Migrations by Air, Land and Sea

Creatures other than man migrate along constant pathways to preserve their species. Dr Anjani Ganase considers some of these corridors of power and how they should be protected, especially in the region most important to our islands, the Caribbean Sea.

 

 Animal migration is the seasonal movement from one habitat to another for the purpose of activities such as breeding and feeding. Migrations can be over small distances, from mountains to meadows; along latitudinal lines; and some circumnavigate the planet.  Many different animals migrate: insects, birds and bats fly, zooplankton moves on ocean currents; and great herds on foot. Here are some quirky facts about animals that travel great distances in air, sea or over land. 

 

Leatherback turtles nest on beaches of Trinidad and Tobago and migrate long distances in the ocean following food.

BY AIR

The longest non-stop bird migration recorded was the bar-tailed godwit travelling across the Pacific Ocean from Alaska to New Zealand. The tagged bird set the record of over 12,000 km in one flight, taking eleven days at a speed of 55 mph without ever touching down. Many bird species travel long distances taking pitstops along the way, on remote desert islands, along shorelines or wetlands. While these pitstops afford essential rest, they can also be dangerous. The Arctic tern follows the polar summers by flying from the Arctic circle to the Antarctic around 30,000 km to enjoy continuous light and lots of food to eat. This evolutionary ability to traverse such distances took millions of years of adaptation, flying about 1000 miles a day bouncing from continent to another land mass while avoiding the treacherous ocean.  Here in Trinidad and Tobago, the Caroni wetlands serve as breeding grounds to the Scarlet Ibis that migrate yearly from Venezuela between October and March. Who knows which is their “native land?” For, while the Scarlet Ibis is TT’s national bird, conservation of these birds goes beyond our national boundaries.

Less obvious fliers are the Monarch butterflies that flutter across the Canada and USA southwards to Mexico over winter to breed. This journey takes about two months. Many theories and studies on the navigation of Monarchs include their ability to use the angle of the sun against the horizon to navigate; while others believe they use the earth’s magnetic field. Monarch butterflies are, unfortunately, under threat from deforestation in both locations, along with the use of pesticides. Monarchs only nest in Milkweed plants and between 1995 and 2003 there was a decline of more than 50 % of these plants in areas in the US that continues to today because of illegal logging. 

 

The Great migration of wildebeest across Kenya's Mara River. Photo by Reginald Victor Runyoro (https://creativecommons.org/licenses/by-sa/4.0/)

 

OVER LAND

Follow the annual migration of the wildebeest and zebras through the several major conservation zones in East Africa: Masai Mara Game Reserve in Kenya, Serengeti National Park, Ngorongoro Conservation Area, Maswa Game Reserve and Grumeti Game Reserve in Tanzania. It is one of the largest land migrations observed and aptly termed the Great Migration. Covering about 2900 km, some 1.5 – 2 million wildebeest, along with zebra take part in the migration grazing on vegetation and in search of water sources provided by rainfall. The migration makes a large clockwise loop where herds head south in August/ September and return north in February/ March along the western margins of the Serengeti National Park. Calving season occurs when the herds are in the south in the Ngorongoro Conservation Area. The migration is followed keenly by large predators, and results in the loss of some 255,000 wildebeest and 30,000 zebra from predation, exhaustion, drowning etc. Some of the most dramatic scenes include the treacherous crossing of the Mara and the Grumeti Rivers filled with expectant Nile Crocodiles, while many big cats wait along the riverbanks.

 

In the USA, the American Bison used to roam the great plains foraging for fresh vegetation. Bison followed specific foraging routes creating their own roads crisscrossing the landscape over years. Their grazing and pooping paths were critical to the health of these ecosystems, encouraging further growth of the nutritious plants, preserving the soil moisture and nutrients. During the 19^th century, the American Bison was hunted almost to extinction. Today, American Bisons are limited to the protected areas, such as the Yellowstone National Park with a population of 3000 which is thought to be the only freely roaming herd. The rest of the bison populations are held in ranches for commercial use.  

 

In Illinois USA, the La Rue-Pine Hills/ Otter Pond Research Natural Area (RNA) oversees and protects the annual great snake migration that takes place during the spring and fall months every year. Many snake (35 species) and amphibian (23 species) species make their way across a major road between their winter grounds (hibernating along rocky cliffs) and summer homes in the swamp. This road is closed to traffic every fall and spring to ensure the safe movement of these animals.

 

Humpback whales, Reunion Island. Credit: François Baelen / Ocean Image Bank

 

BY SEA

Most marine mammals - whales, dolphins, seals and even penguins - migrate for food and breeding. Whales follow the schools of zooplankton to the poles during summer where they can feed and fatten up before they make their way to the tropics for breeding and raising their young. Many fish species travel daily to forage or migrate from nursery grounds to foraging grounds at different parts of their life cycle. Some fish species, such as the Atlantic bluefin tuna can roam the Atlantic Ocean before returning to their natal site to spawn. Even the spiny lobsters in the Pacific make their way in a single file from the shallow California coast during the winter, into the warmer deep ocean. The lobsters clasp on to each other to form a complete chain.  It is thought that they navigate by magnetite in their tissues to allow them to align with the earth’s magnetic field. Here in Trinidad and Tobago, we are visited by marine turtles – hawksbill and green - that forage in our waters but are often born elsewhere in the Caribbean. Trinidad and Tobago is also known for high densities of nesting leatherback turtles that forage as far north as Nova Scotia and east into the Indian Ocean; they return as adults to nest on the beaches of Trinidad and Tobago where they were born.

 

Traveling across geopolitical borders with different regulations for the protection of species and habitat management puts migratory species at risk from exploitation, loss of habitat and declining population. In 1979, the Convention on the Conservation of Migratory Species of Wild Animals was established under UNEP (United Nations Environmental Programme) with the primary aim of the conservation of avian, marine and terrestrial migratory species. The international body works with countries where the migrating species pass; and sets out legal frameworks for the protection of the species and their habitats. There is a priority for species listed as endangered.

 

Trinidad and Tobago along with 129 other countries is a party to the Convention. Interestingly, many large nations, such as Canada, USA, Russia and China have not signed the agreement. Climate change and human activities are rapidly eroding the corridors for migration as seasons shift and become more erratic. There is the increased risk of droughts, wildfire and thermal events in the ocean that leave many migrating animals vulnerable as their food sources and breeding meet ups become compromised. The Caribbean Sea is an area with significant migration of marine and avian life, and all countries that border or are linked by the Caribbean need to create special regional conservation conventions to protect species and limit biodiversity loss.

 

References:

https://www.asiliaafrica.com/blog/the-science-behind-africa-s-great-migration/

 

Vojtěch Kubelka, Brett K. Sandercock, Tamás Székely, Robert P. Freckleton (2021) “Animal migration to northern latitudes: environmental changes and increasing threats” Trends in Ecology & Evolution. (DOI:10.1016/j.tree.2021.08.010)

 

https://education.nationalgeographic.org/resource/migration-collection