Month: August 2024

One of Europe’s most widespread blue butterflies (absent in Ireland), the Chalkhill Blue Lysandra coridon, a relatively large species of blue which in the male has bright pale ‘chalky’ blue uppersides has been studied for its relationship with ants, its conservation needs, especially in England and its genome. Genome refers to the full complement of DNA in a cell.  DNA contains the information needed for an organism to develop. Humans have 23 pairs of chromosomes in the cell’s nucleus. As is the case for butterflies, half are inherited from each of the parents. The haploid number in a human sex cell is therefore 23; the full complement after the fusion or joining of the chromosomes, the diploid number, is 46.

The Chalkhill Blue, however, does not always conform to this neat formula. From Spain to Bulgaria, the chromosome numbers in Chalkhill Blues range from 87-92 with higher counts predominating in eastern Europe. From southern to northern Europe, the chromosomal range is 87-90. Why has the butterfly added chromosomes as it moved north after the last Ice Age, and while moving east?  Was it adapting to a cooler climate? Typically, an animal contains a fixed number of chromosomes; a butterfly with more or fewer than another butterfly might be a different species.

Does this mean that the ‘Chalkhill Blue’ in England and northern Europe is a separate species from the Chalkhill Blue in Northern Spain? Not necessarily, according to Tom Tolman, author of the Collins Guide to The Butterflies of Britain and Europe, expertly illustrated by Richard Lewington.

Tolman states the change in chromosome numbers from Spain to Bulgaria appears clinal. A cline is a progressive, usually continuous change in one or more traits over a geographical or altitudinal range. An example of a cline in Ireland might be the change in the proportion of brown to blue female Common Blue butterflies. In drier coastal areas in eastern Ireland, most females have brown uppersides. As one moves west, there is a progressive change from brown to blue, with some females in the far west showing very little brown on their uppersides.

No one currently believes these blue and brown Common Blues are separate species. In Scotland, the Common Blue produces one generation a year and maintains this characteristic even when bred in the south of England. It is genetically programmed to produce only one generation but this genetic difference with its southern English cousins does not make it a separate species.

Neither are factors such as appearance always reliable species indicators; the Wood White and Cryptic Wood White are identical but separate species. The most accepted definition of a species is based on whether two individuals can or are willing to mate to produce offspring that are healthy and can reproduce.

This occurs when Chalkhill Blue butterflies with slightly different chromosome numbers pair. The additional unpaired chromosome is excluded. While this might result in the loss of some genetic information, such loss is likely selective and does not prevent the development of viable offspring. Nature finds a way.

There is plenty of confusion concerning the Chalkhill Blue.  Many colour forms confuse taxonomists as to what butterfly they are looking at. Some believe that the butterfly that occurs in central Spain, Lysandra coridon caelestissima (Verity 1921 Type Locality: Central Spain, Albarracin) is a sub-species [i]of the Chalkhill Blue; others say it is a separate species. Tolman (2009) accepts the Macedonian Chalkhill Blue Lysandra phillippi as a separate species. Based on appearance, I find it impossible to separate this butterfly from the Chalkhill Blue in England. The Macedonian Chalkhill Blue uses the same larval foodplant, Horseshoe Vetch and is single-brooded, appearing in July and August, like populations in southern England. However, the Macedonian Chalkhill Blue has a much lower chromosome count, ranging from 20-26; in the Chalkhill Blue, this ranges from 84-92.

Just because the Chalkhill Blue has many more chromosomes than the Macedonian Chalkhill Blue does not mean that there are significant genetic differences. Some genes (DNA sequences on a chromosome) are redundant, persisting as a legacy of evolutionary history. In humans, the appendix is largely believed to be redundant, for example.

Some believe the Chalkhill Blue might be producing new species through fission events. Fission occurs when a single entity divides into two or more parts and the regeneration of those parts to separate entities resembling the original. The object experiencing fission is usually a cell, but the term may also refer to how organisms, bodies, populations, or species split into discrete parts.

A study published in February 2024, Comparative genomics reveals the dynamics of chromosome evolution in Lepidoptera, looked at butterfly genomes and found that most have shown remarkable stability, changing little over 250 million years. However, changes occurred in a few species, including the Chalkhill Blue. Most butterflies hold 28-31 chromosomes, not 90 plus as is often the case in the Chalkhill Blue.

