FROM MOUNTAINS TO MOLEHILLS: ADVANCES IN QUANTIFYING CHANGES IN TOPOGRAPHY AND ITS IMPLICATIONS FOR HABITAT FRAGMENTATION AND CONNECTIVITY

Topographic change can cause linkages and segregations of ecological communities thus playing a role in biological evolution and diversity. Examples of the relationship between topography and biology include: the building of mountains and erosion of deep canyons can fragment populations due to the emplacement of large climate gradients that may act as barriers to dispersal; increases in valley relief can generate a greater diversity of available habitats within a region; and changes in topography can cause local and regional shifts in climate, altering the distribution of habitats. Constraining the rates and timing of topographic change can therefore provide important insight into geologic controls on biological evolution. Approaches to quantifying to landscape history have improved greatly over the past few decades allowing improved constraints on (1) the rates of mountain development, (2) the magnitude of changes in local relief (i.e. the depth of valleys), and (3) the timing of significant incision events such as canyon formation. As an illustration of the utility of these approaches, we present a case study from central Idaho in which we constrain the timing of incision along the Salmon River. The incision generated a ~1.5 km deep gorge, separating regions of low relief and limited topographic complexity. We measure erosion rates using cosmogenic nuclides and channel morphology using DEM analysis. These metrics are used to test and calibrate a river erosion model. We recreate the pre-incision landscape by extending the relict topography across the gorge. Using the calibrated model, we then estimate how long it took to generate the modern topography using a numerical approach to quantifying river evolution. Best-fit modeled scenarios suggest canyon emplacement and the potential fragmentation of upland ecological communities at ~8 Ma. This age is in general agreement with geologic constraints and represents a time of significant topographic change in the region. This example illustrates how recent advances in constraining the rate and timing of topographic change presents an opportunity to link geomorphology and evolutionary biology disciplines to understand how biological systems are influenced by landscape history.