Climate change is forcing butterflies and moths to adapt – but some species can’t

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The silver-studded blue butterfly is among that species that may be flexible enough to thrive.
Callum Macgregor, Author provided

Callum Macgregor, University of York

Butterflies are rather like Goldilocks, preferring conditions to be neither too hot nor too cold, but “just right”. Under climate change, the temperature at any given time of summer is, on average, getting warmer, leaving butterflies (and their nocturnal cousins, the moths) with the challenge of how to remain in their optimal temperature window.

One of the main ways in which species are achieving this is by changing the time of year at which they are active. Scientists refer to the timing of such lifecycle events as “phenology”, so when an animal or plant starts to do things earlier in the year it is said to be “advancing its phenology”.

These advances have been observed already in a wide range of butterflies and moths – indeed, most species are advancing their phenology to some extent. In Britain, as the average spring temperature has increased by roughly 0.5°C over the past 20 years, species have advanced by between three days and a week on average, to keep track of cooler temperatures.

Is this a sign that butterflies and moths are well equipped to cope with climate change, and readily adjust to new temperatures? Or are these populations under stress, being dragged along unwillingly by unnaturally fast changes?

In a new study published in Nature Communications, colleagues and I sought to answer this question. We first pulled together data from millions of records submitted by butterfly and moth enthusiasts to one of four recording schemes run by charities or research institutes. This gave us information on 130 species of butterflies and moths in Great Britain every year for a 20-year period between 1995 and 2014. We could then estimate the abundance and distribution of each species across this time, along with how far north they had moved. The data also, crucially, allowed us to estimate subtle changes in what time of the year each species was emerging from the chrysalis as a fully-grown butterfly.

It pays to reproduce quickly

Analysing the trends in each variable, we discovered that species with more flexible lifecycles were more likely to be able to benefit from an earlier emergence driven by climate change. Some species are able to go from caterpillar to butterfly twice or more per year, so that the individual butterflies you see flying in the spring are the grandchildren or great-grandchildren of the individuals seen a year previously.

Among these species, we observed that those which have been advancing their phenology the most over the 20-year study period also had the most positive trends in abundance, distribution and northwards extent. For these species – such as Britain’s tiniest butterfly, the dainty small blue – emerging early in spring gives more time for their later-summer generations to complete their reproductive cycles before the arrival of autumn, allowing more population growth to occur.

Small blue: Britain’s tiniest butterfly.
Callum Macgregor, Author provided

Other species, however, are less flexible and restricted to a single reproductive cycle per year. For these species, we found no evidence of any benefit to emerging earlier. Indeed, worryingly, we found that the species in this group that specialise in one very specific habitat type (often related to the caterpillar’s preferred diet) actually tended to most harmed by advancing phenology.

The beautiful high brown fritillary, often described as Britain’s most endangered butterfly, fits this category perfectly. It is found only alongside the dog-violets that its caterpillar eats, in coppiced woodland and limestone pavement habitats. It’s also a single-generation butterfly that has advanced its phenology. This suggests that climate change, while undoubtedly not the sole cause, might have played a part in the downfall of this species.

The high brown fritillary was once widespread, but is now found in just a few sites in Lancashire and the south-west.
Callum Macgregor, Author provided

All is not lost, however. Many of Britain’s single-generation species show the capacity, in continental Europe, to add a second generation in years that are sufficiently warm. Therefore, as the climate continues to warm, species like the silver-studded blue might be able to switch to multiple generations in the UK as well, and thereby begin to extract benefits from the additional warmth, potentially leading to population increases.

Specialists are at risk

More immediately, we can arm ourselves with this knowledge to spot the warning signs of species that may be most at risk. Clearly the single-generation habitat specialists are of particular concern, as many are already endangered or vulnerable – not just the high brown fritillary and silver-studded blue, but also species such as pearl-bordered fritillary, grizzled skipper and the particularly sought-after white admiral of southern England. Multi-generation species that are failing to advance their phenology might also be threatened: into this category falls another of Britain’s most sharply-declining butterflies: the wall brown.

Using this knowledge to help protect moths and butterflies from climate change is not simply important for the sake of the butterflies and moths themselves – these species also play a number of important roles in our ecosystems. Their caterpillars consume vast quantities of plant material, and in turn act as prey for birds, bats, and other small mammals, while moths even act as pollinators of a surprisingly wide range of plant species, possibly including some important crops.

According to Butterfly Conservation, around two-thirds of butterfly species have declined in the UK over the past 40 years. If this trend continues, it might have unpredictable knock-on effects for other species in the ecosystem. Only by arming ourselves with an understanding of why butterfly numbers are down can we hope to halt or reverse the decline.

Callum Macgregor, Postdoctoral Research Associate, University of York

The Conversation

Wallace, Darwin and the night shift

Last week I had the great honour of attending the Royal Entomological Society’s annual conference, this year hosted by Edge Hill University, to accept the 2017 Alfred Russel Wallace Award. The award is given for the PhD thesis “judged to make the most significant contribution to entomology in the year”, and after several rounds of selection my own thesis, “The role of moths as pollinators, and the effects of environmental change”, was chosen. At the conference, I said that I was particularly proud to receive an award in Wallace’s name, because of his own connections to the subject matter of my thesis – nocturnal pollination by moths. In this blog, I re-tell the story of that connection…

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It is 25th January, 1862, and Charles Darwin is writing to his friend and confidante Joseph Dalton Hooker. Earlier that same day, Darwin had taken delivery of a box of specimens of exotic orchids from James Bateman, a Staffordshire-based orchid specialist.1 Among these, Darwin found one in particular (Angraecum sesquipedale, a species from Madagascar) to be remarkable, due to the great length of the nectary, extending 11.5 inches below the flower. “Good Heavens”, wrote Darwin, “what insect can suck it”.

