Wildlife Migration and the Green-Up

This column was published in the Cranbrook Daily Townsman Tuesday September 14, 2010.


Last week, I wrote about the trade-off that elk and other ungulates make between predation and forage. Many animals are forced to make decisions between accessing high quality food and exposing themselves to higher predation risk. Ungulate species around the world have evolved migration as a strategy to lower predation risk and access higher quality forage.

The issue of non-migratory or resident elk living year-round in the valley-bottom Trench has vexed wildlife managers and ranchers alike. The number of elk migrating to the high country each summer has declined significantly. As efforts continue to encourage elk to resume their seasonal wanderings, new challenges may arise in this effort.

Migratory ungulates in the Rockies follow a spring wave of green-up into the mountains, feeding on nutritious and highly digestible emergent vegetation. But constraints on food availability may hinder elk in the high country compared to their non-migratory cousins who stay put in the Trench. It all ties into plant growth.

Dr. Mark Hebblewhite at the University of Montana notes recent research that suggests changes in historical vegetation growth patterns may further disadvantage migrating elk. He notes that “climate change might potentially lead to reduced benefits for migration” (emphasis his).

If plants grow too quickly, forage quality is reduced. Slow-growing, cool springs keeps grasses lush, digestible and highly nutritious. Conversely, short, rapid-warming springs quickly dry out vegetation with a loss of nutritional value. Research on bighorn sheep and mountain goats in Alberta has shown that compressed green-up can directly reduce lamb and kid survival. It’s not a long stretch to other species using similar habitats and diets: elk and mule deer.

Evidence suggests changes are occurring in high elevation vegetative growth patterns. Global land plant growth increased 6% between 1982 and 1999, with the increase accredited to higher temperature, more rain and solar radiation; all key ingredients to plant growth. When University of Montana scientists Maosheng Zhao and Steve Running set out to update the research, they were astonished to find the same global terrestrial plant growth rate declined, albeit slightly at 1%, between 2000 and 2009. Their research, published in Science last month, found that drought has outpaced temperature increases, particularly in the southern hemisphere, and led to net declines in plant growth in many areas of the world. Elsewhere, including most northern latitude forests such as Canada, net plant growth is still increasing. But the work issues a warning that climate change can cause unexpected and rapid shifts in ecological systems.

A third issue is termed the “trophic mismatch hypothesis,” first described by Penn State’s Eric Post and colleagues for caribou in Greenland. There, changes in green-up time are creating a mismatch between calving times and food availability. In a harsh arctic or high elevation ecosystems where food is often scarce outside a short growing season, timing of birth is very important.

Some estimates place the energetic costs of lactation at some 200% more expensive to mothers than pregnancy. So having an abundant, nutritious food source while young are nursing is essential. Even slight shifts in forage availability is enough to lower calf survival. Whether the same phenomenon is occurring in alpine ecosystems is unknown. It’s a question Dr. Hebblewhite would like to test in the Rocky Mountains.

Timing of ungulate migration is a complex adaptation to various clues. Their ability to respond to change lags well behind plant growth responding to seasonal climate shifts. If changes are indeed occurring in high elevation montane forests, migrating local ungulates will be at a significant disadvantage compared to resident elk feeding on irrigated alfalfa fields in the Trench. Encouraging elk to vacate that food supply and resume traditional migrations from valley bottom to the high country faces its own uphill battle.