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Enlarge / That is an aptly named sheep.

In many areas of the globe, native species have been wiped out of large areas of their range even though some habitats that could support them were left intact or later restored. That has allowed conservationists to reintroduce these species, sometimes with spectacular success. The North American bison, for example, has gradually returned from near extinction largely due to reintroductions from the few small herds that were once left.

But not all of these reintroductions have worked out, and a paper in this week’s Science suggests a reason: over generations, native populations develop a “culture” that helps them to understand when and where to migrate. New populations, dropped into an unfamiliar landscape, tend to sit still and don’t make the most out of their habitat.

Waves of green

Seasonal migrations are common throughout the animal kingdom, and most of the attention is drawn to the more dramatic ones, like the multigenerational travels of the monarch butterfly or the spectacular distances covered by some birds. But many migrations are relatively local, as animals may shift locations without venturing out of their larger habitat. The reasons for this are typically practical: moving to breeding grounds that predators can’t reach easily, for example.

Another motivation is food. The best grazing tends to be on plants that have just started their seasonal growth, and the site of that growth shifts with the temperatures. That typically means such locations shift uphill and toward the poles as the warming of spring progresses. Ecologists refer to following the areas of peak growth as “surfing the green wave,” and it drives the movement of mammalian populations around the world.

But how do herbivores know how to surf? The new research takes advantage of transplanted populations to find out.

This team’s primary research subject is the bighorn sheep, which was wiped out in many locations due to hunting and diseases transmitted by domestic sheep. In recent decades, with the population in many areas rebounding, conservationists have begun restoring it to areas where it was once common by transplanting sheep from existing populations. In some cases, the animals have gone into completely empty habitats; in others, they’ve supplemented a small existing population. (Some parts of the new analysis also include data on moose, which are reintroducing themselves to 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 from native populations and some that had been recently reintroduced. The location of the sheep could be compared to the green wave using satellite data, which can show areas of the habitat literally turning green in the spring. They used this data to produce an idealized surfing pattern, showing what a migration optimized to get the most out of the vegetation change would look like.

Sitting still

In established populations of bighorns, anywhere from two-thirds to all of the sheep migrated to follow the foliage. By contrast, less than 10 percent of the transplanted populations went anywhere. The few that did migrate were all transplants that went into areas with existing populations, suggesting that they learned when and where to move through social interactions with the existing populations. This finding helps settle a debate about how existing populations maintain their migration patterns across generations.

Compared to the foliage-optimized migration pattern, the existing populations did twice as well as the ones transplanted into new habitats. This didn’t mean the migratory animals were completely efficient; things like predators and calving also influence when and where they move. It’s just that established populations balanced their needs much better.

So are transplanted individuals doomed to sit still? To find out, the researchers expanded the study to look at populations that had been transplanted anywhere from 10 to 110 years earlier (this is where the moose data was added). The analysis shows a clear trend: it takes anywhere from 30 years to a century for populations to begin surfing the green wave. So animals do start to work out good migratory patterns over time, but it takes decades for that to become established cultural knowledge in their herds.

The report is interesting science in that it shows the importance of knowledge diffusion within social networks among grazing animals. But it also has practical implications for wildlife reintroduction programs. It may not be successful if we try to reintroduce animals to areas where they once existed if human development has blocked off access to the best migratory routes. And planning for the animals’ return can’t simply involve where they’re being placed—it will need to account for where they’ll be moving decades from now.

(Editor’s note: A Møøse once bit my sister.)

Science, 2018. DOI: 10.1126/science.aat0985  (About DOIs).



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