| Abstract: | The patterns of conifer species diversity in the Great Basin of western North America are investigated. The sources of floristic data are 8 years of collection and observation on over 300 mountain ranges in the region, examination of herbarium material at 15 western herbaria, and extensive review of published floras and distribution maps. All isolated mountain ranges in the Great Basin with at least one montane or subalpine conifer species present were included in the data set, and all neighboring source areas in the Sierra Nevada, Rocky Mountains, Colorado Plateau, and Blue Mountains were sampled, resulting in a total of 160 montane island samples and 40 mainland samples that were used in the subsequent analyses. Physical data for each sample were compiled, as was life history data for each of the species present in the region. Classification, ordination, and regression analyses were conducted to evaluate the species diversity and occurrence patterns present in the region. Explanations for the modern distributions of conifers in the Great Basin were sought by examining four effects on species diversity (area, distance, habitat diversity, and longitude) and the effect of life history attributes (including dispersal) on the number of species occurrences. These effects were considered by testing the predictions of three idealized island biogeography theory models: (1) Immigration, (2) Extinction, and (3) Equilibrium. The Habitat Diversity Hypothesis and the Intermediate Disturbance Hypothesis were also considered, and all hypotheses were treated as alternatives to the Random Placement (null) Hypothesis. The Random Placement Hypothesis was refuted: area-species slopes generated from the data are non-random. All idealized island biogeography theory models failed because mainland slopes are within the range of island slopes. Compounding this failure, additional problems were detected in each of the models as applied to Great Basin conifers, especially the Immigration model, as it made only one correct prediction. The Habitat Diversity and Intermediate Disturbance models were not refuted as long as they were considered not mutually exclusive with the island biogeography models. If considered mutually exclusive, they fail because a significant distance effect was observed. The Great Basin conifer flora is much richer than was previously thought, particularly in terms of the number of taxa present in the region that also occur in Pacific mountains. Although not all of the predictions of either the idealized Extinction, Immigration, or Equilibrium models of island biogeography theory were observed, on balance it appears that both immigration and extinction of montane and subalpine conifers have occurred in the region since the Late Pleistocene Epoch, but extinction has been more important. The failure of all island biogeography theory models to predict the observed island-like species-area slopes of the mainlands is probably because no mainland was actually sampled. Instead, the sites considered as 'mainlands' are part of the Great Basin, which was a montane-subalpine mainland during the Tertiary. Quaternary climate changes caused widespread conifer extinctions and has resulted in a fragmented, island-like pattern of species diversity throughout the region. |
