Spatial heterogeneity in ecologically relevant climate variables at coarse and fine scales

Background/Question/Methods

Climate plays an important role in determining species geographic ranges. With the rapid rates of climate change expected for the coming decades, ecologists have predicted that species ranges will shift large distances in elevation and latitude, and that widespread extinctions could occur as climate shifts faster than species can likely migrate. However, range shift assessments are typically based on coarse-scale climate models that ignore fine-scale climatic heterogeneity and could fail to capture important range shift dynamics. For example, cool microhabitats near the contracting edge of a range could be vital for species persistence while warm microhabitats beyond the advancing edge may be critical for species expansion. Moreover, if climate varies drastically over short distances, species may only need to migrate tens of meters between microhabitats to track their climate as opposed to hundreds of meters upwards in elevation or hundreds of kilometers poleward. Thus, coarse-scale projections of range shifts could lead to overestimation of range shift magnitudes and extinction risks where fine-scale differences in climate are great. To address these issues, we measured climate variables important to species distributions (snow disappearance date and soil temperature) at coarse and fine scales at Mount Rainier National Park in Washington State, USA.

Results/Conclusions

We found that locations separated by small distances (~20m), but differing by vegetation structure or topographic position, experienced differences in snow disappearance date and soil temperature as great as locations separated by large distances (>1km). As expected, snow disappearance was generally latest in areas that receive the most precipitation – high elevations and the windward side of the mountain. Additionally, temperature generally decreased with elevation. However, fine-scale differences in climate were often as great as these coarse-scale differences. For example, snow disappearance date was, on average, three weeks later in topographic depressions than on nearby ridges, which was equal to the average difference in snow disappearance date between locations separated by hundreds of meters of elevation. In addition, the fine-scale variability in climate we observed was strongly correlated with patterns in vegetation, suggesting that fine-scale patterns in climate are ecologically relevant. If the types of microhabitats supporting cool or snowy conditions are common (like the topographic depressions we surveyed), these microhabitats could provide refugia for large numbers of species. Thus, the large degree of fine-scale heterogeneity we observed will likely lead to complex range shift dynamics and have the potential to buffer species from the negative effects of climate change.