How climate change is knocking natural events wildly out of sync
Climate change is throwing off the timing of key events in the natural world, from the flowering of plants to the migrations of birds and mammals. Now, ecologists are warning that this could spiral out of control and cause whole ecosystems to break down
EVERY year in early spring, Japan goes blooming crazy. The first cherry blossoms open in Okinawa in the south in February and the spectacle reaches Tokyo a few weeks later. For the brief period when the trees are in bloom, people gather under their beautiful pink and white canopies for hanami, the traditional custom of flower viewing. It sounds genteel, but wild parties are known to break out.
Hanami has been taking place since the 8th century, but the historical records tell a curious story. For the best part of 1000 years, hanami in Tokyo and Kyoto reliably occurred in the second week of April. By the 1830s, however, it had begun to shift earlier. Last year, Kyoto recorded its earliest-ever full bloom, on 26 March.
The cause of this moveable feast is climate change. Cherry trees open their flowers in response to a few consecutive days of springtime warmth, which is arriving ever sooner. Early flowering brings the risk of a sudden frost, which can kill off the blooms – and the celebrations.
But what’s at play here is much more than an inconvenience for hanami-goers. Similar time shifts are occurring throughout the world with increasingly disruptive effects. “Timing is everything for ecosystem harmony,” says Maarten Kappelle at the UN Environment Programme (UNEP) in Nairobi, Kenya. Although these shifts have been apparent for years, Kappelle and others are warning that the disruption now threatens to completely break down ecosystems, leading to catastrophic losses of species and compromised food security. So how badly out of sync is nature – and can we do anything about it?
Natural historians have long been …
fascinated by the rhythms of life. In the 18th century, Carl Linnaeus, the botanist who devised the way we name and categorise species, kept diaries of when various trees unfurled their leaves, flowered and fruited. In 1853, Belgian botanist Charles Morren coined the term “phenology” for the study of recurring life phases. Around the same time, Henry David Thoreau made six years of detailed phenological records around his home town of Concord, Massachusetts, a labour of love later repeated by botanist Alfred Hosmer from 1888 to 1902.
It was partly these diligent diaries that first alerted modern biologists to the fact that Earth’s ecological clock was shifting. In 2003, biologist Richard Primack at Boston University, Massachusetts, working with Abraham Miller-Rushing at the US National Park Service, compared Concord’s historical data with their own observations and concluded that plants were flowering seven days earlier on average than they did 150 years ago.
Exactly why wasn’t hard to fathom. Meteorological records show that the mean temperature around Concord has risen by 2.4°C since Thoreau’s time, largely due to climate change and urbanisation. And, like cherry trees, many plants use rising temperature as a sign that spring has sprung and it is time to bloom.
Migration to moulting
Since Primack and Miller-Rushing’s early work, we have collected plenty more examples of shifting phenology, and concern among ecologists has come to a head. In February, UNEP published one of its influential Frontiers reports, which it uses to highlight what it calls “emerging issues of environmental concern”. For the first time, phenology is one of the worries the report singles out. It says that 200 species of plant and animal are known to have changed the timing of one or more of their life stages, on average bringing them forward by 2.8 days per decade. The list of species affected includes birds, mammals, insects, fish, crustaceans, molluscs, plants and plankton; the phenological events cover everything from breeding to pollination, migration, moulting and hibernation.
It isn’t just temperature rises that are causing trouble. Changing rainfall patterns are another cause of these shifts, especially in the tropics, where temperatures don’t change much throughout the year. Rainfall patterns are also shifting under the influence of climate change. The third important phenological cue, day length, hasn’t changed, but this often interacts with the others. For example, temperatures may only trigger an event once day length has crossed a certain threshold.
Phenological events are often synchronised between species. The classic example is a food chain comprised of the pied flycatcher (Ficedula hypoleuca), caterpillars and oak trees. Every spring, the birds produce large broods that eat vast quantities of caterpillars – the adults must deliver as many as 60 an hour over the 18 days it takes for the chicks to fledge. But caterpillars are an ephemeral resource, hatching to coincide with the emergence of oak foliage. The birds have thus evolved to breed so that their chicks hatch during maximum caterpillar abundance. The cue they take is temperature, which also precipitates leaf unfurling and caterpillar hatching.
This tightly coupled sequence is being disrupted by climate change. Even though all three events are triggered by rising temperatures, they are responding differently to warming. In some parts of Europe, birds are hatching too late to catch peak caterpillar, reducing the chicks’ chances of survival.
It is problems like these, known as phenological mismatches, that are bringing UNEP out in a cold sweat. We have long appreciated that phenological changes can spell trouble for individual species or pairs of species. But there is a dawning realisation that this is a widespread problem that could presage the breakdown of whole food chains or even ecosystems. “This is truly a global problem affecting plant and animal species in mountains, oceans, tropical and temperate forests and polar regions,” says Kappelle.
Quite how bad things could get, we aren’t sure. “Much of the work so far has been on pairwise interactions like predator-prey or pollinator-plant,” says Marcel Visser at the Netherlands Institute of Ecology in Wageningen, who wrote the phenology section of the UNEP Frontiers 2022 report. “There’s anecdotal evidence that shifts in phenology weaken relationships in a food-web context, but that is a question that very much needs answering.”
Mismatches are especially apparent in the Arctic, where the rate of warming is much faster than the global average. In Greenland, the annual migration of caribou to their summer feeding and calving grounds is triggered by day length, but when they arrive, the burst of nutritious shoots they once relied on has already been and gone because snow is melting earlier. As a result, their breeding success has declined by about 75 per cent.
