'Some species are in really big trouble'

Wall Brown butterfly thought to be on brink of extinction

“The news from the last two years is concerning,” says Jesmond Harding of the health of butterfly populations in Ireland.

The leading butterfly conservationist says that annual statistics collected by the National Biodiversity Data Centre show a general downward trend.

“In 2020 and 2021 their multi-species index showed there was a moderate decline in the number of butterflies flying compared to the baseline year of 2008.

“In terms of individual species trends, no species had a positive trend, and only two species had a stable trend and all the others were either declining or uncertain compared to the baseline year.”

Not to downplay the significance of those findings, Jesmond is more concerned by the likely plight of other species for which there was insufficient data collected to make reliable comparisons.

“A lot of those species are the ones that I would be very worried about, because anecdotally and from my own observations, some species are in really big trouble. The Wall Brown for example used to occur pretty much all over Ireland, it might be extinct from Northern Ireland. Since 2015 two Wall Browns have been seen in Northern Ireland. Two!”

Aside from County Donegal, the Wall Brown’s problem extends across all of Ireland, and is especially acute in the northern half of Ireland. It has been categorised as ‘Endangered’ by the Irish Butterfly Red List compiled in 2010.

He cites a butterfly expert who in 1992 described the Wall as “widely distributed and often abundant across Ireland’.

“There’s no way you could say that now,” laments Jesmond, who runs the Butterfly Conservation Ireland website.

The fact that Wall Browns breed on commonly found grasses such as Cocksfoot and Yorkshire Fog adds to the concern.

“Those grasses are everywhere, so it’s not foodplant availability issue,” he observes.

“When you see butterflies like the Wall Brown, which used to be common and widespread disappearing, it’s a sign of general environmental degradation.

“There are specialist [butterfly] species which live in very niche habitat, and they usually disappear because their habitat was destroyed or changed naturally, that’s easily understood. But where you have widespread species disappearing, that’s much more scary because it means the environment generally is degraded - it’s not a problem with one habitat in one distinct location.”

Nor is the Wall Brown’s plight confined to Ireland as he notes similar experiences across northern Europe and a shocking report compiled in the UK in 2022.

“That recorded an 87% decrease in occurrence, in other words, the places in which it was found over the period 1970 and 2019, and 87% decrease in abundance between 1976, which is their baseline year, and 2019.”

“It has severely declined. I remember as a child there was no problem finding it. It has two generations a year, sometimes it has three. It shouldn’t be in the trouble that it’s in.”

Informed by research in Germany and the Netherlands, Jesmond suspects the problem comes from a combination of agricultural chemicals and climate change.

Butterflies are very sensitive to changes in temperature. Warmth will stimulate butterfly larvae that overwinter to commence feeding.

While they feed on green plant material, many species need to rest on dry leaf litter to obtain the required warmth from the sun to raise their body temperature to the mid-30s to allow them to digest.

“Because of nitrates, and atmospheric nitrogen deposition the soil has more fertility,” explains Jesmond, “the plants are growing more vigorously; they’re growing earlier and they’re growing better. That means that the temperature just above the soil is actually cooling, even in the context of a warming climate, so they can’t digest their food and they starve to death.”

As such, nitrate pollution and nitrogen deposition are impacting those butterfly species that depend on food plants that require low fertility soil such as the Small Heath and the Grayling.

In terms of the impact of the ongoing climate crisis, Jesmond observes we can experience mild “t-shirt weather” in January followed by “a bitterly cold February, and even a bitterly cold March”.

“If you have a gut full of food that you cannot digest, it’s probably going to kill you.”

The few species that benefit from nitrogen-rich plants, such as the Comma Butterfly and the Small Tortoiseshell seem to have had a very good 2023.

Jesmond published a paper revealing his discovery in May 2014 that the Comma Butterfly was breeding in Ireland, specifically in County Carlow. It’s thought they came over from southwest England, arriving first in Wexford.

“Since then it has been rapidly spreading north and west. That butterfly is tracking climate change - it is moving northward elsewhere in the continent.

“I saw a female laying eggs in my garden this summer - if you told me 20 years ago, I wouldn’t have believed it - that butterfly had never been seen here before 1998.”