Caribou Not Keeping Up With Warming Climate

Researchers have traced a sharp decline in survival of caribou calves to climatic warming. While arctic plants have kept pace with the earlier arrival of spring temperatures, the caribou that graze on those plants have not. The resulting disconnection between forage and herbivore is what scientists term a "trophic mismatch".

Female caribou are best off giving birth as tundra plants sprout in spring. That gives new moms the best nourishment plants can offer when it's most needed to support their nursing calves. North of the Arctic Circle, plants peak briefly in nutrition and digestibility soon after emerging from thawing ground.

The trophic mismatch arises from caribou and plants relying on different environmental cues. Caribou depend upon the length of daylight for prompting their migration to calving grounds. Plants, on the other hand, use springtime temperatures to trigger growing.

In Western Greenland mean spring temperatures rose by 4.6 °C between 2002 and 2006. Plants responded by beginning growth 15 days earlier in the year. Meanwhile, the Kangerlussuaq caribou population averaged just over a one-day advance in when it calved.

As the gap between new plant growth and caribou births lengthened, researchers found that fewer newborn calves survived. During warmer temperatures, vegetation was past its prime when females with calves most needed energy.

Calf mortality was seven times higher when the disparity between plant emergence and calving was at its largest, compared with when the two were tightly synchronized. The proportion of females with surviving calves dropped as springs moved ahead.

Another field study at the same Greenland site indicates that a further 4 °C rise in temperature, as is expected, would advance growth of key caribou forage by up to ten days. The research suggests that caribou will face harsh selection pressure to keep up with climate change.