Strengthening the evidence base for temperature-mediated phenological asynchrony and its impacts

The earlier arrival of spring, measured by plants flowering, insects emerging, and the timing of egg laying and migrants arriving in birds, is one of the most obvious impacts of climate change on the natural world. These trends differ between different species’ groups, with plants tending to respond more quickly to warmer temperatures than insects, which have in turn have responded more quickly than birds. This has led many people to suggest that climate change is causing a mismatch (trophic asynchrony) between the timing of predators and prey, or herbivores and the plants that they eat, disrupting food networks and causing species to decline. In the most comprehensive review of the published literature on this topic to date, led by Edinburgh University but involving authors from seventeen other institutions including BTO, five criteria required to demonstrate that warming is having a negative impact on consumers through trophic asynchrony are identified:

• Consumers rely on a seasonal food resource
• The timing of peak consumer demand and peak food availability is diverging through time (asynchrony)
• Variation in asynchrony is linked to temperature
• Asynchrony negatively impacts the fitness of individual consumers
• Asynchrony negatively impacts consumer populations

The majority of the 109 papers reviewed were from North America and Europe, showing a strong geographical bias in published studies; more data are required from aquatic systems and particularly the global south. Most studies described asynchrony in species reliant on a seasonal food resource, with almost two thirds providing evidence that, as expected, consumers were altering their phenology more slowly than their prey as a result of weaker temperature responses. Asynchrony is a widespread phenomenon. However, fewer than 8% of studies considered impacts on species’ populations and for only two species (both birds) were all five criteria documented; the Pied Flycatcher and Great Tit.

This overview highlights the challenge researchers face in understanding how climate change affects complex ecological systems, and identifies some important priorities for future research, including a real need for long-term population monitoring data. Working with our thousands of volunteers, BTO initiatives like the Nest Record Scheme, Constant Effort Sites Scheme and Breeding Bird Survey enable us to do this. For example, we used these data to show that the sensitivity of breeding songbirds to changes in seasonal timing is linked to population change but cannot be directly attributed to declines in breeding productivity. More broadly, these schemes have been instrumental in tracking significant impacts of climate change on species distributions, populations and communities. To summarise, climate change is having widespread impacts on the natural world. This review shows that understanding the mechanisms behind these impacts is challenging and requires a combination of detailed ecological studies and long-term monitoring data to do so.