Global Warming

Climate Change is disrupting the ‘birds and the bees’

Over the last two decades, scientists have found that warmer temperatures are quietly spoiling the mood, making it harder for plants and animals to reproduce.

While humans and many other animals determine sex genetically, many reptiles and some fish use the incubation temperature of the eggs to set the gender of their offspring. This means that changing global temperatures could alter the ratio of sexes produced, making it harder for these animals to find mates.

Eastern three-lined skunk females can partially compensate for temperature increases by digging deeper nests and laying earlier in the season. Nevertheless, according to a study published in 2009, their nests still warmed by 1.5C over 10 years. This shifted the sex ratio towards females.

Not every species is as badly affected. Australian water dragon females have been shown to buffer temperature differences of 4C by nesting in sunnier or shadier locations. When it comes to climate change, behavioural flexibility is often a big advantage.

In the plant world, temperature can influence sex ratios in more subtle ways. For example, the tobacco root plant, which lives in alpine meadows in North America, has been producing ever more male plants over the last 40 years. This may be due to reduced water availability, since females require more water to develop. So far, the extra males have actually boosted seed production, but if the trend continues, the lack of females could eventually leave male tobacco roots feeling a little lonely.

Meanwhile, the majority of the world’s sea turtles use temperature to set the sex of their offspring. “Embryos are laid with no gender,” says Graeme Hays of Deakin University in Australia. “They can develop into males or females.” Warmer eggs develop into females, cooler eggs into males. “Temperature during the middle third of incubation controls the sex,” says Hays, by switching on and off the genes that trigger development into either a male or a female.

The difference between male and female is just a couple of degrees. That means even slight changes in the climate could skew the sex ratio, making males harder to find. Incubation temperatures above 29C are predicted to produce increasingly female-biased clutches.

There is evidence that this shift is already underway. Over the last century, the sex ratios of green turtles, hawksbill turtles and leatherback turtles have become increasingly sex-biased. By 2030, the percentage of male green turtles produced has been predicted to drop to just 2.4%. “All else being equal, a warming climate will produce more female sea turtle hatchlings,” says Hays.

One solution for sea turtles and other reptiles with temperature-based sex determination is to breed at different times of year. This is called a “phenological shift”. “Earlier nesting may mean that sea turtles avoid incubation conditions that are too hot,” says Hays.

Phenological shifts are common, because many animals use environmental cues like temperature and rainfall to time key events like migration, flowering and breeding. Climate change is changing the timing and strength of the seasons, and as these cues change, the annual ebb and flow of the natural world is being disrupted.

In fact, one of the first pieces of evidence for the effect of climate change on living things was the discovery that plants are flowering earlier and earlier each year. In 2002, a landmark study showed that 385 British plant species were flowering on average 4.5 days earlier than in the 1990s. The same story is playing out across the globe: a 2008 meta-analysis looked at 650 temperate plant species in Europe, Asia and North America, and found that spring flowering had advanced by 1.9 days per decade on average.

The biggest concern is that plants and their pollinators might respond differently to climate change, leading to a mismatch that could significantly affect plant reproduction. For instance, in Japan the flowering of the plant Corydalis ambigua has advanced faster than the emergence of its bumblebee pollinators, resulting in a mismatch that reduces seed production in years with an early spring. Such mismatches could have a major impact on certain crop plants.