Widespread extinctions and rapidly diminishing populations are probably what comes to mind when most of us think about the nature of biodiversity change in the 21st century.
This is not surprising. The influence of modern humans on the planet is vast. The growing demand for food, timber, and fuel increasingly erodes and degrades once pristine habitats. Decades of greenhouse gas emissions are causing global temperatures to steadily rise. Catastrophic weather events that used to be once-in-a-generation are becoming the new norm. The magnitude and rate of these changes have even led some scientists to propose a new geological epoch: the Anthropocene.
It is only to be expected that these accelerating impacts will be mirrored by plunging biodiversity trends. Some have estimated that species are being lost at more than one hundred times the natural rate. We may even be in the midst of the sixth mass extinction event.
But despite all this, the response of wildlife to human pressures is by no means universally negative.
Major trunk roads and motorways typically represent a threat to biodiversity. But for some enterprising plants, these have offered an opportunity. The petite white flowers of Cochlearia danica (Danish scurvy-grass) were a sight formerly restricted to Britain’s shorelines. This is a plant that thrives on high salinity (a halophyte). The annual spreading of rock salt has created an ideal novel habitat for this species along main roads, where the turbulence of traffic has aided the dispersal of its seeds.
A much more extensive ‘novel habitat’ is forestry plantations. The post-war demand for domestic timber has led to conifer plantations making up much of the UK’s forest cover. These often consist of non-native coniferous monocultures, which generally don’t support the wealth of life found in semi-natural woodland. Nonetheless, they can offer ideal habitat for opportunists. Some of these species appear to be on the rise because of UK forestry policy. Examples include siskin and coal tit, several moth species (more on those later), as well as certain wood ants.
The most extreme Anthropocene opportunists are found closer to home. Cities are about as far away from a pristine ecosystem as it’s possible to get. Despite this, some species thrive in our concrete jungles.
Taking advantage of the ledges on tall buildings and gorging on feral pigeons, urban peregrine populations are soaring. Our cities seemingly represent superior habitat compared to their traditional haunt of rocky cliffs. Similarly, certain bumblebees are actually doing better in urban areas. The highly successful tree bumblebee appears to be on the rise partly because it’s able to nest in man-made structures. Perhaps the most familiar urbanite is the fox, which has adjusted to the hustle and bustle of our cities remarkably well.
Meanwhile, other wildlife appears to be on the rise because of climate change. British butterflies are generally thought to be benefitting from warmer temperatures. Examples include the Comma, Speckled Wood and Brown Argus, which have all increased their British range over recent decades.
Indeed, a study of over British 3000 species suggested that climate change will present an opportunity for more taxa in terms of future range change than it will a threat.
There is need for caution when interpreting these predictions. Even species that are expanding their distribution, may not necessarily be doing well overall. A species could increase its range (due to warmer temperatures) but decline in abundance (due to unrelated factors, or other aspects of climate change, such as extreme weather events).
A neglected topic
Fascinated by the idea of certain species becoming more common, I decided to take a closer look at success among British moths. I focussed on 51 moths that were increasing in both their abundance and their occupancy (a measure of range) to be sure they constituted true ‘winners’.
These 51 successful moths are rather diverse in terms of their ecologies and life histories. The only thing this group of moths seem to have in common is that, well, they are all becoming more common.
Conservationists often assume that it is the habitat generalists that increase, while specialists decline. But for British moths, at least, there is limited evidence of this so far.
Some of these successful moths are indeed generalists: feeding on a wide range of foodplants and occurring across almost all habitats. But several are restricted to a single foodplant, or typically occur in fragmented habitats (such as mature woodland). The level of habitat generalism for this select group of 51 winners was no higher than for the pool of 330 species from which they were selected (which includes many declining species).
Most of the winners didn’t follow simple paths. Some remained stable for decades until suddenly increasing but then proceeding to decline just as quickly. In a handful of species, there was even evidence of trend reversal. It may be that the success of some of these winners will be short-lived.
This illustrates just how vital it is that we have long-term monitoring of our wildlife. Ideally, this should be able to detect changes in multiple aspects of rarity, e.g. abundance and range.