To investigate the dynamics of fission in the Lysandra genus, the study reconstructed the events that gave rise to the genome structures of Chalkhill Blue and the closely related Adonis Blue Lysandra bellargus. Seven pairwise fusions generated a karyotype (full complement of chromosomes) of n = 24 in the last common ancestor of the family Lycaenidae (the coppers, hairstreaks and blues). Fifteen fissions then generated n = 39 in the last common ancestor of Lysandra. Subsequently, Lysandra bellargus underwent six fissions generating n = 45 and Lysandra coridon experienced at least one fission event in 37 of the 39 chromosomes of the Lysandra last common ancestor. An overwhelming majority of the 90 chromosomes in Lysandra coridon mapped to a single Merian element (Merian refers to ancestral linkage groups of modern butterflies and moths) and show conservation of gene order. The few Lysandra coridon chromosomes that contained segments from more than one Merian element derive from the seven fused chromosomes present in the common ancestor of the family Lycaenidae.

The study states that fusion and fission events are rare and very rare respectively and overwhelmingly occurred in just eight of the 32 ancestral linkage groups modern butterflies and moths are derived from. Other species that have undergone such changes are the Large White Pieris brassicae, Green-veined White Pieris napi, Small White Pieris rapae, Tinea semifulvella, Hewitson’s Tiger-wing  Melinaea menophilus, Melinaea marsaeus, Black-veined White Aporia crataegi, Lesser Marbled Fritillary Brenthis ino, Winter Moth Operophtera brumata, Brown Scallop Philereme vetulata, Wood White Leptidea sinapis and Lilac Beauty Apeira syringaria.

Of these, the Large White, Green-veined White, Small White, Winter Moth, Brown Scallop, Wood White and Lilac Beauty occur in Ireland. Some believe the Green-veined White is in an active state of evolution while others maintain the variations in this butterfly are influenced by ecological factors. Ecological factors that influence the appearance of a species can include climatic influences such as sunshine and rainfall levels, cloudiness, temperature, and features such as elevation and habitat conditions such as the presence of exposed rock, etc.

However, adaptation to ecological conditions can give rise to new species.

Speciation (the process by which populations evolve to become distinct species) can occur in response to changing conditions, such as a changing climate. Mutations (alterations in the DNA) occur naturally in all populations. Better-adapted mutants thrive while less well-adapted individuals gradually, or suddenly, disappear. Less beneficial traits are bred out by natural selection. This underlines the importance of conserving genetic diversity within species, meaning conserving a species across various habitat types, landscapes and continents.

However, we have seen that most butterfly genomes have remained stable over the past 250 million years. This indicates that Lepidoptera are well designed containing within their genome sufficient resilience to adapt to changing conditions to continue their purpose.

We ought to marvel at this winning design. Butterflies have been on the planet far longer than us. The first modern genera [ii] came into existence during the Paleogene era, between 23-66 million years ago. The oldest known still-existing species is the Ancient Metalmark Voltinia danforthi, from Sonora, Mexico. This butterfly evolved as long as 40-50 million years ago. Homo sapiens emerged about 300,000 years ago (Smithsonian Magazine, 2021).

Our destruction of most of the planet’s natural vegetation is challenging 250 million years of Lepidoptera existence, the continuation of one of the most important animal groups on Earth. We need to adapt rather than reshape the environment to suit ourselves.

Key References

Handwerk, B. (2021). An Evolutionary Timeline of Homo Sapiens. Smithsonian Magazine. Available at: https://www.smithsonianmag.com/science-nature/essential-timeline-understanding-evolution-homo-sapiens-180976807/.

McKie, R. (2024) Startling genome discovery in butterfly project reveals impact of climate change in Europe, The Guardian. Available at: https://www.theguardian.com/environment/article/2024/aug/11/genome-discovery-butterfly-project-impact-climate-change-europe-moths?CMP=share_btn_url (Accessed: 19 August 2024).

Tolman, T. and Lewington, R. (2009) Collins Butterfly Guide. Harper Collins, London.

Wright, C. J., Stevens, L., Mackintosh, A., Lawniczak, M., & Blaxter, M. (2024). Comparative genomics reveals the dynamics of chromosome evolution in Lepidoptera. Nature Ecology & Evolution, 8(4), 777-790. https://doi.org/10.1038/s41559-024-02329-4

 

[i] A subspecies is a population occupying a distinct geographical region, separate from other populations of the same species and having constant and clearly different characters; the subspecies can breed with the populations regarded as the species if they meet.

 

[ii] A genus (plural genera) is the group above species but below family, e.g. species: Common Blue Polyommatus icarus, genus: Polyommatus, family: Lycaenidae)

 

All photographs copyright J. Harding.

Butterfly Declines in 2024

How many times have you noticed the absence of butterflies, bees and other pollinators this summer? Months of dull, windy weather are being blamed. Butterfly Conservation UK, who run their Big Butterfly Count across the UK spanning the last two weeks in July and the first week in August, have reported the lowest butterfly count in the 14 years they ran the survey.

So, it is not an impression, it’s fact. Here is an interesting illustration juxtaposing two counts in one of Ireland’s best butterfly sites in 2023 and 2024.

