hroughout the United States, Latin America, Southern Africa, India and Europe, rodents are increasing as crop pests and carriers of disease. Climate variability, changes in land use patterns, and deforestation increase food sources for rodents, and decrease the number of predators. Combined, these factors are contributing to rodent population increases in many areas. A disturbance in one factor can be destabilizing; multiple perturbations can affect the resistance and the resilience of an entire system.

This type of synergy is evidenced in the southwestern region of the United States. Following heavy spring rains in 1993, rodent populations multiplied ten-fold, and a deadly new disease emerged: hantavirus pulmonary syndrome. Some 130 people have suffered from this viral disease and more than half of them have died. A prolonged drought prior to the event appears to have reduced rodent predator populations (owls, coyotes, snakes), while at the same time, the heavy rains provided a crop of grasshoppers and pine nuts, which served as nourishment for the deer mice that carry hantaviruses.

In southern Africa, rodent populations exploded in 1994, following heavy rainfall in 1993 that had been preceded by a prolonged drought. The maize crop in Zimbabwe was crippled; and plague broke out in Zimbabwe, Malawi and Mozambique. In Kruger Park in South Africa a rodent-borne virus was responsible for the deaths of 81 elephants. Hantaviruses have resurged in several European nations, particularly in former Yugoslavia. Plague resurged in India in 1994, following a blistering summer (124° F) and unusually heavy monsoons.