This will also allow us to begin to understand why certain species are on the rise.
Cleaner air and more conifers
The most well-documented group of winners amongst British moths are those with lichen-eating caterpillars.
Lichens are textbook indicators of air quality, as they respond strongly to pollution. Few lichens can survive in air with filled with sulphur dioxide (a gas produced when coal is burnt), for instance. Since industrial times, Britain’s air has become significantly cleaner and lichens began to grow on bare tree trunks once more. This has had knock-on positive effects for those moths that eat lichens, such as the footmen. It is reassuring that policy interventions, such as the Clean Air Act, can lead to such pronounced and rapid recovery of our wildlife.
Another group of moths that are widely thought to have become more common are those that are associated with coniferous trees.
As mentioned earlier, forestry plantations are an obvious novel habitat. These represent large expanses of juicy real estate for those moths that can live there. Many species, originally adapted to feeding on native conifers, have had little problem switching to exotic pines. Indeed, most of the widely planted exotic conifers now support more moth species than Yew (one of Britain’s three native conifers, which is toxic to most animals).
It’s not just forestry policy that has benefitted conifer-loving moths. Britain’s gardeners and urban planners have also played a part.
Ornamental cypresses have become omnipresent. They are fast-growing and are opaque all year round, making them perfect privacy screens. This has provided an opportunity for a small number of moths that feed on cypresses (many of which only colonised Britain during the 20th century). The success of Blair’s Shoulder-knot is unrivalled in this category. The species colonised the Isle of Wight in 1951 and has since spread through urban and suburban areas, moving an impressive 120km per decade. Its caterpillars can now be found munching away in JK Rowling’s infamous leylandii hedge in Edinburgh no doubt.
However, these neat success stories are not the whole picture. Only around ten of the 51 successful moths I looked at feed on either conifers or lichens. A few more species only colonised Britain in the last couple of centuries.
But the positive trends in the remaining 38 species remain something of a mystery.
More cryptic causes of success
Climate change is almost certainly involved in many of the positive trends seen in some British moths (it’s also probably responsible for the negative trends in some others). Why only certain species appear to be ‘climate winners’, remains poorly understood, however.
Climate change appears to be driving more frequent influxes of moths from continental Europe. As a result, new species are increasingly arriving on Britain’ shores. It may be that these influxes are also bringing new genetic diversity of resident species, including races that are better adapted to contemporary conditions.
Given the short generation time of insects, it’s also possible that some species are evolving, with these changes driving their newfound success. Some species may be adapting to exploit warmer temperatures more effectively, or modifying dispersal ability to cope with a fragmented countryside.
Some species may simply be becoming more generalist.
Such shifts are typically hard to identify. There is some anecdotal evidence of this in British moths. For instance, the two lichen-feeders, Scarce Footman (now erroneously named) and Dingy Footman, were said to be quite uncommon and restricted to certain habitats (heathland and fens/marshes, respectively) by Richard South in his 1907 field guide. Both species are now found in a wide range of habitats, including gardens. Perhaps a boom in the population size of these moths (thanks to clearer air) enabled them to breed in a wider range of habitats. Of course, it’s equally possible that his early classifications were simply wrong.
If you’re interested in reading more about successful British moths, the study was published in Frontiers of Biogeography and is available to read for free. You can also check out this Twitter thread which explains the key points. The study only really begins to scratch the surface. There’s so much we still have to learn about our Anthropocene winners.
Successful species are the future
Conservationists have understandably focused on declining wildlife. But I feel there is something to be gained from paying more attention to those species that are bucking the trend. We know so little about how some species are thriving in the face of unprecedented anthropogenic pressures.
There can be a tendency to assume that species thriving in an era of human pressures are undesirable. “Pests” or “weeds” perhaps. And that the positive trends shown by these species are in some way ‘unnatural’, and therefore, a bad thing.
I think this is short-sighted. Biological success should be investigated, not ignored.
It is these species that will become increasingly important for maintaining the healthy functioning of our planet. After all, the ecosystems of the future will be largely made up of the winners. The secrets of their success may hold some of the answers for mitigating the impacts of the biodiversity crisis.