Garden wildlife and the climate crisis

Brian Eversham, Chief Executive of The Wildlife Trust for Beds, Cambs and Northants, takes us through the insects which have grown in numbers in the UK due to climate change.

When talking about the climate crisis, I sometimes meet people who don’t admit the climate is changing, or think it’s just a natural process which has happened before. I disagree [1], and if you look into local gardens, the signs are clear.

Literally hundreds of species have colonised Britain in the last 40 years, nearly all coming from further south in Europe. Most have arrived in southern or south-eastern England and have spread northward and westward. While the largest and most conspicuous are probably birds such as Little Egret, Cattle Egret and Purple Heron, the most numerous are the insects.

It’s sobering to think that whenever I go for a walk in summer, I’ll see dozens of species that the best Victorian naturalists would never have seen in Britain.

Garden arrivals responding to climate change

Whenever you started looking at wildlife in the last century, new species have been colonising. In the 1930s, wasp spiders arrived on the south coast, but it wasn’t till the 1990s, following a run of three hot summers, that they began to spread northward. By 2000 they were in Bedfordshire and Cambridgeshire, and are now as far north as Derbyshire.

Back in the 1940s, Migrant Hawker Dragonfly was well named: a scarce summer visitor from Europe. By the 1950s it was breeding in Kent, and now it’s probably the most abundant large dragonfly as far north as Liverpool and Durham, and it’s still spreading. Most garden ponds probably have them.

Another 1940s colonist was the Hornet Hoverfly. This became established in the London area but did not start expanding its range till about 1995, since then it has reached from Cornwall to Liverpool and is frequent throughout eastern and central England. It is the largest British hoverfly, most abundant in July and August, and often visits buddleia and other flowers. Its larvae scavenge inside wasp and hornet nests.

The first British record of Knopper Gall, the minute wasp whose grubs turns acorns into tough lantern-shaped structures, was in the 1950s, but it was only in 1979 that people noticed it was spreading: by 1995 it had reached Scotland. There was fear that it might prevent oak trees from producing acorns (in the event, numbers fluctuate greatly so we still have good acorn years). You’ll find it on nearly every oak tree. And if you look carefully, an even newer oak gall, the Ram’s-horn gall, arrived in 1997 and is already found from north Wales and Lancashire down to Bath and across to the East Anglian coast.

By the 1970s, people had noticed the Essex Skipper butterfly was expanding from its south-east stronghold. It’s now as common as the very similar Small Skipper in Bedfordshire, Cambridgeshire and Northamptonshire, and has reached Yorkshire and Lancashire. Essex is distinguished by the ‘dipped-in-ink’ black tips to its antennae.

Many non-native plant species reach Britain with human help, and some perhaps by wind-blown seed from mainland Europe. A South African species, Narrow-leaved Ragwort, had turned up occasionally, associated with sheep fleeces, and growing where the waste wool (‘shoddy’) was dumped. But in 1978, it arrived in Kent, perhaps by natural means from France, where it had become widespread. It began spreading in the 1980s, and has since become widespread throughout England and into Scotland, Wales and Ireland, on road verges and railway lines. It’s common on the tracks at Peterborough station, for instance.

The first Tree Bumblebees were seen in Britain in 2000, and it’s now a common species as far north as Edinburgh and Glasgow. It’s very distinctive, with a ginger front, dark middle and white rear, and is the only bumblebee you’re likely to see nesting in a hollow tree – or in a bird-box.

2001 saw the first Ivy Bee recorded from Britain, in Dorset, and by now they are widespread in the south, and have reached north Yorkshire and Lancashire. They are distinctive, being small and neatly-striped brown bees which feed almost exclusively on Ivy blossom, so appear in September and continue on the wing till November. They nest in the ground, mainly in light, sandy soils, so you may see groups of nest holes, or clusters of mating bees, on your lawn.

The occasional migrant Violet Carpenter-bee had turned up in Britain since the 19^th century. They are very conspicuous, being all-black with metallic-violet wings and the size of the largest queen bumblebees. They were not proven to breed here till 2006 in Leicestershire. I heard of a Cambridgeshire record around 2010, and at least 3 were seen in Northamptonshire in 2010. In March 2020, several were feeding together on flowers near Cambridge, so they are clearly hibernating successfully and breeding regularly in this area. Records are widely scattered and it’s not a common insect yet, but has been found as far north as Newcastle upon Tyne and eastern Scotland.

Some species take a few attempts before successfully colonising. Apart from a possibly-mislabelled 19^th century specimen, the first Willow Emerald damselfly was found in Sussex in 1979, but did not become established there. A single empty larval skin was found in Kent in 1992, so it had bred there, but no adults were seen. There were no more records till 2007 when one was seen at a gravel pit in Suffolk. By 2009, several hundred were seen, and they had spread over an area 25km across and were established. They have now spread at far as Lincolnshire and east Yorkshire, though still mainly confined to the eastern half of England^2.²Willow Emerald Watch - British Dragonfly Society (british-dragonflies.org.uk)

They are quite distinctive, being longer-bodied than the other Emerald damselflies, and are on the wing later than most damselflies, numbers peaking in August and September, and often still around in October. They are unusual in laying their eggs in willow twigs over water (leaving rows of distinctive scars on the bark).

