Love them or loathe them, we need insects, yet their numbers are decreasing. From declining monarch butterflies in North America to disappearing bumblebees in Europe, there is mounting evidence that insects are in rapid decline. This should worry us all, for insects are overwhelmingly important; they are food for innumerable larger creatures such as birds and bats, they control pests, recycle nutrients, help to keep the soil healthy and they pollinate three-quarters of the crops we grow. Without insects, life as we know it would grind to a halt.
The causes of insect declines are numerous, with habitat loss and the industrialization of global farming leading the way, assisted by growing use of pesticides, the depredations of invasive species, increasing light pollution, and more. Populations of many insects are now much reduced compared to the past, and most now exist in habitat “islands” — fragments of their favored habitat surrounded by inhospitable terrain.
Sadly, climate change is likely to be the final straw for some. Of course, the climate has changed in the past, many times, and insects have survived. Indeed, insects have come through all five of the previous mass extinction events on our planet (though many individual species must have been lost). However, this time is different. In the past, climate change was usually much more gradual, and species could gradually shift their ranges in response, shifting towards the poles as the climate warmed and returning towards the equator as it cooled. Such movement used to be easy, for there were vast tracts of suitable habitat. Today, to move pole-wards, insects must somehow hop from one patch of habitat to the next, crossing roads, landscapes filled with polluting factories, arable fields sprayed with insecticides, housing estates, and other alien, human-made landscapes.
Insects do not live in isolation, but depend upon particular plants to provide food for their larvae, or nectar and pollen for adults. If these plants haven’t travelled ahead of them — for example via seeds blown on the wild — then they cannot survive, so entire communities of organisms need to make this journey. It seems that they are failing to do so. Studies of bumblebees in North America and Europe have revealed that they are disappearing from the southern edges of their ranges as the climate becomes too warm for them, but the northern edges of their ranges have not moved northwards as we might expect. It has been described as a climate “vice,” the ranges of the bees being gradually squashed from the south. Some mountain-dwelling bumblebees in Colorado and Spain have responded to climate warming by moving higher in the mountains, but eventually they will run out of mountain and be left with nowhere else to go.
Studies of these mountain bees have also revealed that climate change may disrupt the seasons in subtle but important ways. Many insects time their life cycle carefully to ensure that times of activity coincide with availability of a vital resource. As the climate warms, different organisms may use different cues to time their life cycle, leading to mismatches. For example, some mountain plants in Colorado are now coming into flower before the bumblebees that feed on them have emerged from hibernation, meaning that the plants are not pollinated and set few seed.
Bumblebees seem to be particularly sensitive to warming as they are creatures found primarily in cool, temperate climates. Their large size and furry coats are adaptations to keeping warm in cool conditions, and they literally overheat in hot weather. Above about 30 degrees Celsius (30oC), most bumblebee species seem unable to maintain prolonged activity. The recent heat waves that afflicted much of Europe’s temperatures (even in the U.K.) exceeded 40oC, leaving bumblebees unable to gather food for extended periods.
Are bumblebees an exception? Many other insects are more “thermophilic” — warmth loving — and thrive in hot climates. For example, many butterflies are at the northern edge of their range in the U.K., so we might expect them to benefit from a warmer climate. To test this, the charity Butterfly Conservation analyzed changes in the populations of 46 butterfly species that all reach the northern edge of their range in the U.K. — the species we would expect to be enjoying warming. Between 1970 and 2000, three-quarters of these species declined significantly. The pattern differs between sedentary habitat specialists (fussy species with very specific requirements and low mobility, comprising 28 species) and the generalist, highly mobile species (18 species). Of the habitat specialists, 89 percent had declined, while only half of the generalists had declined and a few, including pest species, were thriving. This gives us a clue as to why climate warming has so far not benefited even warmth-loving butterflies. Mobile generalists can more easily move in response to warming, and are more likely to find somewhere where they can survive when they get there.
Of course, climate change is not simply a matter of slight increases in temperature. Perhaps more impactful on wildlife is the increased frequency of extreme weather events, such as droughts, heat waves, wildfires, storms and floods, all of which are likely to become more frequent and more extreme in the future. We have little idea what impact these will have on insects, but of course very few of them will be positive. Fires will obviously kill insects, although the flush of new flowers that follows fires in some ecosystems would benefit some. Summer storms are likely to batter delicate adult insects such as butterflies, and flash floods are likely to destroy underground nests of creatures such as bumblebees. Drought causes water-stressed plants to cease nectar production in their flowers, which will certainly harm pollinators. In prolonged droughts, plants wilt and become unpalatable for caterpillars — for example, in the hot British summer of 1976, many caterpillars of the Adonis blue butterfly died as their food plant, horseshoe vetch, shriveled in the heat. As a result, numbers of the adults were much lower the following year, and some populations died out. With the U.K. now experiencing the worst drought since 1976, it is a fair bet that 2023 will be an especially poor year for butterflies.
Although climate change is undoubtedly bad news for many insects, there is no doubt that some are thriving. Ironically, these tend to be undesirables, from a human perspective. Adaptable, mobile species that can thrive in urban landscapes, such as house flies and mosquitoes, are on the increase. For example, the yellow fever mosquito (Aedes aegypti) seems to have adapted well to urbanization and thrives in cities, breeding in blocked gutters, discarded tires, barrels, buckets, and any other human refuse that traps puddles of water. It is one of the main vectors of several nasty diseases, including dengue fever, chikungunya, Zika fever and of course yellow fever, as its name suggests.
The Anopheles mosquito, the main transmitter of malaria, is also benefiting from the spread of human activities. Cases of malaria tend to become more frequent in areas where forests are cleared for agriculture, because the mosquito likes to breed in sunlit puddles and ditches, which are hard for it to find in dense forest. Climate predictions suggest that malaria is likely to spread to higher-altitude regions of the tropics, for example in Colombia, Kenya and Ethiopia. These regions are densely populated with humans in part because, until recently, they were largely free from malaria. The southern states of the U.S., southeastern Europe, parts of China and the densely populated areas surrounding São Paulo and Rio de Janeiro in Brazil are all likely to become suitable for malaria by 2050. Dengue fever is similarly predicted to become far more common throughout North America, as far north as southern Canada.
There are no simple solutions to these problems. Clearly, we need to place tackling climate change as the single most urgent priority for humankind, both for the sake of the astonishing biodiversity on our planet and for our own wellbeing. Preserving as much nature-rich habitat as possible and attempting to link habitat patches together may help insects and other wildlife to cope in the meantime.
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