Great Migrations: A Thing of the Past?

bison

The largest herd of American bison is found within the confines of Yellowstone National Park. (Credit: Colton Stiffler/istockphoto.com)

When Europeans first came to America, the bison population numbered an estimated 30 to 60 million. The explorers Meriwether Lewis and William Clark commented on the massive herds of bison they passed during their exploration of the Louisiana Territory. In 1839, Thomas Farnham, an author traveling the Santa Fe Trail with the showman Buffalo Bill Cody, noted a herd covering 3510 square kilometers. Unfortunately, mass slaughters of bison in the 1870s nearly led to the animals extinction. By 1889, the bison population only number 1,091 animals. Today, the bison population has rebounded to 500,000a far-cry from historical populations, but safely away from the brink of extinction.

Returning from the brink of extinction came with a cost, however. The majority of bison today are the offspring of bison cross-bred with cattle–many are raised on ranches as livestock. Two hundred years ago, bison roamed across the whole expanse of America. Now, the largest herd of free-roaming plains bison–numbering 4,000–is confined to Yellowstone National Park.

At one time, American bison undertook a great migration during their life cycle. While some bison still migrate, their migration is much shorter, hardly the great undertaking that it once was. Today, scientists wonder whether other migrating animals may face the same fate as the American bison, due to habitat loss, habitat fragmentation, overexploitation, and climate change.

Why do animals migrate?

Migration is an important part of the life cycle of many different types of animals. Blue whales, monarch butterflies, caribou, and elegant terns are just a few of the many animal species that migrate from one place to another during any given year. Survival is the main reason why most animals migrate. Migration patterns are most often associated with seasonal changes or breeding patterns. When climate conditions become too harsh at one portion of an animals territory, it moves to a different portion of its territory where conditions are more favorable. Migration also prevents a species from completely depleting the resources it needs from portions of its territory. Animals also migrate to access breeding grounds. The areas they migrate to are often rich in resources and more protected than the areas where they live during other portions of the year. Salmon, which spend most of their lives in the ocean, return to the same freshwater rivers in which they were born to breed. Young salmon are less vulnerable to predators in river environments than they would be in the ocean.

Because over 90 percent of the world’s elegant tern population breeds on just one island in the Gulf of California, the species is highly susceptible to threats to its breeding habitat. (Credit: Elliott Brooks/istockphoto.com)

How do animals known when to migrate?

Several different cues initiate migratory behavior in animals. These cues include external cues such as photoperiod (length of daylight) and temperature, or internal cues such as the amount of fat stored in the animals body. Animals such as the snow goose know to migrate south when the days become shorter as winter approaches.

Once an animal is cued that it is time to migrate, how does it know where to go? For the most part, animals are born with an innate knowledge of their species migration pathway. Animals use several different methods to navigate along the pathway from one portion of their territory to another. These navigation methods use the position of the Sun, Moon, or stars; major landmarks such as mountains or coastlines; or detection of Earths magnetic field.

What obstacles prevent animals from migrating?

In an article published in the open-access and peer-reviewed journal PloS Biology, authors David S. Wilcove and Martin Wikelski, both of Princeton University, question whether or not its too late to save the great migrations. Wilcove and Wikelski point to four main categories of threats to migratory animals. These categories are habitat destruction, the creation of obstacles and barriers (such as dams and fences), overexploitation (or overhunting), and climate change. While many migratory species are far from becoming extinct, they are noticeably less common than they once were. For example, according to Wilcove and Wikelski, birdwatchers can still see the migratory songbird species they are used to seeing in the spring, they just have to work harder to find the birds than they had to before.

What is the ecological importance of migration?

Both Wilcove and Wikelski believe that protecting migratory animals and their pathways is a significant ecological issue. According to the authors, Protecting the abundance of migrants is the key to protecting the ecological importance of migration. As the number of migrants declines, so too do many of the most important ecological properties and services associated with them.

Migratory animals provide a number of services, such as the addition of nutrients to the ecosystems in which they live. For example, when salmon return from the ocean to rivers to spawn, they transfer nutrients from the ocean to the river system. After spawning, the salmon die, and nutrients are returned to the river system upon their decomposition. These nutrients encourage the growth of phytoplankton and zooplankton, which serve as nutrient sources for other animals, continuing the cycle.

What needs to be done to protect migration pathways?

Saving migratory species is not an easy endeavor. Unlike other threatened animals, migratory species must be protected as a large group to ensure that their migratory behaviors stay intact. According to Wilcove and Wikelski, it is important to understand a migratory species demographic connectivityhow events at any stage in a species migratory cycle affects other stages of the migration. For example, while scientists have studied migratory songbirds at their breeding grounds, wintering grounds, and stopover sites, they remain unsure exactly why migratory songbird populations are declining. Without more intensive studies, it is hard to figure out whether the bird populations are declining because of loss of breeding habitat, loss of winter habitat, increased mortality during migration, or some combination of the three.

Another important aspect that needs to be understood about migration is how animals decide when to migrate, where to migrate, how long to stay, and when to leave. Understanding the decision rules they use is made even more important by the potential impact of climate change on a migratory animals behavior.

One of the biggest challenges to the effort to protect migratory species is the need to protect them before they reach the brink of extinction. It is imperative that lawmakers choose to protect migratory species while they are still abundant–an idea that is counterintuitive to most people. Why protect a species when for all intents and purposes it appears to be far from extinction?

During the zebras’ great migration, the African mammals travel across the Serengeti plains of northern Tanzania before crossing the Kenyan border to reach the Masai Mara National Reserve. (Credit: Liz Leyden/istockphoto.com)

Migratory species are particularly difficult to protect given that their migratory pathways often cross a number of different political (and national) boundaries. Protecting a migratory species includes preserving its habitat at both ends of its migration–and all the areas in between. Doing so will require collaboration between local, national, and international governments, and the people who live within the animals territory as well.

As Wilcove and Wikelski conclude, The challenges–scientific, economic, and social–associated with protecting migratory species are enormous. But so too are the payoffs. We can preserve phenomena that have awed and sustained us since the dawn of humanity. We can protect ecological processes that are integral to many of the planets ecosystems. And we can solve scientific puzzles that have baffled natural historians for millenia. If we are successful, it will be because governments and individuals have learned to act proactively and cooperatively to address environmental problems, and because we have created an international network of protected areas that is capable of sustaining much of the planets natural diversity.

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