Habitat fragmentation is one of the more important contributors to species endangerment, but one form of fragmentation, here called dispersal fragmentation, can often go unobserved for many years after it has occurred. Many species live in naturally fragmented habitats, but the local populations are interconnected genetically and demographically by dispersal through the environmental matrix in which the habitats are embedded. Because of dispersal, the local populations are not truly fragmented evolutionarily or ecologically. However, when human activities alter the environmental matrix such that dispersal is no longer possible, the population does indeed become fragmented even though they initially are present in the same habitats. An example of dispersal fragmentation via an altered environmental matrix is provided by the eastern collared lizard (Crotaphytus collaris collaris). This lizard lives on open, rocky habitats, called glades, that are embedded in the forests of the Ozarks, a highland region located primarily in Missouri and Arkansas in the USA. Forest fire suppression has reduced this habitat, resulting in severe habitat fragmentation, disruption of gene flow, loss of genetic variation within glade populations, and local extinction without recolonization. Beginning in 1982, glade habitats were restored by clearing and burning in the Peck Ranch area of the Missouri Ozarks, a region where the lizards had gone extinct. Starting in 1984, lizard populations were translocated from other Missouri glades onto restored glades at the Peck Ranch. Although these translocated populations survived well on the restored glades, no movement was detected between glades, some just 50 m apart, and no colonization of nearby restored glades, some just 60 m away, occurred between 1984 and 1993. Fragmentation, lack of colonization, no gene flow, and loss of genetic variation still persisted despite translocation reversing some of the local extinction. Fire scar data from trees and tree stumps indicated that forest fires were common in this area prior to European settlement, so in 1994 a new management policy of prescribed burning of both the glades and their forest matrix was initiated. Once the forest had been burned, the lizards could disperse kilometers through the forest, thereby reestablishing the processes of dispersal, gene flow, colonization, and local extinction followed by recolonization. This resulted in a dramatic increase in population size and inhabited area. By incorporating a landscape perspective into the management strategy, the eastern collared lizard has been successfully reestablished in a region of historic extirpation.
Population size, habitat fragmentation, and the nature of adaptive variation in a stream fish
