Consumption of Primary Production by a Population of Kangaroo Rats (Dipodomys Merriami) in the Mojave Desert

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Abstract
The energy requirements for a population of kangaroo rats in the Mojave Desert were estimated indirectly. Energy expenditure for maintenance was 84.1 megacal ha—1yr—1. Total energy expenditure for growth was 1.4 megacal ha—1yr—1, half of which was secondary productivity. Secondary productivity was 0.8% of the total energy flow. This percentage of the total energy flow is lower than for nondesert rodents and may be due to the low productivity of the desert areas in which D. merriami is found. The diet of D. merriami contained an average of 75% seeds. Green herbage formed 35% of the diet during the winter quarter. Kangaroo rats showed a high preference index for Erodium cicutarium seeds, which were larger and had a higher caloric value than the seeds of other annual plants. The average assimilation efficiency was 0.87 for the natural diet. Estimates indicated that kangaroo rats consumed 97.8 megacal ha—1yr—1, 99% of which was from the primary production. Total net primary production (NPP) on the study area was only 1,400 megacal ha—1yr—1; of this, 900 megacal ha—1yr—1 was considered to be available to D. merriami. Kangaroo rats consumed 6.9% of the NPP. This is higher than for any other rodent populations that have been studied and is probably due to the large proportion of available production (AP) in the total primary production (64%). The population consumed 10.7% of AP, which is high when compared to most other rodent species. The high consumption efficiency for D. merriami may be an adaptation to areas of low production, or it may be simply the result of high densities of kangaroo rats in areas of low production. Dipodomys merriami consumed over 95% of the estimated Erodium production and 90% of the total Erodium production. It is estimated that the rats consumed a sufficient number of Erodium seeds in 1970 to reduce the density of that plant by more than 30% in 1971. The D. merriami population was on the brink of overexploitation of its food sources and was potentially food limited. It is probable that desert herbivores in general are food limited since productivity in the desert is so variable.

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