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In recent years, human movements linked to changes in the environment or as a result of fleeing from poverty or war, have grown rapidly and have gained increasing visibility among scientists and policy makers. At the radio, on social media, on television, we often hear of a “migration crisis” when referring to the hundreds of thousands of migrants and refugees who, every year, seek prosperity in Europe coming from Africa, the Middle East and South Asia. Migration is often depicted as a threat, as a maladaptive strategy, and its reach as an unnatural phenomenon. But migration in one of those human patterns that can be seen from the start of human experience to present. Migration, which is the relatively long-distance movement of individuals in search of better conditions, is a phenomenon commonly found in nature. Birds do it. Fish do it. Mammals, insects and reptiles do it. Migration, what’s more natural than that?
Animal migration is surely one of nature most visible and widespread phenomena. Every minute of every day, somewhere, some place, animals are on the move. Migrations are undertaken by all major animal groups; the migrants span the animal kingdom, from birds to mammals, from fish to reptiles and amphibians, to insects and crustaceans. Even sponges and jellyfish larvae make use of this defining animal trait, which is migration, to adapt to imperfect habitats. So why do animal move? Migration is driven by a simple fact: the resources on Earth fluctuate. Warm summer months may be followed by inhospitable cold seasons. Plants – or other meals – may be abundant, but only for a short time. The best place to give birth or hatch young may not be a good place to find food. Thus, animals migrate in search of better resources: these can be warmer temperatures, more and better quality food and the availability of suitable places for reproduction. Often, animals migrate in a group and the social context can be very important when it comes to take a migratory decision.
How far can animal travel?
Migratory birds undertake some of the most extraordinary journeys of any animal and the human-kind has long been fascinated by these long migrations: the first records of bird migration were written as much as 3,000 years ago by Ancient Greek authors such as Homer and Aristotle. The timing of birds’ migration seems to be controlled primarily by changes in day length. Migrating birds navigate using the sky, in particular the sun and the stars, and by sensing the Earth’s magnetic field. But which is the longest distance that birds can cover? The Arctic tern holds the record for the longest-distance migration. This bird travels between the Artic and the Antarctic each single year; it has been calculated that the Arctic tern covers the distance to the Moon and back three times during its lifetime!
Between mammals, humpback whales move for very long distances. Each winter, humpback whales travel from the Antarctic to the tropics to find warmer waters in which to raise their young, as researchers have shown since 2007. In particular, a study published in 2007 by a group of American scientists followed a humpback whale mum and her calf using satellite data. They made a trip of 8300 kilometres in 161 days: it’s really a long way to go for a warm bath! The migration of the humpbacks was considered the longest of any mammal species, at least until 2015. In 2015, indeed, scientists recorded the record (at least so far) of mammal migrations. The record was set by a Gray whale, named Varvara by the scientists who made the discovery. Varvara travelled for almost 22000 km, swimming from Russian waters to Mexico and back!
Between insects, a notable mention goes to dragonflies. Some species travel between Southern India and Southern Africa every year, stopping in Maldives and skirting the East African coast along the way. This migration, which covers between 4,000 and 6000 km, is maybe the longest undertaken by any insect. In insects, migration can involve many generations. The migration of the Monarch butterfly takes four generations to complete, on a 9000 km journey. Starting in Central Mexico and California, the first generation flies to Texas and Oklahoma where it lays its eggs and dies. The next generation takes off further north. The third generation reaches America’s Great Lakes and succumbs before the fourth generation flies all the way back south -up to 5000 km- to lay its eggs.
Can animal mass migrations be driven by social factors?
Migration can be driven by social factors, as in the cases of anadromous fishes. Anadromous fishes are fishes which spend all their adult life in the ocean but that reproduce in freshwaters. In animal documentaries, have you ever seen some images of big fishes swimming and jumping against water flow in streams and rivers? If yes, these fishes definitively belong to the Salmon family. The Salmon family is a family of fish which include, among others, the well-known and highly commercialised Atlantic salmon, the Arctic charr and the Sockeye salmon. These fishes are born in streams and lakes of North America and Europe. Still very young, they migrate to the ocean to feed on small crustacean and other little inhabitants of the rich, oceanic waters. Here they increase tremendously in size and weight, they become holder and sexually mature. Once they reach sexual maturation, salmons return to the same river or lake where they were born. Here they reproduce, and then they die.
