Opinion: new research finds that birds may fail to adapt fast enough to climate change thus increasing the odds of local extinction
Around the turn of the 21st century, frog species started to disappear mysteriously from cloud forests across Central and South America. The immediate finger was pointed towards a spreading fungal skin disease, but closer inspection revealed that warming local temperatures were making life easier for the fungus, spelling trouble for the frogs. As the lead researcher of the study, Alan Pounds, put it at the time, "disease is the bullet killing frogs, but climate change is pulling the trigger".
The frog study exemplifies the so-called attribution problem in climate change research: if a species goes extinct at the same time as climate is changing, can we pin the former on the latter? If an extreme heat wave occurs, can we attribute the event itself to climate change?
The problem is that many co-occurring phenomena could be the reason behind a given event and pure chance also comes in. We cannot easily (or ethically!) do experiments where we manipulate just the climate while keeping everything else constant, and then see what happens.
But one thing we can say for sure is that climate change is loading the dice, such that extreme heatwaves and species extinctions are becoming much more probable. If an athlete takes steroids and wins a race, did the steroids make them win it? We can never say for sure, as the athlete may have simply been in good form on the day, but clearly athletes on steroids will tend to win more races than clean athletes, all else being equal.
Which brings us to the birds. In a new study led by the ecologist Viktoriia Radchuk, which brought together a large team of international collaborators (myself included), data were collated from scores of long-term population studies of many different animal species, the majority of which were birds. The goal was to test for general patterns in how species are adapting to climate change, and four main findings emerged.
Firstly, animal species in temperate regions of the world are on average advancing their phenology, i.e. the timing of important life cycle events like breeding and migration, as their environments warm up. Secondly, these responses are a good thing in that they allow populations to partially track changing seasonal patterns wrought by global warming. For example, breeding earlier when temperatures rise can allow birds to remain in synchrony with an advancing seasonal peak in food supply (e.g. insects that are abundant for only a short window in summer). However, these behavioural adjustments are not always perfect and the birds may shift at a slower rate than their prey, limiting the number of young that can be successfully raised each year.
Indeed, our third result was that, on average, these populations are experiencing natural selection to breed or migrate earlier. This is essentially a Darwinian process, whereby the early bird not only catches the worm, but also produces more chicks who in turn survive better. If breeding time has a genetic basis, then early birds pass on their genes for early-breeding at a faster rate than late birds pass on their late-breeding genes. The population as a whole thereby undergoes adaptation.
This then led to an obvious question: can these populations adapt fast enough to cope with future climate change? This question, of course, can only be addressed by some sort of modelling, just as meteorologists cannot tell you with 100% certainty whether it will rain tomorrow. Rather, they give you a forecast – essentially an educated guess, backed up with tried-and-tested mathematical models of how weather works.
In our case, we used an evolutionary model that compared the observed climate change response of the population in question against a gold standard, "theoretically perfect response". Although it can sometimes proceed rapidly, Darwinian evolution is not without limits, especially if the climate changes too rapidly.
This led to our fourth and final finding. Across the 13 species for which we had sufficient data, the model forecasted that future changes in phenology will likely not be enough for these populations to cope with future warming. In other words, they may fail to adapt fast enough to the accelerating climate change that is coming down the tracks, increasing the odds of local extinction.
Technically, the term extinction means that the whole species dies out everywhere it was formerly found. Our study was focussed on single study populations in given places, e.g. a woodland where the local Great Tits or Blue Tits are monitored in nest-boxes provided by the researchers. As species, however, Great Tits and Blue Tits are found across most of Europe and into Asia and North Africa; thus the loss of local populations does not mean the loss of the entire species.