Answers to Figure Legend Questions

FIGURE 22.4 Primary production is low and plants are sparse in desert ecosystems, so the amount of soil organic matter should also be low. Wetting-drying events should enhance mechanical weathering of soils, producing a range of soil particle sizes.

FIGURE 22.6 Pesticides applied to plants can wash into the organic surface layers of soils, where they can kill both herbivorous animals and soil detritivores. The loss of these animals would effectively lower the rate of decomposition and would thereby decrease soil fertility.

FIGURE 22.12 The simple input-output model depicted in the figure assumes that elements enter the ecosystem primarily through deposition and leave it in stream water. As noted in Figure 22.13, other modes of input and output occur, including inputs through N₂ fixation, outputs in groundwater, and gaseous losses (e.g., denitrification).

FIGURE 22.18 The study of eutrophication in Lake Washington is very convincing, but it lacks an appropriate control. Therefore, it is correlational; that is, it shows a quantitative link between depth of clarity and phosphorus inputs, but that link isn't necessarily causal. Appropriate controls might have included another lake that didn't have sewage inputs, or a lake that continued to have inputs of phosphorus-laden sewage during the time sewage inputs to Lake Washington were halted. (Experiments with appropriate controls have demonstrated beyond a doubt that inputs of phosphorus in sewage entering lakes do cause eutrophication.)