Michael Gray (19/07/2023)
Essex Skipper 8, Cryptic Wood White 2, Large White 26, Small White 12, Green-veined White 12, Common Blue 6, Holly Blue 2, Red Admiral 76, Painted Lady 22 (magnetized to Wild Thyme), Small Tortoiseshell 8, Peacock 31, Comma 6, Silver-washed Fritillary 9, Dark Green Fritillary 2, Speckled Wood 5, Gatekeeper 39, Meadow Brown 20, Ringlet 20, Small Heath 9, Silver Y abundant seen during my exploration of the Raven Nature Reserve from 10.00 am to 6.00 pm, covering about half the open dunes, all the wood down to the southern point, T 113 262, Raven Nature Reserve, Co. Wexford. A mixture of sun and cloud, 50/50.

Michael Gray (16/07/2024)
Essex Skipper 2, Large White 3, Small White 5, Green-veined White 13, Common Blue 1, Silver-washed Fritillary 5, Speckled Wood 5, Hedge Brown/Gatekeeper 17, Meadow Brown 20, Ringlet 82, Small Heath 11, Six-spot Burnet 4, Silver Y 12, Cinnabar larvae several between 11 am and 5 pm at and near T 116 231, The Raven, Co. Wexford. The weather was warm with intermittent sunny spells.

On 19 July 2023, Michael counted 249 butterflies and 19 butterfly species.

On 16 July 2024, Michael recorded 164 butterflies and 11 butterfly species.

Standing in habitats awash with nectar-bearing blooms in the Burren, I needed to search for butterflies there in June, instead of being surrounded by them, as I have been in years past.

The Influence of Weather on Butterfly Abundance

This shortage is not always easily linked to weather-related events; we had high populations of some species last summer, despite the deluges of July and August 2023. Strong declines in a particular year are not disastrous; butterflies have a high reproductive rate allowing for rapid population recovery when conditions are favourable.

A good year, with warm, dry, sunny weather in important months for larval growth for several species, March, April and June following cold winter weather can yield great results. We had warmer temperatures than usual in March and April in the last two years, but June 2024 was cooler than usual with nearly all mean air temperatures below their long-term average.

The greatest difference between these months was in the weather in June. June 2023 was the warmest June on record, and had less rainfall than the average, allowing the caterpillars of species such as the Peacock, Small Tortoiseshell, Red Admiral and Comma to develop rapidly. Species that fly during June (and May 2023, also very dry) had the weather needed to feed and breed.

Is June the key month for these species? The caterpillars of most butterflies are present in June, varying in their development stage according to species. Is there a sensitive phase in larval development during June for some species? Did the extreme rainfall of July 2023 impact populations flying in 2024? During July 2023, rainfall amounts throughout Ireland were high above the long-term average, ranging from 133% to 300% above the long-term average. Extreme precipitation is the number of days above the 97.5 percentile for rainfall during the life cycle period for a particular species at that particular site. A UK study (McDermott Long et al. 2017) found that extreme precipitation is the most frequent cause of population decline in adult butterflies during the first and second-generation adult life stages.

The same study identified the pupal stage of single-brooded butterflies to be vulnerable to extreme precipitation.  Extreme warmth during winter was found to be damaging to butterflies.  Last winter (December 2023 and January and February 2024) are described by Met Eireann as “mild and wet overall”. The study also found that generalist butterflies (our common, widespread species) suffer more than specialist butterflies from extreme climate events (extreme precipitation, drought, extreme cold and heat).

The larva is the growth stage, and larvae are sensitive to weather conditions, often needing precise ecological conditions, including optimum temperature, insolation, surface geology, vegetation type and vegetation structure, and foodplant quality to develop. Adults of some species will move to find the resources needed for sustenance and reproduction and some species can survive long enough to use occasional good weather to breed and lay eggs. Larvae are much less likely or able to move to avoid bad conditions. However, if adult emergence and breeding are delayed by unsuitable weather, the foodplants may be in suboptimal condition. Declines in foodplant quality might be fatal to the successful development of the caterpillar or might result in less healthy adult butterflies. Observers are drawing attention to the number of dwarf butterflies this year.

Interestingly, most butterflies (31/35) are on the wing in June and July. The monthly average of daytime maximum temperatures, sunshine hours, rainfall and windiness are the most favourable during these months. Thirteen species fly in March and 15 in October. After early October the numbers flying collapses, as the conditions and resources needed to sustain adult activity deteriorate rapidly.

When summer weather is poor the four factors contributing to the favourability index are reduced. However, some woodland butterflies are probably better placed to cope with these downturns, given their more sheltered environs; in this suggestion, I am relying on the record of poor summers not reducing Silver-washed Fritillary populations found the following summer. However, in cold summers the Wood White will produce no second generation to speak of, with pupae from the spring flight overwintering until the following spring. This plasticity enables species to ‘wait’ until conditions are at their best before attempting to reproduce but plasticity is limited. The Wood White pupae must hatch the following spring but have around 10 weeks of spring and early summer (June) to select from.

