Migration can be defined as the long-range seasonal movement of organisms, often due to seasonal changes in temperature. It is an evolved, adapted response to variation in resource availability, and it is a common phenomenon found in all major groups of animals. Birds fly south for the winter to get to warmer climates with sufficient food, and salmon migrate to their spawning grounds. The popular 2005 documentary March of the Penguins followed the 62-mile migration of emperor penguins through Antarctica to bring food back to their breeding site and to their young. Wildebeests (Figure 31.1) migrate over 1800 miles each year in search of new grasslands.One of the negative impacts of climate change is the forced changes upon animals’ migration patterns.

Although migration is thought of as innate behavior, only some migrating species always migrate (obligate migration). Animals that exhibit facultative migration can choose to migrate or not. Additionally, in some animals, only a portion of the population migrates, whereas the rest does not migrate (incomplete migration). For example, owls that live in the tundra may migrate in years when their food source, small rodents, is relatively scarce, but not migrate during the years when rodents are plentiful.

KQED: Flyways: The Migratory Routes of Birds

For thousands of years and countless generations, migratory birds have flown the same long-distance paths between their breeding and feeding grounds. Understanding the routes these birds take, called flyways, helps conservation efforts and gives scientists better knowledge of global changes, both natural and man-made.

Migration in humans

Early Hominin Migrations

Human species were migratory from the beginning, moving as small populations of gatherers and hunters within eastern and southern Africa. By following game and the availability of seasonal vegetation from place to place, these small groups of nomads learned about their landscape, interacted with each other, and met their subsistence needs. Their daily needs came through interaction with a changing environment. With the emergence of Homo erectus around 1.89 million years BP (before the present), hominins expanded their territories and began to exhibit increasing control over their environment and an ability to adapt, evidenced by the development of new subsistence systems, including cultivation, pastoralism, and agriculture, and an upsurge in migration within Africa and, eventually, into Asia and Europe. This expansion into new geographical regions was a hallmark of the later human species.

There are several theories on possible migratory sequences within and beyond the African continent. One possibility is that by 1.75 million years ago, Homo ergaster had begun migrating out of Africa, moving northward into Eurasia. Another theory argues that an earlier hominin species, either australopithecine or an early as-yet-unknown species of the genus Homo, migrated out of Africa around 2 million years ago, eventually evolving into the population of Dmanisi hominins who were settled in eastern Europe by 1.85 million years ago, possibly representing another link between H. erectus and H. ergaster. Although settlement dates are currently being retested and reexamined for precision (Matsu’ura et al. 2020), it is known that between 1.3 and 1.6 million years ago, H. erectus settled on Java, an island that is now part of Indonesia. They likely traveled there by a land route, as seas were lower during the Pleistocene Ice Age (approximately 2.588 million–11,700 years ago), allowing for more passage through interior coastal routes. (For more on early human migrations, see The Genus Homo Homo and the Emergence of Us.)

Regardless of the specific time frame and migration pattern, it is well established that there was gene flow between various hominin populations, which indicates that there were migration and exchange. With the migration of these early hominin populations, cultural practices and improvements in toolmaking spread as well. Wherever humans traveled, they carried with them their traditions, intermingling and reproducing both physically and culturally.

Controversies Surrounding the Peopling of the Americas

Current evidence points to the emergence of the genus Homo in Africa. From these beginnings, human populations began moving toward the global north, east, and south in migratory waves. Motivations for these migrations included animal movements, overcrowding and resource scarcity, and, likely, curiosity and adventure. The movement into the Western Hemisphere, into North and South America, occurred significantly later than migrations into Europe and Asia; how much later is a question of enormous controversy today. How did the first peoples make their way to the Americas? When did they first arrive, and how did they migrate within these vast continents? The available evidence is inconclusive, leaving us with one of the biggest enigmas in human evolution. While there is some debate on whether earlier human species migrated into the Americas, the evidence we have today points to members of the species Homo sapiens being the earliest humans to do so. At this point, there is no evidence of any earlier hominin species in either North or South America. The Western Hemisphere was wholly settled by migrants coming from other continents.

There are many theories regarding the first human migration into the Western Hemisphere. Because of changing global climate conditions and the retreat of glaciers toward the end of the Pleistocene epoch, new lands opened to migrating animals and the humans who were likely hunting them (Wooller et al. 2018). As always, because of limited and ambiguous artifact and fossil findings, the primary pieces of evidence are open to multiple interpretations. Upon examining the range of theories, two primary arguments are apparent. Both of these arguments are backed by supporting evidence, and both rely on migratory patterns of H. sapiens in the Americas that have been definitively established. While both migration theories are valid, the question that remains open to argument is which came first, coastal or interior migration?

