Introducing species to an area always carries risks, but some are more inconspicuous than others. When researchers introduced the caterpillars of the Glanville fritillary butterfly (Melitaea cinxia) to a remote island, they were hoping to explore how these winged insects would inhabit the environment. Little did they know that they were delivering a veritable hamper of parasites, as it later emerged that within those caterpillars was a parasitized parasite. What’s more, a bacterium also came along the ride – meaning that where just one species was intended to make ground on a tiny island, instead came four for the price of one. A turducken of host-inhabiting freeloaders, if you will.
The unfortunate series of events played out on Sottunga, a small island in the Åland archipelago. What happened next has been explored in a new paper in the journal Molecular Ecology, which took a look at how all four introduced species reacted and interacted following their arrival at Sottunga. Over the last 30 years, it seems that the island – once uninhabited by Glanville fritillaries and parasitoid wasps alike – has been successfully commandeered by all four introduced species, though some are doing better than others.
As butterflies go, M cinxia is a little lacking in its stamina when it comes to flying across open water. This makes an island population such as Sottunga’s vulnerable to extinction as it can’t be topped up by butterflies holidaying from other locales. Despite this, the population has lingered on.
“The butterflies have gone through many different crashes over the years. Some crashes were linked to extreme weather events in the Åland islands,” said study author Anne Duplouy to IFLScience. “We know that before and after those events, the genetic diversity of the butterflies remains quite high, when we would expect genetic loss. Why this species, this population, keeps high genetic diversity is actually a really good question, and one for which we do not have a very clear answer.”
While it’s hard to imagine an isolated island population having many opportunities in the way of gene-mixing activities, the researchers say that the butterflies’ starting point may have bolstered their stability.
“We have a quite high diversity to start with for sure, which helps buffer the effect of population crashes,” said Dr Michelle DiLeo in a quote sent to IFLScience, “but the dynamics of the Åland population through dispersal and rescue, and the patchiness of the habitat are also playing a big role in the mixing of the butterfly families and thus of the population's high genetic diversity.”
In the other corner, we see the parasitic wasp Hyposoter horticola surviving comparatively better, both in number and diversity. Across the islands where Sottunga’s M cinxia and H horticola originally came from, a strain of the endosymbiotic bacterium Wolbachia is found in this wasp. Interestingly, its presence increases the risk of hyperparasitism, in this case from the hyperparasitoid Mesochorus stigmaticus, a tiny wasp only found exclusively in H horticola.
Understanding this Russian doll of parasitism is a bit of a headache, but the general gist is that H horticola will burst out of an M cinxia caterpillar before it can become a butterfly – unless, that is, it’s infected with M stigmaticus. If the latter scenario is true, then the smallest of the parasitic wasps will burst out of the larger wasp around the same time it’s due to burst out of the caterpillar. All in all, a lot of bursting.
The wasp populations remain strong and with retained genetic diversity, possibly due to blowing across to other islands and mixing with “overseas” populations. Whether or not the parasites’ abilities to move further afield and bolster this diversity has an influence on the diversity of the comparatively land-locked fritillaries isn’t known, but the researchers say it would be an interesting topic for further investigation.
“Parasitism can play a role in the generation of diversity, it is producing selection pressure against which the host needs to reply and sometimes the solution comes from a mutation,” explained Duplouy. “The butterflies in the Åland islands are facing parasitic attacks from three main parasitoid species and a few other less common, it is also facing parasites such as viruses and bacteria on which less research has been done yet. Maintaining the yearly survey of this unique system will be key to such research and discoveries.”
An interesting intersection of host and parasite genetic diversity, then, but not one that has settled the researchers’ fears about the future of Sottunga’s fritillary population. Crashes have become more frequent due to habitat degradation, climate change, and human use of the areas where the host plants and the butterflies live, a trend that is being reflected across Europe.
“There is so much more we can learn from this system,” said Duplouy, “but this would all stop if it ever goes extinct.”