Cold-tolerant wasp spiders spread to northern Europe

Wasp spider female on a web by David Chapman Wasp spiders are becoming a more familiar sight in northern Europe

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Temperature tolerance is key to the spread of wasp spiders into northern Europe, according to scientists.

Since the 1930s the distinctive spiders have expanded their range from the Mediterranean coast to Norway.

Researchers in Germany traced the population boom to breeding between the native European spiders and an isolated colony living near the Black Sea.

Molecular Ecology reports the genetic mixing resulted in generations rapidly adapting to living in colder climates.

Wasp spiders (Argiope bruennichi) are commonly named for their bright, striped abdomens and were recently recorded by the Woodland Trust in Usk, south Wales for the first time.

The first official records of this conspicuous species in the UK were made in the 1920s.

Henrik Krehenwinkel from the Max Planck Institute for Evolutionary Biology, Germany, analysed the DNA of spiders caught across their current range, and museum specimens to understand more about their evolutionary history.

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Piecing together the genetic puzzle, he found that the spiders diverged after the last ice age: part of the population stayed on the Mediterranean while a colony headed east to Central Asia.

While these eastern populations adapted to live in climates as diverse as the tropical south of Japan and cold south-eastern Siberia, the spiders in the Mediterranean remained limited to warm areas.

But, according to the research, rising temperatures across the continent in the last century allowed the Mediterranean spiders to join up and breed with a previously isolated Black Sea population.

"This possibly restored genetic variation within a few generations and allowed for rapid adaptation," said Mr Krehenwinkel.

He theorised that the novel combination of genes resulted in new physical characteristics that helped spiders to survive in different environments.

Out in the cold

To test the whether these more northerly spiders adapted a different temperature tolerance than Mediterranean populations, the PhD student analysed how they reacted when moved into one another's habitats.

Southern spiders could not survive the freezing temperatures in the north, and their counterparts suffered from heat stress in the south.

Mr Krehenwinkel explained that the eastern population had adapted to cooler temperatures and this was passed on to European spiders in the population boom.

Wasp spider on a web Wasp spiders weave ornate webs

The result was the rapid adaptation of hardier offspring that could settle further north than their predecessors.

The spiders found in northern Europe have smaller bodies and are not seen in the coldest months of the year.

Scientists attribute both traits to seasonal changes which do not affect southern species. Spiders found in northern Europe "overwinter", meaning their young are buried during the coldest months; emerging in spring.

The spiders then have limited warm months in which they can mature, which restricts how large they can grow before they reproduce in the autumn and the cycle begins again.

Mr Krehenwinkel described the hatchlings as "highly dispersive", commenting that they can cover huge distances via a method known as "ballooning": riding the breeze on a special parachute made of gossamer silk threads.

"By aerial dispersal, little spiders can cover distances of several hundred kilometres," he told BBC Nature.

"Members of different genetic lineages can thus quickly track warming climate, which increases the likelihood of contact."

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