Declines linked to climate change

In most groups of plants and animals in Britain, about 90% of species have a southern and eastern distribution, so are likely to increase as the climate heats, and only about 10% are northern, western and upland and likely to decline. A few species which were once familiar in gardens have retreated northward in the last 30 years, perhaps most strikingly, Willow Tit, which was once a frequent woodland bird and occasional visitor to gardens. It is now classed as the most rapidly declining bird species in Britain, and its range has shrunk by about 80% since 1990. It is now very scarce or absent as a breeding species east of a line from the Humber to the Isle of Wight, just clinging on in Breckland and in Rockingham Forest.

Until the 1980s, you might have seen Wall Brown butterflies commonly on rough ground, in quarries, and in gardens. It has since declined by almost 90% and disappeared from most of central and southern England. The most likely explanation seems to be a ‘developmental trap’ caused by a warmer climate. In the past, adult Wall Browns flew in July and August, and their caterpillars fed for a while then hibernated as caterpillars. Warmer late summers have meant the caterpillars completing their life cycle and emerging as butterflies in September and October - too late to produce caterpillars able to overwinter.

Change behaviour, change range

A few insect species have become suddenly more abundant by changing their behaviour, and especially their foodplants. The first I remember was Brown Argus Butterfly. Till the 1970s this was a chalk and limestone grassland specialist, confined to habitats where its caterpillar’s foodplant, Rock-rose, could grow. In the 1980s it was first seen egg-laying on crane’s-bills and stork’s-bills, and in the 1990s it expanded its range and filled in the distribution maps well away from chalk and limestone, by switching foodplant.

An even more remarkable success came to one of our rarest and most distinctive bugs, the Box Bug. When I was writing the bugs section of the Insects Red Data Book in the 1980s, this made the top category, Endangered, because it was known only from one site, Box Hill in Surrey, where it has been known since the mid nineteenth century. But in the 1990s it started being found on other trees - buckthorn, hawthorn, apple, even Cupressus, since then it has expanded extraordinarily, and now occurs from Devon to Manchester, and is abundant over central and eastern England.

Ups, downs and what next?

In 2016, I was excited to find a beautiful pale-grey moth, with golden spangling, on a window of the Cambourne office. It was a Small Ranunculus Moth, and the first book I checked said it was extinct in Britain! Its history in Britain is peculiar. In the 19^th century it was widespread in East Anglia and the south-east, but it declined and by 1914 was probably extinct. There were just a handful of records up to 1939, and then none at all till 1997, when a few individuals turned up in Kent. It was proven breeding in Kent in 1998, since when it has recolonised its former range and continued to spread: currently known as far north as Liverpool, and across to Bristol and south Wales. Its caterpillars feed on Prickly and Great Lettuce, which have also spread north and west in the last 40 years, so there’s potential for the moth to reach Scotland this time.

Assisted-passage expanding species

Not all wildlife can make its way to Britain, and across the country, on its own. Some have been helped by people. Flightless species are rarely going to colonise an island under their own steam, so the Spanish and Balkan 3-band Slugs needed the help of gardeners to get here. Buy a pot plant at a garden centre, and there’s a good chance that it comes with free slugs in the soil. Don’t worry, these pink-stripey slugs will help break down dead plants into humus rather than attacking your prize flowers. Though the Spanish 3-band slug has probably been in Britain for a century or more, for most of that time it was confined to heated greenhouses. But in the 1980s it was seen outdoors in gardens, and by 2000 I was finding it in more natural habitats: in the woods at Cambourne, the Spanish 3-band lives in the soil and under logs, while the darker and more energetic Balkan 3-band climbs trees to feed on algae and lichens. Both species are now very common in plastic compost bins, a habitat they share with the Green Cellar Slug.

Flightless insects also need help in crossing the Channel. It’s thought that the Southern Oak Bush-cricket (similar to the native Oak Bush-cricket but with tiny non-functioning wings) arrived on lorries through the Channel Tunnel. It’s been recorded from Cambridgeshire and Northamptonshire (none in Bedfordshire?! Have you looked?) and as far north as Leeds and Hull.

Perhaps the most exciting ‘assisted passage’ colonist turned up in Oxford in 2020, when Praying Mantises were found in several gardens. The females are too heavy to fly more than a few metres, so these must have been aided in their colonisation. Someone might have released ‘pet’ mantis, but I think it’s as likely that an egg pod, a tough and inconspicuous brown structure which might have up to 200 eggs within, was brought back from France attached to a tourist’s garden furniture, or perhaps on a plant-pot from a French nursery. Now they are here, I’d expect them to spread, and they could reach our area in the next decade or so.

Garden wildlife and the climate crisis