The abundance of food in natural 1-ha plot of shortgrass prairie was artificially manipulated by adding alfalfa pellets and whole oats on a regular schedule. The small mammal species naturally inhabiting the manipulated food plot did not respond in either their density or weight to the supplemented food. However, a new specialized seed-eating species, Dipodomys ordii, invaded the food plot and persisted in relatively high density. As a result of this colonization, species diversity was significantly higher on the food plot relative to the unmanipulated control plot. In a second experiment, application of water and nitrogen to two 1-ha plots of shortgrass prairie resulted in increased productivity. However, the increased production on this latter treatment was associated with vegetation growth and thus major changes in habitat structural characteristics relative to the control treatment. Two new species, Microtus ochrogaster and Reithrodontomys megalotis, colonized the nitrogen+water treatment, but other small mammal species "resident" to the shortgrass prairie largely avoided this treatment. As a result of this manipulation, species diversity was significantly lower than the species diversity on the control treatment. These two results agree with the MacArthur prediction regarding how species diversity responds to an increase in production of scarce resources without changes in other habitat variables, and to an increase in productivity associated with changes in other habitat variables.