ANIMALS AND ARID CONDITIONS 369 
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EXPENDITURE 
FOOD INTAKE 
Figure 1. Production of animal tissue (cross-hatched area, labeled 
“Storage””) can take place only when the level of feeding exceeds the 
maintenance level. The line labeled “Basal Metabolism” refers to the 
metabolic level when no food is taken in. Food intake raises the me- 
tabolism, and the quantity of food required for maintenance will there- 
fore exceed the basic metabolic level. With increasing food intake, the 
energy expenditure increases steadily as indicated by the line ‘‘Actual 
Expenditure.” The ‘Maintenance Level’ is where the “Food Intake” 
equals the “Actual Expenditure.” 
This is a simplified illustration and does not refer to any specific data. 
The slope of the curve “Actual Expenditure” includes expenditures for 
obtaining the food, for digestive processes, and waste. If the vegetation 
is scattered, more work is expended to obtain it, and production is 
decreased. 
they are most needed. Production (which in the following refers 
to production of consumable animal foodstuffs) can be increased 
in two ways, either by increasing the amount of feed or by a better 
utilization of the feed available. The first problem, increasing the 
feed, is a problem for hydrologists in irrigation, for agriculturists 
in selection and improvement of plants, and in fertilization, etc. 
In this paper I shall mainly concern myself with the second prob- 
lem, that of utilizing the feed available as natural vegetation on 