• The interior route, also called the Bering Strait theory, is the best-known and most accepted theory for the first human migration into the Americas. The foundation of this theory is the Beringia “land bridge,” which connected northeast Siberia and what is now Alaska when sea levels were lower due to glacial ice formation on the continents. This theory proposes that the earliest human habitants of the Americas crossed this marshy land on foot, most likely beginning around 15,000 years ago based on artifacts and dating sequences. The Beringia land bridge was alternately exposed and submerged multiple times over the earth’s history. According to the interior route theory, the earliest humans crossed this marshy land in pursuit of migratory herds of mammals and then proceeded to filter southward, splitting into multiple groups, some of which penetrated into the interior of North America as they continued to move east and south.

• The coastal route is also based on the migration of a northeastern Siberian population into the Western Hemisphere, but by boat rather than on foot. This theory, sometimes called the kelp highway hypothesis, proposes that the earliest migratory populations followed the continental coastline southward, subsisting on kelp, fish, shellfish, birds, and sea mammals. Research by archaeologist Jon Erlandson (Erlandson et al. 2007; Ocean Wise 2017) suggests that migrants may have followed these food sources all along the continental shelf, a shallow sea area near the shore. Some believe that they eventually reached as far south as Chile, in South America, before breaking into groups and penetrating the interior lands.

Each theory presents its own probabilities and problems in relation to dating sequences and artifacts, and there were possibly multiple early routes for the peopling the Americas. Scientific research does agree on some known facts, however. Genetic sequencing shows continuity between the earliest Americans and populations in northeastern Siberia that indicates the earliest inhabitants of the Americas arrived no more than 25,000 years ago, making the Americas the most recent continental habitation (outside of Antarctica). Humans were already inhabiting Australia by the time other humans first arrived in the Americas.

Archaeological sites in the Americas present fascinating evidence of early human migrations, with the dating sequences continually being retested and revised. Based on some of the early archaeological evidence, scientists had believed that the first American inhabitants were part of what is known as the Clovis culture, identified with a leaf-shaped projectile point used in hunting. As excavations have continued, though, there is growing indication of an extensive pre-Clovis culture, evidenced by a pre-Clovis technology based on gathering, hunting, and fishing, with dates extending back further than 13,200 years before present. Pre-Clovis projectile points are smaller, less standardized, and less worked (flaked), indicating a less advanced tool production. Many pre-Clovis sites are located below the Clovis period occupation. As archaeologists have continued excavations, the dates for earliest occupation continue to be pushed backward.

Why so much debate about the settling of the Americas? There are various reasons for the difficulties in establishing settlement dates. The Bering land bridge was periodically exposed and submerged under water during periods of glacial growth and retreat. Using core samples obtained by drilling down into the shallow sea floor, archaeologists have found evidence of large mammals and even fluted points (hunting tools) in and around the Aleutian Islands, through which the land bridge would have crossed. Establishing and cross-checking dates, though, has been difficult because most evidence is now submerged. This is a challenge also for the coastal route theory, as coastlines have receded since the end of the Pleistocene, and encampments would have likely been small, possibly temporary sites. Many sites are likely now submerged offshore (Gruhn 2020).

Among the best-known pre-Clovis sites are the following:

• Monte Verde Site, Chile. This is one of the most studied pre-Clovis sites. An extensive array of artifacts has been found at Monte Verde, including hearths, wooden and stone tools, animal bones, and even human footprints. The dates assigned to these artifacts, as early as 16,000 BP, put this site within the range of pre-Clovis dates seen in North America.
• Debra L. Friedkin Site, Texas. This pre-Clovis site has a dating sequence of 13,500 to 15,500 BP. A wide range of pre-Clovis tools have been found here, including partially flaked tools, blades, and scrapers.
• Cactus Hill Site, Virginia. A well-document Clovis site has been identified at Cactus Hill, but below this level of artifacts, there is evidence of pre-Clovis projectile points. Although controversial, these points have possible dating sequences of 18,000–22,000 BP.

Based on this new evidence, scientists now agree that the Americas were first settled by a pre-Clovis population. How they arrived, when they arrived, what movements they made, and in what order they made them are major archaeological questions today. What we can conclude is that human populations continued to migrate after peopling the Americas.

Temperature and habitat

Temperature affects the physiology of organisms as well as the density and state of water. Temperature exerts an important influence on living things because few living things can survive at temperatures below 0 °C (32 °F) due to metabolic constraints. It is also rare for living things to survive at temperatures exceeding 45 °C (113 °F); this is a reflection of evolutionary response to typical temperatures near the Earth’s surface. Enzymes are most efficient within a narrow and specific range of temperatures; enzyme degradation can occur at higher temperatures. Therefore, organisms either must maintain an internal temperature or they must inhabit an environment that will keep the body within a temperature range that supports metabolism. Some animals have adapted to enable their bodies to survive significant temperature fluctuations, such as seen in hibernation or reptilian torpor. Similarly, some Archaea bacteria have evolved to tolerate extremely hot temperatures such as those found in the geysers within Yellowstone National Park. Such bacteria are examples of extremophiles: organisms that thrive in extreme environments.

