Some species struggle to adapt to climate change
Change is afoot. Climate patterns across the globe are shifting as the planet steadily warms. That climate change is occurring is largely a settled issue in the scientific community. The debate has now shifted to questions of degree. How much will climates change? How will climate change impact living creatures?
Ecosystems are already feeling the strain. Summers are hotter, droughts are drier, and ocean acidification is well under way. But, preliminary studies suggest some species can adapt, changing food sources or migratory patterns to survive in altered environments.
Many plants, for example, are taking advantage of mild winters and early spring thaws by growing and flowering earlier than usual. This shift of springtime growth has real and dramatic consequences for the herbivores that feed on these plants. Will they change their behaviors to adapt to an earlier arrival of their smörgåsbord?
A long-term study published recently in PLoS Biology sheds light on this question. French and British scientists collected nearly three decades of reproductive data on roe deer, an important European herbivore. Roe deer mothers give birth to their young in spring for one very important reason: mothers rely on the rich, nutritious, and easily digestible springtime diet of young plant shoots to help them produce enough milk for their hungry offspring. But, the window for this vernal bounty is narrow; mothers who give birth too soon or too late miss the fresh vegetation and have less milk for their fawns. Critically, these deprived offspring are less likely to survive beyond their first year.
The French and British team analyzed a well-studied population of roe deer from the Trois Fontaines, a forest preserve of over 1,300 hectares in France’s historic Champagne province. They had access to 27 years of demographic data for local roe deer, including the number of offspring born to each mother and fawn survival rates. Over that same time period, they also knew the timing of springtime forest growth (based on meticulous records of flowering dates for nearby vineyards).
Over the 27 year period (from 1985 to 2011), average springtime temperatures in the Trois Fontaines increased by more than 2.5°F. As a direct consequence, the fresh shoots of the season arrived two weeks earlier in 2011 than they did in 1985. But, amazingly, the roe deer in the preserve did not shift the timing of reproduction over the same period. As a result, mothers missed out on the rich diet of young forest shoots, and produced less milk for their offspring; fewer roe deer fawns survived their first year.
This worrisome trend dented roe deer population growth in the Trois Fontaines. In the long term, as temperatures continue to rise, the gap between springtime growth and roe deer births should widen further. This may dampen the roe deer population, depriving a complex forest ecosystem of a critical herbivore.
The findings of this study stand in stark contrast to observations from other animal species. For example, many birds have adjusted breeding times and locations to cope with changing climate. But, the roe deer of the Trois Fontaines are a reminder that not all species are currently capable of recognizing and adjusting to the impacts of humankind on global ecosystems. That may change; roe deer, for example, could migrate to areas where springtime growth dates are more synchronized with their reproductive cycles. But, for enclosed preserves like Trois Fontaines, this is not a viable option.
Why were roe deer unable or unwilling to adjust their reproductive cycles? One possible explanation might involve the cues they use to time reproductive activities. Some species use temperature as a sign that spring has arrived and it is time to breed; this strategy has built-in flexibility for adapting to climate change, since warmer weather and an early spring go hand-in-hand. But, roe deer appear to use photoperiod (day length) as a trigger for their reproductive behaviors. This may explain why roe deer mothers continued to give birth at the same time over three decades, even as their bountiful feast emerged earlier and earlier each year. After all, day length depends on Earth’s axial tilt and rotation, something humans have not yet learned to alter.