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Lecture6WarmDeserts6slidespp - Whatisadesert WFC 50 Natural...

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1/27/2009 1 WFC 50 – Natural History of California’s Wild Vertebrates Warm Deserts Douglas A. Kelt What is a desert? Defined by aridity, but is it “heat” or “low precipitation” that leads to deserts? In fact, deserts have low precipitation but also high variability in precipitation. n 0.8 The most tangible factor defining a desert is the lack of available water – hence, even very cold places (some polar regions) are considered deserts. Negev Desert Median anual rainfal (mm) 0 25 50 75 100 125 Coefficient of variation 0.3 0.4 0.5 0.6 0.7 y = -0.004 x + 0.80 r 2 = 0.78 Many scientists use ca. 250 mm as a threshold. Consider the consequences of such ppt distributed evenly through the year vs. in a small number of pulses! Types of deserts in California Warm / Hot Precipitation typically as rain in winter (no summer rains) Usu. lower latitude Hot summer days but often very cold nights Sahara, Namib, Kalahari, Arabian, Iranian, Australian, Sonoran, Mojave, Chihuahuan Cold Precipitation often winter snow (summer rains rare) Higher latitude or altitude Summers warm to hot, but winters cold (to very cold) Great Basin , Patagonian, Turkestan, Gobi What causes deserts? e.g., what limits rain? 1. Global circulation of atmosphere * Leads to formation of deserts at about 30°N (and S). 2. Oceanic circulation * Not a strong factor for North American deserts, but in some regions water movements lead to “upwelling” of very cold water from deep in the ocean. This cools the air near the ocean, and leads to dense fogs, but since cooler air holds less water than warm air, the air is dryer and does not bring moisture onto the land. 3. Rainshadow * Air masses drop moisture on mountains and the descend the other side with less. 4. Midcontinent (e.g., Gobi Desert) * Some deserts are just a long way from water sources and hence do not receive water inputs. Global distribution of deserts Great Basin Gobi Turkestan Iranian Mojave Sonoran (Colorado) Chihuahuan Monte Patagonian Sechura Atacama Sahara & Somali Australian Victoria Great Sandy Gibson Simpson Namib Kalahari Arabian Thar Great Basin Desert California deserts Mojave Desert Sonoran Desert
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1/27/2009 2 Great Basin Desert California deserts Atlas of the Biodiversity of California CDFG 2003 Mojave Desert Sonoran Desert Mammals in California deserts . . . At any given point, you may not encounter high numbers of species, but this map is deceptive in giving the impression that deserts are relatively devoid of life. Of the 17 mammal species found only in California (e.g., endemic), two are found only in desert regions. We’ll talk about them both very soon. Perhaps not surprisingly, bird species richness also is relatively low in deserts, and this is particularly true in summer. However, note that “relatively low” includes much higher numbers of species than for other vertebrate groups.
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Lecture6WarmDeserts6slidespp - Whatisadesert WFC 50 Natural...

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