In the Canadian Arctic, meanwhile, Ross’s geese and lesser snow geese hatchlings also miss out on peak vegetation, which has reduced their reproductive success. To add insult to injury, goose eggs have become an important food source for polar bears forced to leave disintegrating seal-hunting ice and repair to land ever earlier.
There are many other examples in the Arctic and, increasingly, elsewhere, and potential for more as the climate hots up. “We certainly see that there is huge capacity for ecological mismatch,” says Amanda Gallinat at the University of Wisconsin-Milwaukee. “It absolutely can happen that interacting species shift at different rates or in different directions. So yes, I think it is a danger to biodiversity.”
It is often not so much that phenological mismatches alone drive biodiversity loss, she says, rather that they exert extra pressure on an already stressed system. One strand of Gallinat’s research concerns North American birds flying south for the winter. In a 2020 study, she found that warming caused the birds to set off later, but made the fruits they rely on along the way ripen earlier, robbing the birds of vital calories – and also depriving the fruit trees of seed dispersal services. Lots of birds die during these migrations at the best of times because it is such a long, demanding journey. The birds are also under pressure from habitat loss, which remains the leading cause of biodiversity loss worldwide. Ecological mismatch could be the final straw.
Well, in theory. Despite decades of study, our knowledge of mismatches and their impacts is still rudimentary, says Gallinat. “There’s enough information for us to know that it is a danger, in part because it has potential impacts that can ripple throughout ecological communities,” she says. “But the extent to which those ripples have been demonstrated is still so limited. There’s a lot left to learn.”
To that end, she and others, including Primack, are developing a new discipline called macrophenology to integrate all of the local and species-level findings into a global understanding of phenological change. The idea is to use new analytical tools and data sources, such as remote sensing and citizen science, to investigate the impact on bigger scales. “There’s so much value to local-scale studies,” says Gallinat. “But if we aren’t able to connect them to one another and identify some patterns or how transferable a single result is from one place to another, then it really limits our ability to predict what might happen next.”
Beyond the Arctic
We can make educated guesses about what will happen next. If the fast-warming Arctic is a guide to our future, phenological mismatches are going to become another major driver of nature loss. Aside from reversing climate change, it is difficult to see what to do about it. “It is really tough to know how best to respond to an ecological mismatch once we’ve identified it,” says Gallinat.
It is, however, possible that the Arctic isn’t a canary in a coal mine. Ecological mismatches identified thus far tend to involve specialist species heavily reliant on a single food source, and such interactions are unusually common in the Arctic. It may be that the majority of the world’s food webs aren’t like this and have sufficient redundancy to absorb the blow. “If one resource comes out of sync, a more generalised consumer might shift their diet to consume more of something else,” says Gallinat. We may even be able to engineer this by, say, planting later-ripening fruits along bird migration routes.
Another defence, says Kappelle, is to give nature a hand by conserving large populations of wild species in order to maintain high levels of genetic diversity. Evolution may then ride to the rescue. Natural selection can sometimes generate new adaptations in the space of a few generations. Great tits have had a similar problem to pied flycatchers in that they hatch too early to catch peak caterpillar. But some populations of great tit in the UK, Czech Republic and Belgium have already reset the timing of their brood to get back in sync with the caterpillar boom, probably through adaptation, says Visser.
But we cannot rely on this. Phenological mechanisms often involve multiple interacting genes and such systems are slow to evolve. On top of that, climate change is outpacing anything that evolution can achieve. It is happening so fast that “many plant and animal species are not able to adapt in a timely manner”, says Kappelle. “In the end, we have to recognise that the only way to effectively reduce the negative effect of mismatched shifts of ecological events worldwide is to rapidly reduce carbon emissions and reduce climate change.”
On the plus side, says Visser, nature’s shifting rhythms are a powerful reminder that we have to get on with that. Having to mow the lawn earlier each year is the kind of change people notice. “Their response is: ‘that’s not natural’. They feel that things are getting out of balance.” Time, indeed, for a change.
In time for dinner?
In nature, key events in the lives of animals and plants need to synchronise, so that, for example, caribou migrations coincide with the sprouting of new shoots in spring. When events like this don’t hit the same beat, it is called a phenological mismatch (see main story). This may soon pose a danger to humans directly – and the threat is to our dinner plates.
Many staple crops use natural cues such as temperature and day length to decide when to germinate, fruit and set seed, but climate change is making their biological clocks go haywire. Shifts have been observed in cereals, soya beans, cotton and fruit, often affecting quality and yield.
Fruit trees are particularly vulnerable, says Marcel Visser at the Netherlands Institute of Ecology in Wageningen. Flowering is cued by rising temperature, which happens increasingly early, but makes the blossoms vulnerable to sudden frosts. “This year in the Netherlands, it was really warm towards the end of February, all the fruit trees were blossoming and then we got a cold spell and that led to lots of damage.” Many fruits also rely on pollinators, which are experiencing their own phenological shifts.
These agricultural changes aren’t yet a major threat to food security, says Visser. But according to the UN Environment Programme, they are already complicating efforts to adapt food production to climate change.
Fisheries are also floundering phenologically. Stocks are typically exploited when fish populations are at a seasonal peak, but many are shifting their spawning season, creating phenological mismatches between predator (in this case humans) and prey. Alaska pollock – commonly encountered inside the bun of a McDonald’s Filet-O-Fish – are spawning early and missing out on plankton blooms. Off southern California, more than half of commercially important species are spawning too early or late to fully exploit their food supplies. And in 2012, a marine heatwave in the North Atlantic caused lobsters to migrate inshore earlier than normal, leading to a record catch that, paradoxically, almost destroyed the industry as supply vastly outstripped demand and prices collapsed.