significance that most large herbivorous mammals, including camels, donkeys, goats, sheep, and horses, have been domesticated historically in deserts and semiarid regions and are common as domesticated livestock today. Other large, nondomesti-cated ungulates such as gazelles, ibexes, and oryxes are generally extinct or at least rare and endangered.
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