The Motion of the Seasons

Published on: 16 February 2016

Climate change is effecting the timings of biological events

By Matthew Creasey

University of Exeter Scientists led by Dr Malcolm Burgess measure amount of excretion by caterpillars to investigate how climate change is affecting the timings of important biological events.

“On Saturday [the very hungry caterpillar] ate through one piece of chocolate cake, one ice-cream cone, one pickle, one slice of Swiss cheese, one slice of salami, one lollipop, one piece of cherry pie, one sausage, one cupcake and one slice of watermelon. That night he had a stomach ache!” Not surprising really.

Across northern Europe, as spring takes over from winter, caterpillars start hatching from their eggs and begin the all consuming challenge of consuming all they can in order to grow sufficiently to pupate and metamorphose into their adult form, a butterfly or moth.

Of course what goes in must come out, and all this munching has predictable consequences – caterpillar poo, otherwise called frass. What may be more surprising however, is that this frass is proving to be a key resource for research into the effects of climate change.

By measuring the amount of frass being excreted by caterpillars in British woodlands, a team of scientists led by Dr Malcolm Burgess of the University of Exeter are investigating how climate change is affecting the timings of important biological events, and trying to predict what type of butterfly our world is likely to metamorphose into as global temperatures continue to increase.

A challenging climate

For wildlife, one of the most direct consequences of climate change is a phenomenon called trophic mismatch. This is when the lifecycles of species at different levels of the food chain get out of synch, and this is the focus of Dr Burgess’ research. Most hungry caterpillars hatch in early spring to coincide with the emergence of the fresh green leaves (rather than the cake and cheese favoured by their names sake) and spend about forty days gaining the weight required to pupate. Birds like the blue tit (Cyanistes caeruleus) must lay their own eggs so that the chicks are growing at roughly the same time as the caterpillars. Too early, and the growing grubs will not yet have emerged. Too late and they will have already eaten so much that they will have fallen to the woodland floor and pupated. The arrival of warmer spring temperatures triggers the leaves, and therefore the caterpillars to emerge.

This is where climate change becomes critical. In recent decades, spring has been starting steadily earlier, and the caterpillars and blue tits are having to adjust accordingly. Potentially even more challenging is the fact that patterns of climate change may be different across different geographic areas. For migratory species this makes judging the arrival time of spring especially difficult. The pied flycatcher (Ficedula hypoleuca) for example spends the winter in sub-Saharan Africa, travelling to woodlands across Northern Europe to breed. But are climatic conditions in their wintering grounds of Guinea, Liberia and the Cote d’ivoire a good indicator of the climate in breeding sites in the UK, Finland and Norway?

Pied flycatchers seem to be in some trouble, and populations have declined in the UK by 53 per cent since 1995. So how well are the flycatchers, and other migratory species adapting to changes in climate, and is the increasing difficulty of timing their migration and breeding right, responsible for the bird’s decline? This is what Dr Burgess and his colleagues are trying to discover.

The study

And so we come to caterpillar poo. The amount of frass falling from the woodland canopy has been shown to be a good indicator of caterpillar timing. During the spring, the amount of frass shows a sharp peak, and then declines as the caterpillars start to pupate.

Dr Burgess and his team have a looked at data from across Britain, at a number of woodland sites, in which both blue tits and pied flycatchers breed. By placing plastic trays around these sites, they catch the frass, and so are able to measure the amount of falling over time during the season. From this they can infer the availability of caterpillars feeding on the leaves above, and by monitoring this in the same woodlands over a number of years, they can begin to see how the timing of the caterpillar peak changes.

The team have collected the caterpillar data since 2009. Then to relate these findings to the birds, the researchers use data from nestbox schemes, some dating back to the 1950s, and closely monitor the nests of both the blue tits and pied flycatchers at the study sites. Nest boxes are visited repeatedly throughout the season, and scientists record the date the eggs were laid and hatched, how many chicks hatch and successfully fledge the nest, and the body condition of the chicks across the nestling period. They can then combine the two data sets.
By comparing caterpillar abundance against the progression of the breeding birds, they can assess how effectively the birds are synchronising their nests with the caterpillars, if this is changing over time in line with changing temperatures, and what the effects are for their reproductive success.

The findings

As we would predict, earlier and warmer springs are resulting in an earlier peak in caterpillar abundance. The peak is also getting shorter. What the team has also found is that there is high variation in the onset of spring between years, and between woodlands. In fact at a single site, the peak has varied by as much as 42 days, coming in April, May and June in different years.

And how are the birds responding to this? As we have seen, the blue tits are adapting quickly to the changing temperature and hence caterpillar patterns. But what of the migratory flycatchers? Flycatchers breed a little later than blue tits and always have done, but they seem to be tracking caterpillars right across Britain. But they are only well matched to caterpillar availability in colder years, and are especially mismatched in warm years which are now more frequent. But that said, they are responding.

At the main study site on Dartmoor, the birds can now lay their eggs almost two weeks earlier than they did in the 1950’s. However, the data also show that although the flycatchers are adapting, they are less well synchronised with the caterpillar peak than the blue tits and great tits. So it does indeed seem like the migratory species are finding it harder to adapt.

The implications

This mismatch is likely to be having an effect on the number and quality of flycatcher chicks produced, and so could be at least partially responsible for the decline in populations. There is also no reason to expect the advance of spring to slow.

At the recent climate talks in Paris, 195 nations agreed to a target of “(holding) the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C”. However, some have serious doubts about our ability to achieve these targets, and even if we do, we are only beginning to build a picture of what sort of effects we should expect with warming of this magnitude.

So just like the caterpillars, the development of Earth will be highly temperature dependent.

Global temperatures have already risen by 0.5°C over the past 100 years. The warmer it gets, the less recognisable our metamorphosing planet will become. This makes the work of Dr Burgess and his colleagues critical. It is only by learning how species are responding to climate change now, that we will be able to predict how they are likely to respond to the further rises in temperature predicted, and therefore the consequences of global warming for biodiversity on Earth. Metamorphosis is a bizarre and beautiful process. It suggests great possibilities for what is to come. What sort of world do we want to see emerge?

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