EN
In this paper we present a hypothesis about the loss of organisms associated with hosts in small populations. Data for fungi specific to arcto-alpine plants are presented. We found that the number of fungal species on plants is directly proportional to the size of the host population. This conforms to MacArthur and Wilson's theory of island biogeography. Once the host population is below a critical size, some fungal species are lost. We term this process 'symbiotic drift.'