“Super-slugs” are invading British gardens – and we don’t know how to stop them

Mmmmmm, slimy. Image: Xauxa Håkan Svensson.

The Daily Mail calls it a “slime wave”. The Sun calls them “an army”. Either way, both papers have reported 500bn slugs are set to invade British gardens, after a mild winter created perfect breeding conditions. The Conversation

So is the UK really about to be overwhelmed by slimy slugs? The simple answer is no, but there could be something far worse in store.

Headline numbers alone aren’t necessarily something to get in a lather over. A typical garden can contain several thousand slugs, and the “500bn” figure is derived from estimates of maximum numbers per area. In any case, slug numbers can rise and fall a great deal across time and space, in natural cycles, and even astonishingly dramatic increases are not always cause for concern. Like waves crashing against a beach, the rise is often transient and local – usually slug numbers will drop back to normal, with the disturbance hardly noticed beyond a few local gardeners.

What is more problematic is the progressive, sustained and perhaps less spectacular rise in numbers which, tsunami-like, is maintained for far longer, and spreads widely throughout the countryside. This is Britain’s real slug invasion. So what can we do about it?

The trigger seems innocuous enough in isolation: a few non-native slugs from continental Europe have accidentally been introduced. Several of these species have close relatives in the UK, so similar in fact that only specialists can tell them apart, and they can interbreed freely. Of course, many animals can create hybrids without presenting a threat, but what makes slugs different – and these hybrids so worrying – is their interesting and deviant sex lives.

This is a hybrid between a ‘Spanish stealth slug’ and the UK’s common black slug. Image: Les Noble/author provided.

Slugs are hermaphrodites, which means the same individual exists as both sexes; they first develop as males, before experiencing a true hermaphrodite phase to become female. This means they can dispense with normal mating requirements, and this is where the consequences of the difference between British and continental species becomes significant.

Why British slugs are different

When slugs colonised the UK after the last ice age, they found an island recently covered with ice sheets, where the biological diversity remained poorer than continental Europe. In these circumstances, the ability to self-fertilise was a good evolutionary strategy, one which ensured reproduction even when slug populations were devasated by harsh weather.

A downside of such continued close inbreeding (and mating with oneself is as inbred as it gets) is a rapid loss of genetic variability, and some British slug species eventually came to consist of almost genetically identical individuals. This meant they were more vulnerable to parasites and pathogens that could rapidly evolve to overcome their defences.

Meanwhile, in continental Europe, slugs were becoming more diverse, as balmier weather meant parasites and pathogens were a bigger issue than finding a mate. These slugs tended not to self-fertilise, and were genetically highly variable. This made at least some of them more resilient to attacks from parasites – a possibility not afforded to the inbred British slugs.

Echoes of these different past environments resonate in contemporary species. British slugs, adapted to a variable climate and dearth of mates, have fallen into the clichéd “No sex please, we’re British” mould, producing fewer, bigger eggs later in life by self-fertilisation. Continental slugs, meanwhile, adapted to resist rapidly evolving enemies. Their strategy is therefore to produce many smaller eggs earlier in life, which maximises genetic diversity and compensates for losing many individuals to infection.

The ‘Spanish slug’, one of Britain’s key invaders. Image: tviolet/creative commons.

These different adaptations weren’t an issue until humans disturbed the natural order by moving slugs back and forth as stowaways in commercial produce. As a result of this, we’ve seen widespread breeding between British and continental species. These new hybrid “super-slugs” are highly fertile, and their genetically diverse offspring are adapted to cope with both the British climate and parasites and pathogens, most of which remain in continental Europe anyway.


Fighting the slug invasion?

Legislation aimed at environmental protection has led to the EU banning commercial use of molluscicides (pelleted chemicals which poison slugs but cause collateral damage to other wildlife). Instead, the emphasis is on using natural enemies like nematode worms, though these are generally ineffective against the larger invasive hybrids.

Nonetheless, the increased slug biomass could still host important veterinary or agricultural parasites and pathogens, spreading more plant and animal diseases. Remarkably, despite their obvious presence in our gardens, we remain startlingly ignorant of the fundamental biology of slugs; evidenced by recent work which increased the number of identified British species by more than a fifth.

So where are we going with this phenomenon? Studies have already found invasive slugs and snails can destabilise ecosystems and reduce biodiversity in the US and Scandinavia. Something similar is happening here in the UK.

The good news is that our research suggests population sizes do eventually begin to decline, after 30 to 40 years. The ecosystem may eventually rebound from this slug invasion, but it remains to be seen how long it will take and what the lasting effects will be for the spread of diseases, ecosystem services, or British biodiversity.

Leslie Noble is a reader in zoology at the University of Aberdeen.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

So what was actually in Grant Shapps’ latest transport masterplan?

A tram in Manchester. Image: Getty.

Poor Grant Shapps. This weekend, the UK’s transport secretary unveiled a fairly extensive package of measures intended to make sure Britons can keep moving about during the Covid-19 crisis. On Saturday, he fronted the government’s daily afternoon press briefing; on Sunday, he did the rounds of the morning political shows. 

And were those nasty mean journalists interested in his plans for bicycle repair vouchers, or the doubling of the A66? No they were not: all they wanted to ask about was reports that the Prime Minister’s senior advisor Dominic Cummings had breached the lockdown he himself had helped draw up. The rotten lot.

This is, from some perspectives a shame, because some of the plans aren’t bad. Here’s a quick run down. 

  • The government is releasing a total of £283m to increase frequencies on bus (£254m) and light rail (£29m) networks, enabling more people to travel while maintaining social distancing. 

  • It’s deploying 3,400 people – British Transport Police officers; staff from train operators and Network Rail – to stations, to advise passengers on how to travel safely.

  • It’s promising to amend planning laws to enable councils to reallocate road space and create emergency cycle lanes, using a £225m pot of funding announced earlier this month. 

  • It’s also spending £25m on half a million £50 bike repair vouchers, and £2.5m on adding 1,180 bike parking spaces at 30 railway stations.

All this sounds lovely, but announcements of this sort tend to throw up a few questions, and this is no exception. The UK is home to over 2,500 railway stations, which must raise doubts about whether a few extra bike parking spaces at 30 of them is going to be enough to spark a cycling revolution. And councillors say that £225m for new cycle lanes has been slow to materialise in council bank accounts.

As to the money for public transport: that £29m will be shared between tram networks in five English conurbations (Greater Manchester, the West Midlands, Tyne & Wear, Nottingham, Sheffield). Just under £6m each doesn’t sound like the big bucks.

Then there’s the fact that all of these pots of money are dwarfed by the £1bn the government is planning to spend on turning the A66 Transpennine route across the north of England, from Workington to Middlesbrough, into a dual carriageway. Which puts the money allocated to cycling into perspective.

That said, it is refreshing to see the government taking an interest in cycling at all. Also, Grant Shapps genuinely tried to distract the nation from a huge political scandal by talking about bike repair vouchers, and you’ve got to give him credit for that.

More details of the plan on gov.uk here.

Jonn Elledge was founding editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.