Climate change is causing species’ ranges to shift. While observations of species range movements and local extinctions are common, few studies have tested causes of these range shifts. An understanding of range movements is predicated on what factors limit a species’ geographical range. A possible explanation for range boundaries comes from the Dobzhansky-MacArthur hypothesis (DMH), which proposes that species’ ranges are most limited by inhospitable abiotic conditions (e.g., low temperature) at their high altitude/latitude range boundaries, and by biotic interactions (e.g., herbivory) at their low altitude/ latitude boundaries. In mountain ecosystems, plant distributions show natural boundaries at elevation limits that are replicated across peaks, providing a tractable system to test the DMH in the context of climate change. The abiotic limitations of species’ high range limits are well supported. So, we focused on the biotic limitation aspect of the DMH and performed a “space-for-time” experiment across species’ ranges to ask: Do low elevation competitors suppress the fitness of alpine plant species more than high elevation competitors? We transplanted three alpine plant species into a modified response surface competition design (intra- and interspecific competition) below their current range boundaries (novel competitors) and within their current range (current competitors).
Results/Conclusions
Thus far, interspecific competition suppressed the growth of all three focal species. These effects were stronger in the novel, low elevation community than in the resident, high elevation community, consistent with the DMH. Intraspecific competition was weak, with no consistent effect on growth of the three focal species. Because focal species showed high survival and growth in the absence of competition in the more benign climate of low elevations, our results suggest that climate warming will initially bolster growth of alpine plants. However, depending on the velocity at which currently low elevation plant communities move up mountains, benefits of a more favorable climate may be negated and possibly exceeded by the amplification of competition.