Changing the nature of the seasons

Photo: bluebell shoots, a harbinger of spring in the UK, by Alicia Hodson

Photo: bluebell shoots, a harbinger of spring in the UK, by Alicia Hodson

Across the seasons, animal migrations, periods of hibernation, and the metamorphic life cycles of insects play a vital role in ecosystem functioning, resulting in a cyclical transfer of nutrients and energy. They also strongly affect predator-prey relationships and spark excitement in animal enthusiasts. Like a reflex, mass migrations occur to keep a population in favourable conditions, so as one location becomes, for example, too warm or too cold for survival, the individuals of the population collectively move to a location which suits them better during a given time of year. In a way, for non-migratory insects, overwintering as an egg or dormant larvae occurs for the same purpose – to help an animal avoid life in unfavourable conditions. Debate surrounds how such mass movements and transitions may be repeated consistently every year, but it is believed to be due to their sensory systems responding to a number of environmental cues, possibly a combination of changing temperature, following wind directions, movements of the sun, polarised light, and in some cases of migration, employing the magnetic poles, or spatial memory for direction. Therefore you can probably imagine that abnormalities in the seasonal changes of the environment in temperate parts of the world, such as the UK, may largely affect the lives of many animal species. Consequently, along with the publicising of climate change, in recent years there has been a lot of news surrounding unusual appearances of several animal species, notably migratory birds and insects, whether or not such sightings are a cause for concern.

During the winter months, we have several avian visitors to the UK. In the beginning of January of this year, twitchers were taken by surprise when Wax Wings were spotted at Rayleigh Weir in Essex, a roundabout junction between main roads with several large stores and large car parks – not exactly a bird enthusiasts’ prime spot for a hide. These birds are winter visitors, residing in the UK between October and March, and they feed on the kind of berries that are abundant in this kind of environment. So, this is not a completely inexplicable sighting, however out of place it may seem. Additionally, Blackbirds sometimes lay eggs in January which may worry some people as the young have a slim chance of survival in the chill of British winter, but this is considered relatively normal and not due to abnormalities in the climate, rather it is believed to occur when young pairs are testing breeding boundaries.

On a more concerning note, on the 29th December 2016, a Clouded Yellow butterfly was spotted in Southbourne Undercliff, Bournemouth (credit to @cloudedyellows on twitter), one of the latest ever recorded. This species is known to be able to survive over winter as larvae in Bournemouth, but generally they are only a summer visitor north of the Alps, migrating from the north of Africa and southern Europe, and typically in flight in the UK between early April to early November. Therefore seeing an adult in flight in the UK during late December should be treated as an abnormality.

The most common explanation for seeing a butterfly in flight so late in the year is that it has been disturbed during hibernation, perhaps from physical agitation, or an unexpected rise in temperature. The species of butterfly known to consistently hibernate in the UK are the Brimstone, Peacock, Small Tortoiseshell, Large Tortoiseshell, and Comma. However, we are starting to see some additions to this list. Red Admirals are primarily migratory, originating in central Europe, but along with the Clouded Yellow, in recent years there have been adults successfully overwintering here in the U.K. The Red Admiral now overwinters in the south of England so frequently that it is considered resident. I myself was seeing Red Admirals in flight well into November and early December, notably on particularly cold days, and they were seen flying during a relatively warm period in January. Unfortunately, as is often the case, this warmth was followed by a very cold period, accompanied by snow, giving rise to a cause for concern. When insects emerge from hibernation too soon due to a brief period of unseasonably warm days, there’s a chance the weather will turn again, dropping to freezing temperatures and killing a lot of exposed insects – as it did earlier this year. The result is less adult insects survive the winter to emerge once temperatures become more suitable for them. In turn, this leads to fewer individuals being present in the spring and summer months to reproduce. Additionally, the usual emergence time of pollinating insects from hibernation tends to coincide with the blooming of flowers. Therefore if such an insect emerges too early in the year, it will not be able to find the food source it needs to replenish its energy levels following several weeks of dormancy.

So although it may be very uplifting to see a butterfly or a bee on a winter day, spotting an insect too late or too early in the year may be a sign of climate change, and has a detrimental knock-on effect on insect population sizes for an indefinite amount of time, leading to a loss of pollinators, decomposers, cleaners, and prey.

If you sight any unseasonable wildlife activity, be sure to record it with the national authorities, such as Butterfly Conservation, the RSPB, or the Royal Entomological Society.

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Alicia Hodson

Alicia Hodson

BSc (Hons) Zoology graduate from the University of Bristol, former long-term volunteer keeper for the Bug World department of Bristol Zoo, and currently doing voluntary conservation work with the Essex Wildlife Trust. Main passions in Entomology. Absolute naturalist.
Alicia Hodson

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