While we’ve all been arguing about carbon emissions, renewable energy and, indeed, whether or not global warming is even taking place, the natural world has been quietly getting on with the business of adapting as best it can to the changing climate.
Numerous aspects of the biology and ecology of animal and plant species are tied to temperature, from growth and reproduction to the timing of migration and even basic survival. So it’s little surprise that an increasing number of biologists are uncovering evidence that for vast numbers of species, climate change is already a fact of life.
The study of the timing of natural seasonal events – such as the first snowdrop of spring, or the first returning migratory bird – and how they vary over time and between locations is known as phenology. With data collected by a combination of gentleman naturalists and a series of mass volunteer programmes, the Nature’s Calendar programme, run by the Woodland Trust, is one of the world’s most detailed phenological repositories, with records that date as far back as 1736.
These records show a very clear signal, according to the programme’s manager, Dr Kate Lewthwaite. ‘To put it really simply, spring’s coming earlier, on average,’ she says. ‘This is particularly notable with very temperature-sensitive events that you see in early spring, such as bulbs coming into flower and insect activity.’
Autumn, on the other hand, has become what Lewthwaite calls ‘a season of two halves’. ‘What you might call autumn fruiting – when you see the first blackberry, hawthorn berry, rowan berry – that’s coming earlier because of the connection between flowering and fruiting,’ she says. ‘But the late-autumn events, such as leaves turning colour and leaves falling, are coming quite a bit later. So the effect overall is that we have an extension of the growing season at both ends.’
Looking in more detail at the data collected since 2000 – when Nature’s Calendar began its current programme of mass volunteer data collection on 88 spring events and 63 autumn events, covering a wide variety of trees, shrubs, flowers, birds, insects and amphibians – it’s possible to see a number of distinct changes. ‘For me, one of the most striking shifts we’ve seen is that autumn fruiting seems to be advancing quite dramatically,’ Lewthwaite explains. ‘Even over just 12 years of data, rowan has advanced its fruiting date by a month. Spring insect activity is also advancing – on average, across all of the groups we record, by three weeks, which is astonishing, while plants are advancing their spring behaviour by between ten days and two weeks.’
THE SAME BUT DIFFERENT
Birds have also shifted the timing of their behaviour in response to climate change, most noticeably in the timing of their migration. British birds are extremely well monitored, mostly by volunteers, explains Dr Jenny Gill of the University of East Anglia. ‘We have fantastic armies of these people, who generate really powerful long-term data sets,’ she says. ‘And what they show, across a large range of species, is changes in the timing of migration. Lots of species are advancing their migration – getting earlier. Very, very few, if any, are getting later. But the extent to which those advances are occurring is varying quite a lot between species. The link to climate change is much, much more difficult to establish conclusively, but there are very strong correlates with climatic change.’
A recent study in which Gill took part has helped to elucidate the mechanism that’s allowing the timing to change. ‘What we see in our Icelandic godwits is that individual birds are unbelievably consistent in the timing of their migration – to within a few days,’ she says. ‘And lots of other studies are showing the same thing. Which begs the question – how do you get population-level changes if individual birds are doing the same thing every year?’
The answer, she explains, is that the young birds coming into the population must be doing things differently to their predecessors. ‘What’s changing is the timing of breeding,’ she continues. ‘They come back at the same time, but often they nest earlier in a warm year. There’s a gap between arrival from migration and nesting, and the size of that gap can vary hugely, depending on the weather conditions. That, we think, is the direct impact of climate change – in a warm year, they can nest early. That means that their chicks hatch early, and we think that there are cascading effects from hatching early that end with them being early arrivers.’
Gill suggests that it may be that the warmer weather is leading to earlier vegetation growth. The birds conceal their nests in tall grass and in warm years, they don’t have to wait as long for the grass to get long enough for them to begin nesting. It could also be related to the timing of insect activity, which will affect how quickly they feed and assimilating the energy they need to reproduce. ‘Those changes to what happens to them in the breeding season, facilitated by climatic change, can have these knock-on consequences to the whole migratory cycle,’ she says.