Angraecum_sesquipedale.png
(L-R) a drawing of Angraecum sesquipedale by W.H. Fitch, included in: James Bateman (1876) A second century of orchidaceous plants. Curtis’s Botanical Magazine; two flowers of A. sesquipedale (© Michael Wolf | Wikimedia Commons | CC BY-SA 3.0); a plate commissioned by A.R. Wallace depicting the hypothetical hawkmoth pollinator of A. sesquipedale, drawn by T.W. Wood and included in: Alfred Russel Wallace (1867) Creation by Law. The Quarterly Journal of Science, 477.

Darwin must have acted quickly. In the same letter to Hooker he had described his “infinite satisfaction” at the prospect of correcting the proofs of his next books, Fertilisation of Orchids, “in 2 or 3 weeks”. Yet when Orchids emerged less than four months later on 15th May, Darwin had revised the text, incorporating a discussion of A. sesquipedale and a rough hypothesis as to its insect pollinator. Reasoning that the longest tongues among English insects belonged to hawkmoths (family Sphingidae), he wrote:

in Madagascar there must be moths with probosces [sic] capable of extension to a length of between ten and eleven inches

Darwin’s prediction sparked debate; among those who sprung to his defence was Alfred Russel Wallace. Contrary to popular opinion, correspondence between Darwin and Wallace was friendly and reasonably frequent after they jointly proposed evolution by natural selection (van Wyhe & Rookmaaker 2015). Five years after the publication of Orchids, Wallace’s Creation by Law asserted not only that Darwin’s predicted moth could safely be assumed to exist, but further, that naturalists visiting Madagascar “should search for it with as much confidence as astronomers searched for the planet Neptune”. Wallace picked out one moth in particular to back up this statement – Xanthopan morganii, then known from tropical Africa (as Macrosila morganii), which had an exceptionally long proboscis.

Xanthopan_morgani.png
Xanthopan morgani; a specimen from continental Africa. Malagasy X. m. praedicta have proboscides several inches longer still!

Several decades later, Walter Rothschild (2nd Baron Rothschild) and Karl Jordan described a newly-discovered subspecies of X. morganii, from Madagascar. Remarkably, the subspecies had a tongue that was, by several inches, longer than that of its continental cousins. In honour of the predictions made by first Darwin and later Wallace, the new subspecies was named X. morganii praedicta.

DarwinWallaceRothschild
(L-R) Charles Darwin, Alfred Russel Wallace, and Walter Rothschild

At this point, the trail went cold for the best part of the 20th century. We had the flower, and we had found the moth with a proboscis to match it, but the final proof of their association was lacking because nobody had ever seen the two interact. That state of affairs lasted until the early 1990s, when Professor L.T. Wasserthal (then of the Friedrich-Alexander University Erlangen-Nuremburg) finally photographed, beyond doubt, the pollinator of A. sesquipedale. It was, indeed, X. morgani.

BotanicaActa
Making the front cover, a photograph of Xanthopan morgani praedicta visiting the flower of Angraecum sesquipedale finally resolves the mystery!

As a final chapter, the video below is well worth a watch: both for its remarkable footage, shot in 2004 by Dr Philip DeVries of the University of New Orleans, of this pollination interaction in action, and for the sheer joy and excitement shown by Dr DeVries and his colleagues when their hard work finally paid off!

If you want to learn more about this story, I owe a debt of gratitude to the review of Arditti and colleagues (2012), on which I drew heavily whilst researching this blog.

1: The relationship between Darwin and Bateman must have been complex. Bateman was, at this time, one of Britain’s foremost experts on, and cultivators of, orchids, and it is hard not to assume that he and Darwin corresponded while the latter prepared Fertilisation of Orchids. However, this box of specimens, and a brief subsequent exchange of letters, appears to be the only surviving evidence of direct interaction between the two (Darwin Correspondence Project). Bateman lived at Biddulph Grange in Staffordshire, designing (with his wife Maria) its famous landscape gardens, which are now owned by the National Trust. The gardens featured a Geological Gallery, seen as a direct attempt to refute Darwin’s theory of evolution by natural selection by drawing connections between evidence from the fossil record and the account of creation given in the book of Genesis.

References

Arditti, J., Elliot, J., Kitching, I.J. & Wasserthal, L.T. (2012) ‘Good Heavens what insect can suck it’–Charles Darwin, Angraecum sesquipedale and Xanthopan morganii praedicta. Botanical Journal of the Linnean Society169, 403-432.

Darwin Correspondence Project, “Letter no. 3356,” accessed on 3 April 2018, http://www.darwinproject.ac.uk/DCP-LETT-3356. Also published in The Correspondence of Charles Darwin, vol. 10.

Van Wyhe, J. & Rookmaaker, K. (2015) Alfred Russel Wallace: Letters from the Malay Archipelago. Oxford University Press, Oxford.