He is best known for his work in advocating the theory that human overhunting of large herbivores during the Pleistocene caused Siberia's grassland-steppe ecosystem to disappear and for raising awareness as to the important roles permafrost and thermokarst lakes play in the global carbon cycle.

[9] Founded in 1977,[4] the Northeast Science Station boasts three laboratories, a network of field sites, tools for data analysis and communication, transportation, accommodation for visiting researchers,[7] and a year round staff of six.

[14][15] Zimov began the Pleistocene Park project in 1988 in northeastern Siberia near the Northeast Science Station in Cherskii, Sakha Republic, Russia.

[3] Concurrently, many of the large herbivores that roamed Siberia during the Pleistocene, including mammoths, woolly rhinoceroses, bison, horses, musk oxen, elk, saiga, and yaks, vanished from the region.

[3] According to Zimov, reintroducing large herbivores to Siberia would initiate a positive feedback loop promoting the reestablishment of grassland ecosystems: "The animals, their hooves, they disturb the moss and let grasses grow instead.

"[16] Current efforts in the park include reintroducing surviving Pleistocene megafauna into the fenced enclosure until they reach densities to change the vegetation and soil to a steppe grassland ecosystem.

[3] Pleistocene Park currently covers an area of 160 km2 (62 sq mi)[16] and contains less than 100 large mammals[16] representing six major herbivore species (horses, moose, reindeer, muskox, elk, and bison).

[4] Zimov's goal for Pleistocene Park is to increase the number of large herbivores to 20 per square kilometer before then reintroducing predators, including wolves, bears, and Siberian tigers.