Changing Seasons Threaten Species, Ecosystems

Seasonality shapes much of life on Earth. Most species, including humans , have synchronised their own rhythms with those of Earth's seasons.

Plant growth cycles, the migration of billions of animals, and even aspects of human culture - from harvest rituals to Japanese cherry blossom viewings - are dictated by these dominant rhythms.

However, climate change and many other human impacts are altering Earth's cycles. While humans can adapt their behaviour by shifting the timing of crop harvests or Indigenous fire-burning practices , species are less able to adapt through evolution or range shifts.

Our new research highlights how the impacts of shifting seasons can cascade through ecosystems, with widespread repercussions that may be greater than previously thought.

This puts species and ecosystems at risk the world over. We are still far from having a full picture of what changes in seasonality mean for the future of biodiversity.

Almost every ecosystem on Earth has seasons

From tropical forests to polar ice caps and abyssal depths, the annual journey of Earth around the Sun brings distinct seasons to all corners of the planet.

These seasonal rhythms shape ecosystems everywhere, whether through monsoonal rains in equatorial regions or the predictable melt of snowpack in mountain ranges.

But the seasonality of these processes is changing rapidly due to local human impacts. This includes dams in many rivers, which completely and abruptly disrupt their natural flow , and deforestation , which changes the timing of the onset of the rain season.

These local influences are compounded by climate change, which is systematically modifying seasonal patterns in snow cover , temperature and rainfall around the world.

From the earlier seasonal melting of glaciers and the snowpack to the disruption of monsoonal rain cycles, the effects of these changes are being felt widely.

Many important ecological processes we rely on could be affected. A mismatch between plankton blooms and the life cycles of fish could affect the health of fisheries. Tourism dependent on seasonal migrations of large mammals could suffer. Even the regulation of the climate system itself is tightly controlled by seasonal processes.

Changing seasonality threatens to destabilise key ecological processes and human society.

Evolutionary adaptations to seasonal fluctuations

The seasonal rhythms of ecosystems are obvious to any observer. The natural timing of annual flowers and deciduous trees - tuned to match seasonal variations in rainfall, temperature and solar radiation - transforms the colours of whole landscapes throughout the year.

The arrival and departure of migratory birds, the life cycle of insects and amphibians, and the mating rituals of large mammals can completely change the soundscapes with the seasons.

These examples illustrate how seasonality acts as a strong evolutionary force that has shaped the life cycles and behaviour of most species. But, in the face of unprecedented changes to Earth's natural rhythms, these adaptations can lead to complex negative impacts.

For instance, snowshoe hares change coat colour between winter and summer to blend in with their surroundings and hide from predators. They are struggling to adapt to shifts in the timing of the first snow and snowmelt. The impact of changing seasonality on hare populations is linked with changes in predation rates. But predators themselves may also be out of sync with the new onset of seasons.

Our research highlights that these kinds of complex interactions can propagate impacts through ecosystems, linking individual species' seasonal adaptations to broader food web dynamics, or even ecosystem functions such as carbon sequestration.

Although biologists have studied seasonal processes for centuries, we know surprisingly little about how they mediate any ecological impacts of altered seasonality. Our findings show we are likely underestimating these impacts.

The distinct mechanisms involved deserve further attention. Until we account for these complex processes, we risk overlooking important ecological and human consequences.

The more we understand, the better prepared we are

Understanding the extent to which impacts of altered seasonality can interact and propagate from individuals to whole ecosystems is a big challenge. It will require different types of research, complex mathematical modelling and the design of new experiments. But it is not easy to manipulate the seasons in an experiment.

Scientists have come up with inventive ways of experimentally testing the effects of altered seasonality. This includes manually removing snow early in spring , manipulating rainfall patterns through irrigation and moving plants and animals to places with different seasonality .

Some researchers have even recovered seeds from centuries-old collections to sprout them and look at how recent changes in climate have affected plant populations.

These efforts will be of great value for forecasting impacts and designing effective management strategies beneficial for ecosystems and humans alike. Such efforts help to anticipate future shocks and prioritise interventions.

For instance, understanding the mechanisms that allow native and non-native species to anticipate seasonal changes has proven useful for " tricking " non-native plants into sprouting only in the wrong season. This gives an advantage to native plants.

Similarly, studies on the molecular mechanisms involved in the response to seasonality can help us determine whether certain species are likely to adapt to further changes in seasonal patterns. This research can also point out genes that could be targeted for improving the resilience and productivity of crops.

Not only are we likely underestimating the ecological risks of shifting seasons, we tend to forget how much our everyday lives depend on them. As Earth's rhythms change, the risks multiply. But so does our opportunity to better understand, anticipate and adapt to these changes.

Daniel Hernández Carrasco receives funding from a Doctoral Scholarship by the University of Canterbury.

Jonathan Tonkin receives funding from a Rutherford Discovery Fellowship administered by the Royal Society Te Apārangi and the Centres of Research Excellence Bioprotection Aotearoa and Te Pūnaha Matatini.