It was a good year for picnickers, but a bad one for sea swimmers: in the National Trust’s annual survey of how the weather and climate has affected Britain’s flora and fauna, jellyfish were judged to have flourished while wasps suffered a sharp decline.

A sunny winter was followed by a late spring, which meant summer migrant birds were late, as were the leaves on the hedges. A pretty wet and windy summer followed, but September and October came good, leading to prolonged flowering for many summer plants and a longer season for insects.

As so often in recent winters, violent storms – including Storm Desmond at the beginning of December – wreaked havoc on human infrastructure, but it is too early to assess the possible long-term damage to plants and wildlife.

Bracket fungi on a tree at Lydford Gorge, Devon. It was a disappointing autumn for fungi enthusiasts. Photograph: John Millar/National Trust

Matthew Oates, a National Trust nature and wildlife specialist, said he was most struck by the proliferation of jellyfish and the paucity of wasps. “We’ve seen unprecedented jellyfish invasions,” he said. “This may be due to overfishing and warming seas, which has led to huge plankton booms and reduced the number of predators.”

Some believe the idea of a jellyfish invasion is a media-driven “silly season” story. At the height of summer, naturalist Steve Backshall argued in the Guardian that it was not uncommon for “smacks” or “blooms” of jellyfish to become stranded on British shores. He also pointed out that the barrel jellyfish that appeared this summer only had a mild nettle-like sting.

More disturbingly, there was hardly a wasp to be seen this summer. “Wasps had another poor year, particularly in the south-west,” Oates said. “This represents a wider decline in our insect populations, thought to be a result of confounding weather alongside the possible effects of pesticides used in farming.

“We need to ask what’s happening to our wasps. Many might welcome their dwindling numbers, but the ecological world is a delicate one, and with our two species of common wasps incredibly scarce in many districts for the second consecutive year, we have to ask what impact this is having.”

The survey made a number of cheerful discoveries, including the appearance of the red-shanked carder bee at Birling Gap in Sussex, which is now part of a conservation management plan. The long-tailed blue butterfly, an extremely rare migrant, returned to the south-east, breeding again on the white cliffs of Dover.

Long-tailed blue butterfly. Photograph: Matthew Oates/National Trust

The good news

Conservation projects in places such as Malham Tarn and Upper Wharfedale in North Yorkshire helped the barn owl population flourish.

There were spectacular appearances of barrel jellyfish, particularly around the south-west of England and Wales. As climate change causes sea temperatures to rise and plankton blooms become bigger and last longer, there are likely to be more jellyfish appearing even further north.

A lack of stormy weather or frosts in early autumn ensured it was a fantastic year for autumn tints and produced a superb apple crop.

The elegant seabird the little tern enjoyed its best year on Blakeney Point in Norfolk since 2011.

There was another record-breaking year for breeding guillemots on the Farne Islands off Northumberland.

BBC Springwatch enthusiasts noted a huge increase in goldfinches in gardens.

The not so good news

