Seasonal Mismatch - The Fight To Adapt To Advancing Spring

For the relaunch of Scitable and its 12 shiny new blogs bursting with fresh content, I was pondering what to discuss in my first post and thought: what better to symbolise this new beginning than Spring!

Here in the northern hemisphere after a long, hard winter, we all crave those first signs of spring, looking out for the first bright-faced daffodils to herald the apparent re-awakening of nature with their distinctive yellow trumpets.

Despite the delayed spring observed this year in large parts of Europe and North America, the general trend over the last several decades is that spring is actually arriving earlier^1,2. This may seem good news to those suffering from lingering winter blues, but for wildlife it can pose a huge problem.

For many organisms, environmental cues trigger critical life-history events. For example, the blooming of flowers is influenced by changes in photoperiod (the amount of daylight), while the emergence of certain species from hibernation may be triggered by seasonal temperature changes. The study of such periodic natural phenomena, especially in relation to climate, is called phenology, and it's an increasingly important field in the face of our changing climate. Problems occur for organisms when there is a phenological mismatch, meaning basically that things are out of sync. Climate change can really wreak havoc by causing these mismatches. But how?

Here's how.

Many plants and animals share phenological relationships - a synchrony in seasonal timing of life-history events - for example the arrival of a migratory bird species during the seasonal peak of its main food source. Advancing springs, a marker of climate change, are causing a "mismatch" in the timing of such events. In the case of a migratory bird, this could mean that it arrives at its spring feeding grounds to find that it has missed the annual peak in food abundance - meaning less food for its chicks and increased competition for resources. So, the major concern with phenological mismatch is that relatively rapid environmental change will out-pace the rate at which organisms are able to adapt.

But which species will be worst affected, and how?

Generalist species are likely to adapt more easily to rapid changes in climate than species with specialized habitat or dietary requirements. Population flexibility will also play an important role. For example, in the case of breeding mismatch, the more flexible a population is in the duration of its reproductive period, the better that species is buffered against environmental changes³.

You might logically conclude that, in the face of a strong mismatch, populations of negatively affected species would decline, but an interesting new study of a population of birds called great tits has shown that population growth rates actually remained unchanged in the face of phenological mismatch.

Buying time for evolutionary adaptation

To guarantee as many juicy caterpillars as possible for their demanding chicks, great tits (or at least a particular Netherlands population I read about here) endeavour to match their breeding times with the pronounced seasonal peak in caterpillar abundance. Obviously the more caterpillars they eat, the stronger the chicks and the better their chance for survival.

The challenge these great tits are now facing, however, is that the peak in caterpillars has been advancing at more than twice the rate of great tit laying dates (despite intensifying selection on great tits for earlier laying). Scientists at the Netherlands Institute of Ecology (NIOO-KNAW), lead by Dr Tom Reed and Prof Marcel Visser, have been studying great tit populations at the Hoge Veluwe National Park in the Netherlands for many years. After analyzing 37 years of data, they discovered that a mismatch of almost two weeks has developed since the 1970s, meaning the great tits are lagging behind in the race to adapt to rising spring temperatures.

As a result, an international team of scientists, lead by those at NIOO-KNAW, seized this opportunity to test the effect of climate-induced phenological mismatch on population growth - something which few studies have done before. Using almost four decades of life-history data they found, as expected, a "directional selection" for earlier laying dates. This means that females laying eggs earlier in spring were more successful (produced more surviving offspring) than late birds. What they did not expect to find, however, was that the total number of birds in the new generation stayed the same.

The conundrum they now faced was: Why is population growth not lower in years when a large fraction of females lays too late?

"The answer is that for selection it matters which birds survive, while for population size it only matters how many survive." - Dr Tom Reed and Prof Marcel Visser (NIOO-KNAW)

The idea is that the fitness losses associated with the mismatch are being counteracted by fitness gains due to a widely observed phenomenon called "density-dependent compensation" (DDC). If fewer fledglings are produced by late females, this may be off-set by increased survival of earlier chicks due to reduced competition (there being fewer other fledglings around to compete with for food). So even if fewer chicks fledge in years with high mismatch, the ones that do fledge have better chances of survival thanks to "relaxed competition" (see diagram below). DDC therefore buffers the population from otherwise more rapid declines.

Unfortunately, however, it's not all good news - simulations carried out by the researchers predicted that a gradual decline in population will occur if population mismatch continues to increase. With rises in temperatures predicted to continue, these birds are merely "buying time" to adapt to their mis-timed food source.

We still don't know whether any other species are benefiting from such short-term buffers. But as a solution DDC will only ever buy so much time. In the long-term it will be a fierce evolutionary race if climate change continues to follow its predicted trend.

References:

Image credits:
1. MRHSfan on Flickr
2. AlexRK on Flickr
3. ed_needs_a_bicycle on Flickr
4. Diagram: Author's own