But how do they know when it’s time to move back to the rivers? Which type of signals in the environment (i.e. triggers) are used by these animals to start their travel? A study published in 2017 in the peer-review journal Animal Behaviour by a group of American scientists and based on over 20 years of data, aimed to shed lights on these questions. The researchers found that, in absence of any obvious environmental trigger, such as changes in temperature or light, is actually the social dynamics that induce fish to migrate. In particular, the researchers found that salmons synchronise their travel, by moving back to the rivers altogether. When a fish see the others migrate, is likely to follow them and to migrate itself. This study demonstrates the social component of migration; until now, most research on fish migration viewed the process of moving as an individual process. This study shows that the social context in which animals live matters when it comes to take a migratory decision. So why do salmon migrate all together? There are multiple reasons for which salmons migrate in mass. First, moving as a coordinated group helps them to reduce the risk of being predated. In particular, a mass migration reduces the possibility of being predated by bears, which are waiting for the salmons jumping upriver. Also, travelling together likely increase the possibility of encountering receptive partners once arrived at the spawning site. Once fishes arrive at the spawning site, indeed, females start the nest preparation, which involves creating a depression in the gravel bed of the river by literally piling gravel stones using their tales (see supplementary material). Once the nest is ready, females complete spawning within a few more days. Thus, females are only available to spawn for a very short period and thus the males who arrive together with them have more chance to reproduce. Arriving later and alone, in this case, means that there won’t be any female left to court and to mate with!
Also between the crustaceans, there are some cases of mass migrations. The spiny lobster in the northern Bahamas, for example, migrates in mass during autumn. Large numbers of lobsters move from the shallow waters near the coast to reach deeper, oceanic waters where they reproduce. This mass migration usually takes place at night, and it goes on for several weeks. Lobster starts to become hyperactive when the summer storms cause the coastal waters to become cooler. When this happens, lobsters start to literally queue: one by one, one following the other, they move to deeper waters. Scientists have proven that a lobster which becomes active induce others to queue up and to follow. Which benefits do lobster gain by moving together in a queue? Scientists have shown that, by queuing, lobsters reduce the risk of being dragged away by the fast and strong currents which are common in the Caribbean. Being together reduce indeed the hydrodynamic drag on each individual!
Are there any benefits arising from migration?
There are ecological benefits connected to animal migrations. Several studies have investigated the ecological importance of animal migrations. The migrations of flying insects, birds and bats mean that a large number of living organisms move from the north to the south of the world. Because many migrant species, especially insects, are extremely abundant, seasonal migrations profoundly affect natural communities, scientists have shown. The effects that animal migrations have on the Earth ecosystems include changes in both predation and competition dynamics, but they are not limited to these. Moving animals simply transfer an enormous quantity of energy and nutrients between regions.
Let’s take a closer look to this, by focusing again on the salmons. By migrating upstream, spawning, and dying, salmon transfer nutrients from the ocean to the rivers. A portion of the nutrients is delivered in the form of faeces, sperm, and eggs from the living fish; much more comes from the decaying carcases of the adults. Phosphorus and nitrogen from salmon carcases enhance the growth of the small, microscopic, planktonic communities of rivers and lakes, which provide food for smaller fish, including young salmon. Thus, young salmon are literally sustained by their parents.
Is animal migration an endangered phenomenon?
Around the world, many of the most spectacular migrations have either disappeared due to human activities or are in alarming decline. Birdwatchers in Europe, for example, complain that fewer songbirds are returning each spring from their winter quarters in Africa. Indeed, a recent continent-wide analysis of European breeding birds concluded that long-distance migrants (i.e., those species that breed in Europe but winter in sub-Saharan Africa) have suffered severe population declines, more than related nonmigratory species.
And what about the salmons? Prior to European settlement, 160–226 million kilogrammes of salmon migrated each year up to the rivers of Washington, Idaho, Oregon, and California. Today, after decades of dam construction, overfishing, water withdrawals for irrigation, logging, and streamside grazing by livestock, salmon populations have crashed. The total biomass of spawning salmon in the Pacific Northwest is now estimated to be only 12–14 million kilogrammes. Scientists have calculated that the rivers of the Northwest receive just 6% of the marine-derived nitrogen and phosphorus they once received from the abundant salmon population. How this may be affecting the ecology of the region’s rivers or adjacent farmlands is largely unknown. The causes of all these declines vary depending on the species and the locale, but in general, the threats to migrant animals fall into four categories: habitat destruction, the creation of obstacles and barriers such as dams and fences, overexploitation, and climate change.
Protecting the abundance of animal migrants is the key for protecting the ecological importance of migration. As the number of migrants declines, so do many of the most important ecological properties and services associated with them. Increasing attention is given by the scientific community and policy makers to protect animal migrations. The challenges—scientific, economic, and social—associated with protecting animal migratory species are enormous. But so too are the payoffs. We can preserve phenomena that have sustained us since the dawn of humanity. We can protect ecological processes that are integral to many of the planet’s ecosystems from which our own survival depends of.
If we are successful, it will be because governments and individuals have learned to act proactively and cooperatively and because we have created an international network that is capable of sustaining much of the planet’s natural diversity. Two questions remain open: will we be able to do so? Shouldn’t we use a similar type of international, cooperative approach when it comes to human migrations as well? So far, these questions remain unanswered. A good starting point could be to begin to consider human migrations as a resource and not as a threat: after all…migrations, what’s more natural than that?