The cover image for our May issue features a female Broad-tailed Hummingbird (Selasphorus platycercus) visiting flowers of the plant Ipomopsis aggregata in Gothic, CO, USA. This image relates to the research article: Comparative impacts of long-term trends in snowmelt and species interactions on plant population dynamics, by Diane Campbell, Mary Price, Nickolas Waser, Rebecca Irwin, and Alison Brody. Here the author Diane Campbell tells us the story behind the image.

Populations of plants and animals are subject to multiple potential threats from both changing aspects of the physical environment and interactions with other species. Climate change has reduced the extent of snow cover and led to a trend towards earlier snowmelt in the mountains of North America. That earlier snowmelt can lead to prolonged drought conditions with the potential to impact plant populations. But the majority of natural plant populations also have seed production that is limited by low levels of pollination by animal pollinators, such as the hummingbird pictured, or predation on seeds from pre- and post-dispersal seed predators. Our research over two decades near the Rocky Mountain Biological Laboratory in Gothic, CO compared the impacts of these abiotic and biotic threats to population growth for the subalpine plant Ipomopsis aggregata. Long-term data sets like this one are rare and tremendously powerful in their ability to detect the direct and indirect effects of climate change on plant reproductive success. 

Earlier snowmelt driven by climate change has a bigger impact on long-term population persistence of I. aggregata plant populations than do low pollination and seed predation. We combined long-term demographic data with field experiments using supplemental pollination in 13 years spread across a 21-year period. Supplemental pollination increased seed production by an average of 28%, while eliminating pre-dispersal seed predation would increase it by 44%. Early snowmelt, however, correlated not only with greatly reduced seed production, but also reduced seedling emergence and reduced survival of vegetative plants to the next year. Incorporating the data into an integral projection model, the projected biotic impacts of pollen limitation and seed predation on population growth rate were small compared to factors associated directly with snowmelt date. Providing full pollination could not rescue these populations, as it would delay the date when earlier snowmelt will cause populations to fall below replacement by only 14 years. Pollen limitation has become progressively less important over time, as abiotic conditions worsen, suggesting that natural selection on floral traits may weaken with climate change. These results highlight the value of studying both abiotic factors and biotic interactions to understand how climate change will influence populations.

Diane Campbell, University of California, Irvine, USA.

Read the full research article here: Comparative impacts of long-term trends in snowmelt and species interactions on plant population dynamics