In many areas of the world, native species have disappeared from large areas of their range even though some of the habitats that could support them are left behind sooner or later. That has allowed conservationists to reintroduce these creatures, sometimes with surprising success. The North American bison, for example, has gradually returned from near extinction largely due to introductions from the small herds that were once left behind.
But not all of these innovations have worked, and a book in this week Knowledge suggests a reason: over the generations, the native population developed a “culture” that helped them understand when and where to go. New residents, dropped into an unfamiliar landscape, tend to sit still and not make the most of their habitat.
Waves of green
Seasonal migrations are common throughout the animal kingdom, and much attention is drawn to the more spectacular ones, such as the multi-generational journeys of the monarch butterfly or the incredible distances covered by some birds. But most migrations are relatively local, because animals can change locations without leaving their large habitat. The reasons for this are often practical: moving to breeding grounds that predators cannot reach easily, for example.
Another factor is food. The best grazing tends to be on plants that have started their growing season, and the point of growth changes with the temperatures. That means such conditions move up and toward the poles as spring warming progresses. Scientists refer to following areas of high growth as the “green wave,” and it drives the movement of mammal populations around the world.
But how do herbivores know how to surprise? A new study is taking advantage of the living population to explore.
The main research topic of this team is the big sheep, which is being destroyed in many areas due to hunting and diseases spread by domestic sheep. In recent decades, with populations in many areas recovering, conservationists have begun reintroducing it to areas where it was once common by moving sheep from existing populations. In some cases, the animals have gone into completely empty habitat; in others, they have supplemented an already small population. (Some parts of the new analysis also include data on moose, which also reproduce in many areas of the US, such as New York.)
To track the animals themselves, the authors took advantage of GPS collars fitted to more than 200 bighorns, some native populations and some recently reintroduced. The positions of the sheep can be compared to green waves using satellite data, which can show the areas of the habitats turning green in the spring. They used this data to develop an accurate navigation model, showing what the optimal migration to get the most out of the vegetation change would look like.
That is sitting
In the established population of bighorns, anywhere from two-thirds to all of the sheep leave to follow the foliage. By contrast, less than 10 percent of the population moves anywhere. The few who moved had all moved to areas with existing populations, suggesting that they learned when and where to move through social interactions with existing residents. This finding helps resolve the debate about how existing populations maintain their migration patterns across generations.
Compared to the foliage-optimized migration process, existing populations performed twice as well as those transplanted into new habitats. This does not mean that migratory animals are completely efficient; things like predation and calving also influence when and where they live. It’s just that established populations balance their needs better.
So are living people doomed to sit still? To investigate, the researchers expanded the study to look at populations that had been planted anywhere from 10 to 110 years ago (this is where the moose data was added). The analysis shows a clear trend: it takes anywhere from thirty years to a hundred for residents to start surfing the green wave. So the animals began to work out better migration patterns over time, but it took decades for that to become an established culture in their herd.
The report is scientifically interesting in that it shows the importance of knowledge dissemination within social networks among grazing animals. But it also has practical implications for wildlife rehabilitation programs. It may not be successful if we try to reintroduce animals to their original habitats if human development has blocked access to the best migratory routes. And planning for the animals’ return can’t just be about where they’ve been placed — it will need to consider where they’ll be living decades from now.
(Editor’s Note: We accept once bitten my sister.)
Knowledge2018. DOI: 10.1126 / science.aat0985 (About DOIs).