Great tits depend on a good supply of winter moth caterpillars during spring to successfully raise their chicks to adulthood. Climate change is causing the start of spring to occur earlier and earlier each year, which causes these caterpillars to hatch earlier. In turn, the great tits must also start breeding earlier so their chicks hatch when there are the most caterpillars. This great tit – caterpillar example is illustrative of the general phenomena of temporal asynchrony that is caused by changes in climate and impacts many species across the world.
This research used data from a long-term monitoring study of great tits and caterpillars from Wytham Woods in Oxford, UK, which has been running since 1960.
Researchers used these data to create a statistical model to project how the timing of caterpillar abundance and great tit breeding might change by the end of this century. This model also considered the effect of other weather conditions and winter food supplies, and looked at the impact all these factors had on the great tit population.
Lead researcher, Dr Emily Grace Simmonds, said, ‘This new model is exciting because it lets us look at the mechanisms behind the change in timing as well as the result. It tracks both evolutionary change and flexible changes in behaviour.’
These projections showed that great tits did not respond to warming temperatures as quickly as the caterpillars they rely on. This caused great tits chicks to hatch later than the time when most caterpillars were available.
Research found thresholds of asynchrony between caterpillars and great tits, beyond which the great tits have an increased risk of extinction. According to these projections, once great tits begin breeding 24 days after the caterpillar peak, rapid extinction is inevitable.
Dr Simmonds comments. ‘What I find most interesting is the idea that populations that seem stable could suddenly go extinct. That matching with a food source can act as a threshold, before a certain point things are ok and then quite suddenly they are not.’
The higher numbers of simulated populations going extinct under a high greenhouse gas emission scenario occurs very rapidly, even when the population has previously been stable.
While some extinctions of simulated populations occurred in all scenarios of climate change, in models running a high emissions scenario the species went extinct 17% of the time, compared to under 5% when using low or medium emissions scenarios. Therefore, reducing climate change seems to be a route to mitigate some of the risk to the population. It could allow more time for evolution to occur as well, as this was shown to be slow in our model.
There are also some elements that this model does not include, which could help prevent extinction. Great tits could reduce their risk of extinction by switching to a different food source or the caterpillars could increase in numbers. But we should also be cautious that changes in climate could be impacting populations that we don’t currently think of as at risk.
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