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160 Co+rservation Genetics of laesen Fishes rescued, literally at the last minute, and held in captivity until spring flow resumed. Likewise, existence of the Devil's Hole pupfish (Cyprinodon diabolis) was jeop- ardized when the water level in its single habitat was being lowered by groundwater pumping. Only a Su- preme Court decision to halt development in the region restored flow and maintained this habitat (Pinter 1979, 1985 ). Thus, the goal for conservation efforts of fishes in this model is simply to maintain strong, isolated popu- lations. Amore complicated version of the Death Valley model occurs with Cyprinodon macularius in the vi- cinity of the Salton Sea sink in southern California. Using Turner's (1983) electrophoretic data, Echelle, Echelle, and Edds (1987) were able to partition total genetic diversity into H~ and D~~ components. Polymorphism Natural Flow 0.3\~ ~ ~ ~ ~ ~ / ~ 0.8 / ~`l~ I I 0.01 J Meffe snd Vrijenhoek within Lakeshore colonies accounted for about 70% of the total genetic variance, and differences among colo- nies accounted for about 30%, contrary to expectations of the Death Valley model. However, this is easily ex- plained in the context of the hydrologic history of this region. These populations have been repeatedly sub- jected to isolation and possible reconnection during several rounds of drying and flooding in the lake basin. According to Turner (1983), Holocene Lake Cahuilla occupied this basin, followed by drying, followed by Wisconsinan Lake Leconte, followed by drying once again. From 1905 to 1907, the basin was flooded with water that broke out of the Colorado River irrigation system. Apparently, these cycles of reconnection have permitted the maintenance of significant variation within local populations. The recent flooding of the Sal- STREAM HIERARCHY MODEL F 0.6 Disrupted Flow B fi` 1• PUMPING ~~'~ PUMPING C ~ DAM ~ .Oi~ / i ~ DA M ~ o.o lo.~ 1 \ / \ ~ ~ / ~ / ~ ~ ~~ / ~~0.0/ ~~ 1 0.001 0 DA M Ht = H~ + Der + Drs + Dst Figure 2. Graphic representation of the Stream Hierarchy model of poputation structure (Left) Hypothetical natural situation. Sites A~ represent 10 natural populations separated by various probabilities of connectivity. Sites ~ B, C ~ F, and H are springhead~ I is a marsh, and the remainder are stream segments Numbers refer to hypothetical probabilities of connection between that habitat and habitats further upstream over the long terrrg and should not be interpreted as estimates of gene flow (m). (Right) Hypothetical disruption of natural situa- tion. In this case, dams and pumping of springheads have severely altered habitats and probabilities ojgene flow. Two populations (D and I) are extinct and most others have reduced connectivities Conservation Biology Volume 2, No. 2, June 1988