None of which is necessarily a bad thing, but scientists are concerned because populations of the species that aren’t advancing their migration as much are declining. ‘One of the other patterns we see in Iceland is that the species that are advancing their migration less are the ones that are travelling farthest south,’ Gill explains. ‘They also arrive back later and they have a smaller gap between arrival and laying. They simply don’t have any capacity to advance their laying. That may well be why they’re not advancing their migration. Weather conditions in Iceland are changing but they’re not able to respond to that, and that may be related to some of these population declines that we’re seeing.’
The distributions of many plant and animal species are determined by temperature, and so, as the temperature has gradually risen, many species have exhibited a shift in their ranges – primarily involving movement into areas that were previously too cold, either to higher altitude or higher latitude.
Examples have been coming in thick and fast, particularly from North America. A 2009 US Forest Service study of 40 major tree species in 30 eastern states concluded that the trees’ ranges had moved, on average, about 100 kilometres north in less than a century. Similarly, more than 60 per cent of the birds that the US National Wildlife Federation tracked in a recent study had expanded their range northward by an average of 56 kilometres in the past 40 years. And in the Sierra Nevada Mountains, 14 small mammal species were found to have extended the elevation at which they can survive by an average of 500 metres.
A meta-analysis of data from studies such as these, comprising more than 2,000 animal and plant species, was published in Science in 2011. It found that over the past 40 years, on average, the species had moved to higher elevations at 12.2 metres per decade and, more dramatically, to higher latitudes at 17.6 kilometres per decade.
BUTTERFLIES AS BAROMETERS
In the UK, one of the groups that has shown dramatic shifts in distribution is the butterflies. ‘Butterflies are very sensitive to environmental change, and we have lots of data on them because they’re quite charismatic,’ says Dr Tom Oliver of the Centre for Ecology & Hydrology in Wallingford. Indeed, according to Richard Fox, the surveys manager at Butterfly Conservation UK, ‘British butterflies are probably the best-monitored invertebrates in the world.’
These facts, combined with their short generation times, mobility and high habitat specificity, make them good barometers of climate change. ‘Everything about butterflies is very closely entwined with climate and, on a short-term level, the weather; everything about their lives is governed by temperature,’ says Fox.
And this has been reflected in their changing distributions. ‘Lots of species are shifting their ranges northwards,’ says Oliver. ‘They’re limited at the northern end of their range by a number of aspects, such as summer temperatures. As the conditions have warmed over the recent decades, the species have shifted northwards.’
And according to Fox: ‘Many of those that have spread like that have also increased their abundance, which is also likely to be related to climate. For example, the brown argus has gone from being a fairly restricted butterfly to something that’s turning up in farm fields all over the general countryside.’
And as with other animal groups, some species have also changed their phenology. ‘Butterflies are tending to emerge earlier and to fly later into the year,’ says Fox. ‘So their flight period – the length of time that they’re around as adult butterflies – has tended to increase. A few species have taken it a step further and started to produce additional generations during the year as well.’
At a finer scale, some butterfly species are even beginning to alter their ecological requirements. ‘The silver-spotted skipper, for example, was highly threatened and restricted to unimproved chalk grasslands in the southeast of England,’ Fox explains. ‘But in the past 20 years or so, it has improved the size of its range and the number of populations has also increased quite a lot.
‘This is a butterfly that’s very dependent on high temperatures,’ he continues. ‘The females will only lay eggs when the temperature is above about 25°C. Back in the 1980s, they would only lay eggs in the hottest parts of the nature reserves in which they were found. They would be living on the south-facing slopes – the slopes that get the most sunshine – and they would only live there if the grass was grazed incredibly short.’
However, as the surrounding climate has warmed, the butterflies have begun to move out and colonise other areas. ‘They’re increasingly found on east- and west- and even north-facing slopes, and the females are willing to lay their eggs – and the caterpillars survive – in longer turf,’ Fox concludes.
While this may all sound rather benign – species just moving about to new areas as the temperature warms – there are good reasons to believe that the situation is more problematic. ‘In today’s world, habitats are so fragmented that that process is becoming more difficult for species,’ explains Oliver. ‘They’re really hindered in their ability to respond to climate change through shifts in distribution.’
For species that live at high altitudes, the problem is even more acute. All over the world, there are species that are confined to the peaks of mountains. These are typically species whose ranges have contracted as the Earth came out of the last ice age, slowly shifting uphill as the climate has warmed until they’ve gone as high as they can go. Because their mountain homes are separated by areas of unsuitable habitat, it’s difficult for them to move to find new, more suitable homes. So, as it warms further, these species will eventually run out of suitable living space and disappear forever.
