When organizing a long car drive, there are two conflicting people: those who just want to get to our destination, and those who will take it a bit slower. Some of us grab something to go and often try to limit as many stops as possible to get to our destination faster. If we have to make a stop, it will be through the fastest food place possible. On the other hand, some of us will always plan to find a sit-down restaurant for our meals.
We often don’t realize how much our eating habits control the way we travel, and how the way we travel controls our eating habits; this relationship affects our departure time, arrival time, and quality of food we are eating. We are not alone: mule deer and many other migratory ungulates plan their spring migration around food and when it becomes available. But, our world is changing, and the landscapes that ungulates migrate through and what they eat along the way are changing as well.
Surfing the green wave
Just as our food habits are an important factor in our health, nutritious protein-rich food is essential to the health of the mule deer. A lack of protein-rich spring greens can—along with other factors—affect their survival the following winter, as well as the health of their offspring. Like green vegetables are better for us than tree bark, green protein-rich plants are more nutritious to ungulates than dry, brown plants. Mule deer time life events and their migration around flushes of nutritious spring green-up that progress across the landscape. Scientists have called this behavior surfing the green wave, and this style of movement helps herbivores have more opportunities to eat nutritious food while migrating.
This flush of green across a landscape is made possible because plants have their own internal timing, a term referred to as phenology. It varies from species to species, but plants start growing when the environment signals to them that daylight and temperatures are increasing. Plants early in life are especially nutritious, since they are full of protein but haven’t yet started to degrade, as they do later in the summer. The high-quality nutritious spring green-up is only available for a short window of time at a specific location. Mule deer migration allows them to exploit this resource in a given spot, as well as track the progression of young and nutritious plants by surfing. By lengthening their exposure to high-quality food, they can increase their energy intake and fitness.
Drought affects plant phenology
Drought has caused migrating mule deer to have less exposure to spring green-up as they travel to their summer ranges. Changes in climate can cause soil moisture to decline, increase plant growth, and cause plants to green-up out of order. Altering the timing and pattern of spring green-up makes it more difficult for ungulates to eat high-quality food while migrating. The changes in plant green-up are happening quickly, but ungulates are not able to respond as quickly. These effects are also seen in other ungulate species. For example, reduced elk pregnancy and recruitment have been linked to increased drought intensity and shorter spring green-up periods.
The change in plant phenology is an important example of how climate change can negatively affect migratory species by reducing when key food resources are available. Although drought will likely continue to disrupt plant green-up patterns, conservation efforts that eliminate movement barriers and prevent habitat fragmentation can help ensure that migratory ungulates have the necessary access to forage along their migratory routes.
Compendium by Elsa Freise.
Want to read more? Check out these papers:
- Aikens, E. O., K. L. Monteith, J. A. Merkle, S. P. Dwinnell, G. L. Fralick, and M. J. Kauffman. 2020. Drought reshuffles plant phenology and reduces the foraging benefit of green-wave surfing for a migratory ungulate. Global Change Biology 26:4215-4225.
- Middleton, A. D., M. J. Kauffman, D. E. McWhirter, J. G. Cook, R. C. Cook, A. A. Nelson, M. D. Jimenez, and R. W. Klaver. 2013. Animal migration amid shifting patterns of phenology and predation: lessons from a Yellowstone elk herd. Ecology 94:1245-1256.