Human activities have plenty of obvious impacts on wildlife. Overhunting and overfishing, habitat destruction and the side effects of climate change are all notable forces that have cause wildlife populations to shrink or move. Perhaps a less talked-about, but nonetheless significant, impact of humans on wild animals, however, is our ability to alter — and even destroy — their migration patterns, a phenomenon that could have serious ecological consequences.
“A lot of attention has rightfully been focused on protecting endangered species, preventing species from becoming extinct,” said David Wilcove, a professor of public affairs and ecology and evolutionary biology at Princeton University. “That is indeed a top priority. But there’s a growing interest in the idea of protecting animal migrations both for their ecological importance and for their cultural importance and for protecting these species while they’re still abundant.”
The newest instance of humans versus wildlife migration has cropped up in a new paper, just published in the journal Science Advances on Friday. The paper documents the migration behavior of young white storks and observes that the presence of human societies can have a significant influence on where the birds stop along their migration route and for how long — in some cases even causing the storks to halt their journey entirely in favor of spending their winter feeding at landfills.
“It seems that the waste that is provided by humans is a big food source for some of the wintering populations,” said the paper’s senior author Martin Wikelski, director of the Max Planck Institute for Ornithology in Germany.
The researchers used GPS devices to follow the migrations of eight populations of juvenile white storks, a species found throughout Europe, western Asia and parts of northern Africa. They were interested in observing how different migration behaviors in each of the populations might affect the amount of energy the birds expend during the journeys and, thus, their chances of survival.
The storks in this study came from a variety of home bases, including Greece, Poland, Russia, Spain, Germany and Uzbekistan. In general, different populations are known to migrate south into different areas during the winter, some into sub-Saharan Africa, and others overwintering north of the Sahara. The notable exception is the Uzbekistan population, which researchers have observed no longer migrates at all, although it used to overwinter in the Middle East.
In general, the researchers were able to confirm the idea that the less energy expended by the birds, the greater their chance of survival during the migration. And they also observed that the birds are strategic when it comes to minimizing their energy expenditure. One of the most notable observations was the storks’ tendency to take advantage of human resources along the way — even if the result was a disrupted or halted migration.
The researchers found that human population density along the migration route had a high influence on the storks’ behavior. Storks that traveled through heavily populated landscapes tended to stop frequently and be highly active during these pauses, presumably taking advantage of human resources by moving around and feeding during these pauses.
In a few cases, the birds’ migration was entirely disrupted by human presence. A handful of young storks from southwest Germany halted their migration in Morocco instead of continuing down through Africa with the rest of their kin, choosing to spend the winter feeding on garbage dumps in the area.
And the Uzbek storks, who no longer migrate at all, are believed to have changed their behavior over time in response to human activity. In Uzbekistan, there are numerous fish farms and ponds that provide an easy food source for the birds, minimizing the need to migrate south in the winter, Wikelski said.
The question that remains is whether these human-influenced changes in migration patterns are good or bad for the animal kingdom. The Uzbek storks seem to be getting a good deal out of their situation — and it’s also notable to point out that the garbage-eating German storks who paused their migration had a higher survival rate than the ones who continued on with their migration. As the authors wrote in the paper, “Feeding on anthropogenic food sources such as landfills seems to be beneficial because birds can shorten their migration distance and decrease their daily energy expenditure.”
So on one side of the coin, these migratory changes actually seem to be a boon for individual birds, giving them a better shot at survival through the winter than they would have making the long haul south. But on the flip side, disrupting a long-standing migration pattern can have bigger consequences for the ecological webs these animals belong to.
“If you have large numbers of animals on the move, they’re going to affect the ecosystems that they move through,” said Wilcove, the Princeton professor, who was not involved in this particular study.
For example, migrating birds, specifically — storks included — often eat large quantities of insects during the journey, providing a valuable pest-control service to plants along the way. Halting a migration could take this service away and allow insect populations in certain areas to get out of balance. Migrating animals can also play an important role in seed dispersal and nutrient cycling in the ecosystems they move through.
So there’s a clear benefit to protecting the longstanding migratory patterns of wildlife. But unfortunately, it’s not just storks that are changing their behaviors. Humans have had clear influences on the migratory patterns of other species as well, Wilcove said.
One notable example he pointed to is the effect of dam-building in the Pacific Northwest, which has severely disrupted the migration of salmon in the region. This is a problem because salmon are an important component of the nutrient cycle.
Salmon are born in freshwater, but migrate out to sea while they’re still young and spend the majority of their lives in the ocean. When it comes time for them to reproduce, they migrate back upstream, spawn and die soon after. Their bodies then decompose, and their nutrients are distributed into the freshwater system. But as salmon are blocked from moving through certain parts of the freshwater systems in the Pacific Northwest, scientists believe there’s been a drop in the nutrients, notably nitrogen, that help sustain plants that grow within these areas, Wilcove said.
Another perhaps less dramatic example is the behavior of geese in the United States. “By virtue of the creation of corporate lawns and golf courses and ponds, we’ve basically short-circuited the goose migration, and a lot of these resident geese simply weren’t here 40 years ago,” Wilcove noted. “That’s a case where large population of an animal has essentially stopped migrating.”
The issue of altered migration habits is starting to draw the attention of ecologists — but the question of how to preserve the migratory habits of wildlife is a complicated one. The first step is ensuring that animal populations that are already abundant stay that way, Wilcove said.
“You can’t wait until a species is rare and then start to worry about it if you want to protect the great migration,” he said. Humans should also become more aware of the activities that seem to be having the greatest effects on animal movements. The issue is a challenging one, though, as migrating animals tend to cross borders. Protecting them “requires a much greater degree of coordination across jurisdictions than we’ve seen,” Wilcove said.
The new paper on storks highlights many of these concerns with its observations of changing stork behavior. But it also leaves some room for optimism. As Wikelski pointed out, the study indicates that the storks, at least, are resourceful and good at changing their behavior in ways that benefit their immediate survival. These behavior changes may not necessarily be good for the wider ecosystems they belong to in the long run, but they do send the message that certain types of wildlife may have hope of adapting to an increasingly anthropocentric world.
The challenge for humans will be to figure out how the behavior changes we induce will affect the world’s ecological webs in the long run.
“I see a certain amount of hope in this study in the sense that it shows the ability of animals to adapt to a changing world,” Wilcove said. “I also see an element of sadness that they have to do that — and a great deal of uncertainty as to whether we can likely assume that all of our migratory animals are going to be able to figure out how to continue to travel in the face of all the changes we are causing.”