Arctic tern. Photo by K. Pikner, Wikimedia Commons
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“Vagility” is a word that I first came across in a February 19, 2018 article in the Science section of the New York Times. Its word root, “vagile,” means “free to move about,” and its application in ecology refers to the degree by which an organism is able to move around in its environment. In other words, the freedom an organism has to migrate.
There are many examples of and also many types of migration. Some animals migrate over very long distances (like arctic terns that breed in the tundra around the Arctic Ocean and then spend their “winters” in the “summer” over the oceans around Antarctica. It is said that no other animal species sees as much sunlight in its life as the arctic tern!). Other animals fly or walk just a thousand miles or two as they move along their yearly migration routes (many of our summer birds (like the scarlet tanager, Baltimore oriole, rose-breasted grosbeak) spend their winters in South America and return here to the north to breed in the summer). The herds of wildebeest, zebra and antelope in East Africa circle the Serengeti and Masai Mora ecosystems completing an 1800 mile annual path in their search for fresh grass and safe places to give birth to their young. Wapiti and pronghorns migrate up and down the slopes of the Rockies making use of the abundant grasses and protective landscapes of the mountain slopes in the summer to breed and then more sheltered lowlands to survive their brutal winters. Eastern towhees (a bird Deborah and I have been watching recently in and around our house) spend their winters in nearby sheltered valleys and then spread out through the eastern forests to mate and rear their young in the summer.
Wildebeest. Photo by J. McCarthy, Pixabay
All of these species (and so many more!) rely on their freedom of movement (their vagility) in their environment to survive, feed and breed. These movement patterns may be hardwired into the species’ DNA or they may be facultative responses to conditions of their environments. Abundant winter food, for example, can reduce the migration urge in many species (we have talked about this in previous blogs for American robins and sharp-shinned hawks). Other species, though, migrate regardless of food abundance (ruby-throated hummingbirds, for example, will migrate south in the early fall even if abundant nectar feeding stations are available).
Photo by Ongayo, Wikimedia Commons
Human activities and constructions, though, can interfere with a species’ ability to migrate. Highways, fences, farm fields, oil and gas wells, urban and suburban buildings and streets can all get in the way of natural species movements. Even if they don’t stop migration completely, these human-made impediments can slow down or restrict an animal’s freedom of movement.
A large, global study on animal movements was just published in Science (26 January, 2018). In this study researchers fixed GPS collars on 803 individual animals of 57 different species that ranged in size from pocket mice to elephants. Their movements were precisely recorded and then analyzed taking into account the degree of human development in the specific animal’s environment. Overall, it was found that animals in human developed environments move between one third and one half the distances of animals living in environments not developed by human beings.
The proximate benefits of migration to each species are fairly obvious: the species finds a safe habitat in which to reproduce and new food resources to sustain it. It also spreads out its resource use so as not to overly degrade its environment. But there are also more distal benefits of migration that include seed and nutrient transport sometimes over vast distances. The long term consequences of restricting an animal’s vagility, then, may not only be increased mortality and decreased fecundity of individuals of that species, but also a spiraling decline in the viability of their entire ecosystem.
As Dr. Matthew Kauffman (a wildlife biology professor at the University of Wyoming and one of the authors of this Science article) put it: “Wild animals on an intact landscape move in sync with their needs. When you develop the landscape, that leads to less movement and they are less in tune with the naturally occurring pulse of the landscape.”
Image by T. Knepp, USFWs
Pacific salmon species exhibit remarkable vagility. They hatch from eggs that have been fertilized in freshwater streams and lakes that, depending on the salmon species, may be located hundreds of miles inland and often high up in the mountains of the coastal ranges. Then they are eventually swept downstream into the Pacific Ocean. These salmon may live 3 to 6 years and spend anywhere from 3 months to 3 years in freshwater before they make the transition to marine existence. Adult Pacific salmon at the end of their lives make the often incredibly laborious return to their birth streams or lakes to spawn and then die. Since only a very small percentage of adult salmon that attempt this return migration are successful, this vagile event represents a powerful, evolutionary filter that selects for only the most vigorous and evolutionarily “fit” individuals.
These two phases of salmon migration are not only integrally important to the survival of the salmon themselves, but they also contribute to the overall vitality of these Northwestern ecosystems especially through the delivery of high quality and abundant food to the myriad of salmon predators that inhabit the terrestrial and aerial habitats along the streams.
Wild-bred salmon, though, are declining in the Pacific Northwest primarily due to human mediated impacts. Hydroelectric dams block many of the rivers, spawning habitats are becoming significantly degraded, and water quality all along the streams and even out in the ocean has been compromised by excessive pollution. Further, wild populations of salmon have also been harvested historically in very unsustainable ways.
Photo by R. Hartnup, Flickr
To compensate for these precipitous declines in wild salmon, salmon hatcheries have been established throughout the Northwest to breed, feed and release smolts into the region’s streams. Not surprisingly, though, these hatchery-raised salmon are different from the wild-bred varieties.
Although the hatchery-bred salmon look like and act like their wild counterparts, their survival rates in both freshwater streams and in the open ocean are much lower. Further, if a hatchery-raised salmon does survive long enough to spawn, it produces fewer offspring than the wild-raised fish.
Researchers at Quebec’s Laval University wanted to know if hatchery life was affecting gene selection and expression in the salmon. A 2012 study suggested that hatchery conditions (confined spaces, crowded conditions, high levels of fish waste in the water) might be selecting for specific genes that were counter-adaptive to open stream and ocean existence. The constant influx, though, of wild genes into the hatchery stock made a gene-level explanation unlikely. So the Laval research team looked not at the genes of the salmon but, instead, at their epigenetics.
Methylation of genes occurs during embryonic development and continues after birth. When a gene is methylated its activity is reduced. Depending on the magnitude of the methylation this reduction in activity may be a complete shutting down of the gene. The Laval team found that hatchery-raised salmon had 100 genes that were methylated compared to wild salmon stock, and that ninety percent of these genes were hypermethylated. Some of the hypermethylated regions contained genes that affected the salmon’s immune system and neuromuscular control. Also, regions regulating feeding behaviors and appetite were significantly impacted. Possibly, these epigenetic alterations were making the hatchery-raised salmon less able to survive and thrive in the wild ecosystems into which they were introduced.
Results of this Laval University study were published in the Proceedings of the National Academy of Sciences of the United States of America (5 December, 2017).
So animals need to have the freedom to move about in their environments. When these movements are restricted, both the animal species and its environment are stressed. We need to be careful with our solutions to these migrations problems, though. Often even simple, logical solutions have novel and unexpected consequences.