Migrating Robins and Hibernating Marmots

In this post I will explore the problem of phenological (seasonal) changes affecting altitudinal migrants and hibernating species. A paper by Inouye et al. (2000) studied the effects of climate change on american robins (Turdus migratorius) and yellow-bellied marmots (Marmota flaviventris) in the Colarado Rocky Mountains. 

The site was the Rocky Mountain Biological Laboratory in Gothic, Colarado at an elevation of 2,945 m. Temperatures can reach -40 celsius and the area experiences over 7 months of snow cover a year. Annual observations showed that the beginning of the growing season at the site had not significantly changed over the past 25 years (P = 0.9) although air temperatures have slightly increased. This may be due to the slight increased volume of snowfall occurring each year.

Robins are an example of altitudinal migrants, who migrate to lower altitudes during the winter months when food is unavailable. The date that robins have been sighted has been proven not be be statistically significant (P = 0.110) but Inouye et al. (2000) believe it to be biologically significant. First sightings of robins moved on average 8.4 days earlier over the 26 years from 1974-2000. This difference has been attributed to the changing phenology of low altitude areas, initiating an earlier spring migration. High altitudes are not shifting phenologically at the same rate as the lower altitudes, causing potential problems to altitudinal migrants over food availability. Robins arriving in the Colarado Rockies will have to wait longer for the snow to melt for food to become available. While robins are able to make the 10-20 km journey back to the low altitudes, if the resources are only available for a certain period of time the robins could find themselves running out of food.

Another way species adapt to extremely low temperatures is hibernation. Marmots in the area typically begin hibernation in August/September after a period of fat accumulation  and body mass gain. The video below shows how Vancouver Island marmots hibernate and how important the period after emergence is.

Marmots are beginning to emerge significantly earlier (P = 0.029), on average 23 days over the past 23 years. This change is believed to be attributed to local air temperature which has been seen to have increased, approximately 1.4 degrees, which is also believe to be the cause of marmot emergence. As they are emerging earlier when there is still significant snow cover, food is scarce and additional stress may be put onto the marmot’s bodies to maintain a high body temperature while their fat reserves run low. While they are not able to eat immediately after emergence, after they reactivate their digestive system feeding is crucial. Prolonged snow cover may also caused decreasing litter size and frequency of reproduction.

Climate models predict increased winter precipitation in the Rockies of the magnitude of 20-70% and problems with altitudinal migrants and hibernating species will persist and most likely worsen. This is just one example of differing local and global changes in climate that can cause problem to migrating species. Spatial variability of changing temperatures may yet become a bigger problem than global mean temperature rise when studying highly mobile species.