“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.

 
 
 
 

The Tory manifesto promises to both increase AND decrease the rate of housebuilding

Housing secretary Robert Jenrick. Image: Getty.

In his 2014 Mansion House speech, the then-chancellor George Osborne expressed with uncharacteristic honesty the motives at the heart of how the Conservatives see British housing politics: “The British people want our homes to go up in value, but also remain affordable; and we want more homes built, just not next to us.”

Five years later these contradictions remain unreconciled and present in their manifesto, which contains two different and contradictory – but clearly extensively targeted and focus-grouped – sets of policies.

The Conservatives have two housing targets. The first is to make significant progress to hitting “our target of 300,000 houses built a year by the mid-2020s”. The second is their aim to build “at least a million new homes” during the next parliament, which implies a target of 200,000 homes a year. This is not only 100,000 lower than their initial target but also lower than the current rate of housebuilding: 213,660 new homes a year. They have therefore implied at separate points in the same manifesto that they intend to simultaneously increase and decrease the rate of housebuilding.  

There are similar conflicts in their approach to planning. They intend to make the “planning system simpler” while simultaneously aiming to introduce community-led design standards for development and planning obligations to provide infrastructure for the local community.

None of this is unsurprising, The Tories don’t seem to know if they want to build more houses or not – so of course they don’t know whether they wish to make it easier or harder to do so.  

Politicians like obfuscation on housing policy to placate NIMBY voters. Take for example prospective Conservative MP and ‘environmentalist’ Zac Goldsmith’s crusade to save treasured local car parks. The manifesto can equally be accused of pandering to NIMBY instincts, protecting their shire voters from all housing, including ones they might actually need or want, by promising to protect the greenbelt.  

Instead, Conservatives intend to foist development on Labour-leaning inner-city communities and prioritising brownfield development and “urban regeneration”. This requires massive, infeasible increases in proposed density on brownfield sites – and research by Shelter has shown there are simply not enough brownfield sites in cities like London. Consequently, it is not clear how such a policy can co-exist with giving these inner-city communities rights on local design. Perhaps they intend to square that circle through wholesale adoption of YIMBY proposals to let residents on each street opt to pick a design code and the right to turn their two-storey semi-detached suburban houses into a more walkable, prettier street of five-storey terraces or mansion blocks. If so, they have not spelt that out. 

Many complain of NIMBYism at a local level and its toxic effects on housing affordability. But NIMBYism at the national level – central government desire to restrict housebuilding to make house prices rise – is the unspoken elephant in the room. After all, 63 per cent of UK voters are homeowners and price rises caused by a housing shortage are hardly unpopular with them. 


There is anecdotal evidence that protecting or inflating the value of homeowners’ assets is central to Conservative strategy. When George Osborne was criticised for the inflation his help to buy policy caused within the housing market, he allegedly told the Cabinet: “Hopefully we will get a little housing boom, and everyone will be happy as property values go up”. More recently Luke Barratt of Inside Housing noted that most Conservatives he spoke to at the 2018 party conference were scared “they’d be punished by their traditional voters if the values of their homes were to fall”. He was told by a Conservative activist at the conference that, “If you build too many houses, you get a Labour government”.

But the senior figures in the Conservative Party are painfully aware that the continuing housing shortage presents major long-term problems for the Party. As the manifesto itself acknowledges: “For the UK to unleash its potential, young people need the security of knowing that homeownership is within their reach.” Perpetual increases in house prices are incompatible with this goal. The problem has greatly contributed to the Conservatives’ severe unpopularity with a younger generation priced out of decent accommodation. 

Equally, there is increasing evidence that ‘gains’ from rising house prices are disproportionately concentrated in the south of England.  The differences in housing costs between regions greatly reduce labour mobility, suppressing wage growth in the north and midlands, which in turn leads to greater regional inequality. The policy of coddling southern homeowners at the expense of the economic well-being of other regions is a major long-term stumbling block to Conservative desires to make inroads into the ‘red wall’ of Leave-voting labour seats outside the south.

Before dealing with the issue of where housing should go, you must decide whether you want to build enough housing to reduce the housing crisis. On this issue, the Conservative response is, “Perhaps”. In contrast, even though they may not know where to put the necessary housing, the Labour Party at least has a desire in the abstract to deal with the crisis, even if the will to fix it, in reality, remains to be seen. 

Ultimately the Conservative Party seems to want to pay lip service to the housing crisis without stopping the ever-upward march of prices, underpinned by a needless shortage. Osborne’s dilemma – that the will of much of his party’s voter base clashes with the need to provide adequate housing – remains at the heart of Conservative housing policy. The Conservatives continue to hesitate, which is of little comfort to those who suffer because of a needless and immoral housing shortage.

Sam Watling is the director of Brighton Yimby, a group which aims to solve Brighton’s housing crisis while maintaining the character of the Sussex countryside.