Answers to Review Questions

1. Extreme environmental conditions, such as high and low temperatures or droughts, are important determinants of mortality in organisms. As a result, distributions of species often reflect extreme environmental conditions more than average conditions.

2. Differences in the intensity of solar radiation across Earth's surface establish latitudinal gradients of surface heating. Greater heating in the tropics results in rising air currents, which establish large-scale atmospheric circulation cells, called Hadley cells. The warm rising air also promotes high amounts of precipitation on the tropics. Polar cells form where cold, dense air descends at the poles. Between the Hadley and polar cells are the Ferrell cells, driven by the movement of the Hadley and polar cells and the exchange of energy between equatorial and polar air masses. The temperate zone is found at mid-latitudes in association with the Ferrell cells.

3. Salinization is a progressive increase in soil salinity due to surface evapotranspiration of water. Desert areas have high rates of evapotranspiration and little precipitation to leach salts to deeper soil layers. Some desert soils also have impervious soil layers underlying the surface layer that impede leaching, increasing the potential for salinization.