When I first moved to my current house some 20-years ago, my next-door neighbour had a buddleia growing in their garden which overhung part of my driveway. In the summer, the shrub was a resting spot for a multitude of red admiral butterflies, which exploded into a swarm if I got too close. Sadly, that buddleia was a casualty of renovation work a few years ago but even if the shrub had survived, I suspect the butterfly swarm might still be a thing of the past. Every year the UK organises the big butterfly count where over 100,000 members of the public score the number and species of butterflies in a particular area over a given time. This year they reported the lowest average number of butterflies since recording began in 2010.
The red admiral is a migratory butterfly, happy to inhabit that buddleia in the summer but then heads south for the winter. If conservationists are to identify the causes of the falling population, then they need to understand the red admiral’s migratory behaviour, and that’s not such an easy thing to do. The traditional approach is to capture butterflies, place labels on their wings before releasing them back into the wild, then record the labels as butterflies turn up in other geographic locations. This is a hit and miss method because information on migration routes rely on butterfly labels turning up in the expected location. Moreover, insect migration, known as migration phenology, can be deceptively complex, sometimes involving journeys of thousands of kilometres over more than one generation. Wing labelling therefore provides a snapshot in time, rather than overall and longer-term migration phenology data. Instead of relying on wing labels, recently scientists have turned to isotope analysis to attain more nuanced migration phenology data.
I have mentioned isotopes in previous blog posts, and they are essentially different versions of the same chemical element, distinguished only by the atomic mass. Hydrogen, for example, has an atomic mass of one because its nucleus has one proton. An isotope of hydrogen called deuterium on the other had has an atomic mass of two, because its nucleus has one proton and one neutron. Scientists distinguish isotopes by given the total number of protons and neutrons as a prefixed superscript to the elemental symbol. Hydrogen is 1H and deuterium is 2H for example. All the chemical elements have a range of isotopes. For instance, the most common isotopes of oxygen are 16O, 17O and 18O (all oxygen atoms have 8 protons, 16O all has 8 neutrons, 17O has 9 neutrons and 18O has10 neutrons). The relative combinations of isotopes in molecules depends upon their surrounding physical conditions. The relative amounts of hydrogen and oxygen isotopes in water, for example, reflect the conditions of evaporation from oceans and precipitation as rain. Pour a glass of water from your tap and the blend of 1H216O, 1H217O, 1H218O, 2H216O, 2H217O and 2H218O depends upon where on planet Earth your tap resides.
Butterflies incorporate hydrogen isotopes into wing keratin, which is a fibrous protein found in hair (and butterfly wings). Once formed, keratin is stable, locking in the isotopic pattern. The 2H to 1H isotope ratios vary according to rainfall and local temperatures and so can be used as regional markers. To compare isotopes in butterfly wings to specific regions requires data on geographical isotope patterns. This is achieved through painstaking sample collection and analysis over wide landscapes, to build what’s called an isoscape.
Traditionally considered to have regular migration patterns, red admirals go to northern Europe in the spring where they reproduce with the next generation heading south for the winter. Although hydrogen isotope analysis confirmed this general concept, it also revealed complexities not previously realised. Analysis of 2H showed red admirals migrate from the south probably as far away as north African locations in the spring, but in August they originated further from the north in what are two distinct migration patterns. In spring, butterflies migrating from the south carried eggs but we find no eggs in August on butterflies coming from the north. Differences in 2H patterns measured in central Europe in spring compared to winter, are consistent with the butterflies reproducing in the Mediterranean during winter. In contrast to the migratory patterns of spring and winter, in autumn 2H isotope patterns indicated butterflies are of a more local origin. Surprisingly, they may be two populations of red admiral because the 2H isotope patterns in those from western Europe rarely correspond with those of north-eastern Europe migrating in the autumn. Based upon isotopic patterns therefore, the migration phenology of the red admiral is far more complex than first realised.
The reason why some butterflies seem to be on the decline remains obscure, but the red admiral is not alone in this respect. In North America, monarch butterflies that overwinter in the Oyamel forests of Mexico have suffered an 80% decline and the possibility of extinction looms. Phenology studies on monarchs, prior to 1997, was carried out using traditional wing labelling techniques but despite hundreds of thousands of butterflies being tagged, they only ever found 125 in Mexico. In 1998 a Canadian research group turned to using isotopes of hydrogen and carbon to trace the origins of monarch butterflies. They measured 2H and 13C in the wings of monarchs captured in Mexico and compared the isotope abundances to isoscapes across North America. Data for 2H reflected local rainfall and 13C data correlated to milkweed, the principle food source for monarch caterpillars. They determined that around half of the monarchs that overwintered in Mexico originated from the Midwest corn and soybean belt of the United States, Nebraska, Kansas and Texas on the western edge and the coastline to the east. Once these butterflies migrated to the Oyamel forests they bred and the next generation returned to its origins in the Midwest. Some conservationists believe that pesticides and genetically modified crops are responsible for the butterfly decline but others point to limiting factors on the migratory routes (now there’s a better understanding of those routes). Either explanation is possible but isotope analysis turned up another possible explanation.
In 2018, 2H and 13C isotope analysis on captured monarchs in south Florida showed there were two populations, one resident to Florida and the other migratory. Not all monarchs are migratory therefore, and it seemed Darwinian evolution was at work because the resident population had a smaller wing size. The migratory population mostly originated from the Midwest and the Texas-Oklahoma border, which correlates to the origins of the Mexican population. This raises the intriguing possibility monarchs are not so much on the decline but perhaps relocating away from Mexico and towards Florida. Given the complexity of red admiral migration so elegantly revealed by isotopic analysis, I’d like to think perhaps those butterflies which once rested on the buddleia overhanging my garden have just moved on to more suitable locations as our climate warms. Without evidence perhaps this is wishful thinking but I’m sure the answer is hidden somewhere within the isotopes.