The temperature (of both water and air) can limit the distribution of living things. Animals faced with temperature fluctuations may respond with adaptations, such as migration, in order to survive. Migration, the regular movement from one place to another, is an adaptation found in many animals, including many that inhabit seasonally cold climates. Migration solves problems related to temperature, locating food, and finding a mate. For example, the Arctic Tern (Sterna paradisaea) makes a 40,000 km (24,000 mi) round-trip flight each year between its feeding grounds in the southern hemisphere and its breeding grounds in the Arctic Ocean. Monarch butterflies (Danaus plexippus) live in the eastern and western United States in the warmer months, where they build up enormous populations, and migrate to areas around Michoacan, Mexico as well as areas along the Pacific Coast, and the southern United States in the wintertime. Some species of mammals also make migratory forays. Reindeer (Rangifer tarandus) travel about 5,000 km (3,100 mi) each year to find food. Amphibians and reptiles are more limited in their distribution because they generally lack migratory ability. Not all animals that could migrate do so: migration carries risk and comes at a high-energy cost.

Some animals hibernate or estivate to survive hostile temperatures. Hibernation enables animals to survive cold conditions, and estivation allows animals to survive the hostile conditions of a hot, dry climate. Animals that hibernate or estivate enter a state known as torpor: a condition in which their metabolic rate is significantly lowered. This enables the animal to wait until its environment better supports its survival. Some amphibians, such as the wood frog (Rana sylvatica), have an antifreeze-like chemical in their cells, which retains the cells’ integrity and prevents them from freezing and bursting.

Climate change and effects on migration behaviors

Climate change, and specifically the anthropogenic (meaning, caused by humans) warming trend presentlyescalating, is recognized as a major extinction threat, particularly when combined with other threats such as habitat loss and the expansion of disease organisms. Scientists disagree about the likely magnitude of the effects, with extinction rate estimates ranging from 15 percent to 40 percent of species destined for extinction by 2050. Scientists do agree, however, that climate change will alter regional climates, including rainfall and snowfall patterns, making habitats less hospitable to the species living in them, in particular, the endemic species. The warming trendwill shift colder climates toward the north and south poles, forcing species to move with their adapted climate norms while facing habitat gaps along the way. The shifting ranges will impose new competitive regimes on species as they find themselves in contact with other species not present in their historic range. One such unexpected species contact is between polar bears and grizzly bears. Previously, these two distinct species had separate ranges. Now, their ranges are overlapping and there are documented cases of these two species mating and producing viable offspring, which may or may not be viable crossing back to either parental species. Changing climates also throw off species’ delicate timed adaptations to seasonal food resources and breeding times. Many contemporary mismatches to shifts in resource availability and timing have already been documented.

Range shifts are already being observed: for example, some European bird species ranges have moved 91 km northward. The same study suggested that the optimal shift based on warming trends was double that distance, suggesting that the populations are not moving quickly enough. Range shifts have also been observed in plants, butterflies, other insects, freshwater fishes, reptiles, and mammals.

Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations. Some climates will completely disappear. The accelerating rate of warming in the arctic significantly reduces snowfall and the formation of sea ice. Without the ice, species like polar bears cannot successfully hunt seals, which are their only reliable source of food. Sea ice coverage has been decreasing since observations began in the mid-twentieth century, and the rate of decline observed in recent years is far greater than previously predicted.

Species adapting to climate change

The same way the musicians of an orchestra rely on a conductor to remain synchronised, migratory species rely on environmental cues, such as day length and temperature, to decide when they need to start moving from one area to the next. But because different species rely on different environmental cues to time their life cycles (e.g. breeding), not all species will adjust to climate change at the same rate. There is consequently a high likelihood that climate change will disrupt these synchronous movements that the animal kingdom has developed over thousands of years (Renner and Zohner, 2018). This disruption of timed aspects of a species’ life cycle, such as migration and breeding, is called phenological mismatch or trophic asynchrony. Researchers have already seen signs of phenological mismatch: some migratory birds that overwinter in the tropics have started to migrate to their European breeding grounds at earlier dates than before (Both et al., 2006; Vickery et al., 2014). If these trends hold, they may soon start breeding before peak food availability, which could lead to lower fitness of offspring.

References

Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction

https://bio.libretexts.org/Sandboxes/tholmberg_at_nwcc.edu/General_Biology_I_and_II/06%3A_Unit_VI-_Ecology/6.2%3A_Conservation_and_Biodiversity/6.2.05%3A_The_Impact_of_Climate_Change

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.06%3A_Innate_Animal_Behavior/45.6B%3A_Movement_and_Migration

https://socialsci.libretexts.org/Bookshelves/Anthropology/Introductory_Anthropology/Introduction_to_Anthropology_(OpenStax)/10%3A_The_Global_Impact_of_Human_Migration/10.02%3A_Peopling_of_the_World