Perhaps the Silver-washed Fritillary caterpillar enjoys a phenological advantage over other woodland butterflies; its caterpillar feeds from March to early June only. This makes it vulnerable only to poor weather during three months, unlike the Speckled Wood whose larva occurs in most of the year and Wood White which exists in the larval form in May/June and August/September.

A feature that has been noted this year is the appearance of dwarf butterflies. This has been observed in Speckled Wood, Holly Blue, Small Tortoiseshell and Peacock butterflies. This is likely the result of insufficient food for the larva. In short, the food available might have been in short supply, food was unsuitable, or both. Some species can still pupate and produce adults when the larva is insufficiently fed but there might be species less able or unable to do this.

Does poor weather per se explain the shortage of butterflies?

The answer to low figures might lie partly in poor weather in combination with the quality of the habitat and where the habitat is located. In a cold, windy summer, butterflies that use species-rich grassland in highly exposed coastal locations, upland slopes and open areas in the Burren will struggle. In these circumstances, butterflies congregate in the most sheltered areas, making them more vulnerable to predators. Most observers have favourite places to record butterflies, especially those doing transect walks. A population can collapse on one site but not nearby. This year, for example, the Marsh Fritillary was numerous in Lullybeg but numbers were far lower in adjoining Lullymore.  Recorder bias and location bias can distort results.

Therefore, a wider scope is needed to assess population trends. That is why the records sent to Butterfly Conservation Ireland matter. We are receiving evidence from various locations around Ireland to show the population trend of most butterflies is down compared with previous years, especially in the summer months.

The Role of Predators

Another factor in low abundance might be predation levels.  Butterflies have many natural enemies. Many are obvious: birds, frogs, dragonflies, spiders. However, a greater number of predators are obscure. Many parasitic wasp species destroy vast numbers of butterflies and in some years these predators reach a peak that collapses butterfly numbers. This phenomenon has been thoroughly studied in the common brassica-eating whites, the Marsh Fritillary and Holly Blue. In the latter two, the loss can be so high that populations disappear from some sites for years. In landscapes with well-connected suitable habitats, butterflies will return, and populations recover. We might be seeing the peaking of parasite populations for some butterflies dragging numbers down.

Weather is the likeliest cause of 2024 Declines

However, because all butterflies flying in summer appear to be in decline, it appears most likely that a common factor is driving declines. The likeliest is our weather. All months from July 2023 to April 2024 had above-average rainfall except January. As we have seen, extreme rainfall damages populations.

Longterm Declines 2008-2023

There is little doubt that populations have crashed spectacularly in 2024. Looking at the population data for our butterflies during the period 2008 and 2023 tells us that we are seeing sustained declines. The National Biodiversity Data Centre report for 2023 shows that common species that are widespread and well-monitored are in trouble. The Large White (-70%), Small White (-69%), Orange-tip (-65%) and Green-veined White (-82%) are in the ‘strong decline’ category with populations lower than the baseline year 2008.

The strong decline of -82% in the Green-veined White since 2008 is particularly disturbing for this is rated as among Ireland’s most widespread butterflies, recorded in over 86% of Ireland’s 10 km squares. It breeds in wet and humid grassland, vegetated damp ditches, hedgerow margins and marshes, along the banks of waterways, woodland glades and rides and wilder gardens. These habitats are common so one would expect the butterfly to thrive.

However, the Green-veined White is not flourishing in areas away from the direct influence of modern farming. Its distribution in Ireland is probably unchanged but its abundance has reduced. The reasons for the decline of the common white butterflies are unknown but what should create alarm is the fact that the Green-veined White, a widespread butterfly that has many larval foodplants and is not a habitat specialist is undergoing a severe thinning. In a sense, this is more concerning than a rare butterfly being in decline.

A Major Crisis?

Rare species can decline when their habitat, occupying small areas is shrinking or suffering neglect but that does not apply to the widespread Green-veined White or other common whites. We need to consider whether our entire countryside is being degraded. The failure of our landscape to support healthy populations of unfussy, common species must sound the sirens.

Time to worry.

Let us hope that butterfly lover Matthew Oates is correct in his statement, “Never underestimate a caterpillar.”

Key References

Judge, M and Lysaght, L. (2024). The Irish Butterfly Monitoring Scheme Newsletter, Issue 16. National Biodiversity Data Centre.

Long, O.M., Warren, R., Price, J., Brereton, T.M., Botham, M.S. & Franco, A.M.A. 2017, “Sensitivity of UK butterflies to local climatic extremes: which life stages are most at risk?”, The Journal of animal ecology, vol. 86, no. 1, pp. 108-116.