And then there are creatures that find it difficult to move at all. Many coral reefs have been growing in the same locations for tens of thousands of years or even longer. And many coral species are extremely sensitive to changes in water temperature. When it gets too hot for them, the individual coral polyps expel the symbiotic photosynthetic algae that live in their tissues. While many can and do eventually recover, many colonies die as a result.
The most extensive and devastating example of coral bleaching in recorded history took place in 1998, when an extraordinarily strong El Niño event raised water temperatures well above normal. Every coral reef region in the world was affected by bleaching, with the phenomenon being reported in 60 countries.
Indian Ocean corals were particularly badly affected, with more than 70 per cent mortality reported in some countries. Overall, about 16 per cent of the world’s coral was killed.
Anecdotal reports suggest that bleaching is becoming both more frequent and more widespread. When I spoke to Lyle Vail, director of the research station on Lizard Island on the Great Barrier Reef, in 2011, he offered a bleak perspective. ‘We’ve been here 21 years, and in the early days, summer was always the time you pined for, because it’s warm and all that,’ he said. ‘But now, you look forward to it with trepidation, because you know that some bleaching is going to occur – it’s happening almost every year now.’
There is, however, a glimmer of hope here, with some evidence of corals shifting their ranges in response to climate change. A study of temperate reefs in Japan, published in Geophysical Research Letters in 2011, found that four major coral species categories, including two species that are key for reef formation in tropical areas, had shown poleward range expansions since the 1930s.
And there’s further concern because in the natural world, everything’s connected. Species don’t exist in isolation – they’re all tied up into complex food webs. This means that as species alter their behaviour and/or their ranges, it can set off a series of cascading effects.
Since the turn of the millennium, a range of seabirds, including puffins, guillemots and kittiwakes, have had a run of poor breeding seasons, particularly in Scotland and especially in 2004 and 2005. Colonies are producing fewer young, primarily because of a lack of food for both adults and young.
The preferred prey for many of these species is the common sand eel. As the relatively shallow North Sea has heated up, it has changed the phenology and the distribution of the local phytoplankton and zooplankton. In warmer years, the cold-water zooplankton on which the sand eels feed are replaced by less nutritious warm-water varieties, causing a decline in sand eels, which means less food for the birds.
Meanwhile, in North America, forests are being devastated because of the changing dynamics of a pest species. Native to the forests of the continent’s western regions, the mountain pine beetle has recently extended both its latitudinal range and its altitudinal range. During the 1970s, the beetle was only found at altitudes of less than 3,000 metres; today, it can be found at 3,350 metres and higher. It has spread into northern British Columbia and eastward in the boreal forest of north-central Alberta.
When outbreaks occur, it can lead to the loss of millions of trees. The area of forest devastated in the past decade or so is ten times that of any previous epidemic recorded, and it isn’t over. Overall, the damage stretches from California across the prairies to the east, and from New Mexico to northern Canada. In British Columbia, which saw its first outbreak
in 1990, the insect has killed about half of the total volume of commercial lodgepole pine. It has also killed virtually all of the mature lodgepole trees in northern Colorado and southern Wyoming.
The reason it has been able to spread is that the winters have been getting warmer. In the past, cold winter temperatures would kill most of the beetles, restricting their spread. But now, winter temperatures often aren’t cold enough to kill them, and in some areas, the beetles are beginning to produce two generations per year, drastically increasing the damage.
It’s clear that there’s no room for complacency. As the world warms still further, more and more species will be pushed out of their current ranges, with unpredictable consequences. ‘Climate warming is only one aspect of climate change and for warmth-loving insects, it might be beneficial, but we only have quite a small window of opportunity because eventually, summers will become too dry and too hot,’ Oliver says. ‘In the next decade or so, species have the opportunity to expand with the warming, but they’re being hindered by the way we manage the landscape, the degree of intensive farming we have. And eventually, climate change will start affecting species in negative ways.’
Gill offers the same prognosis. ‘For some of the [bird] species that we’ve been working on, it’s quite positive at the moment,’ she says. ‘but very rapidly, we’re going to end up in a situation where changing ecological patterns will start to have an impact on those populations.’
This story was published in the March 2014 edition of Geographical Magazine