Corvids hold the key to saving the world’s forests

Imagine you’re a tree. Like every other organism on this planet, you’ve got two life goals. The first is to survive long enough to reproduce, and the second is to actually reproduce. Your aim is to cast out your seeds as far as possible so that your descendants will have room to grow. After all, if you just drop your seeds straight to the ground, your children’s nascent root systems will compete with your own mature one and will have trouble surviving. The solution? Strike a bargain with an animal. You provide the food, and the animal provides the baby carriage to transport your precious cargo a safe distance away. According to a new study, some of the best animal couriers around are corvids, the family of birds that includes crows, ravens, rooks, jays, and magpies.

But the immobile tree makes a gamble: by making their seeds tasty and nutritious, birds are highly motivated to gather up as many as possible. Instead of scarfing them all up at once, they cache them and return later to feast. But even the cleverest western scrub jay or Clark’s nutcracker won’t quite remember where he or she hid every single seed. Some of those seeds will go un-eaten and, with luck, they will become new plants. Those plants will grow to become trees which will provide food for the corvids’ own descendants. The two sets of organisms have come to rely on each other. That’s why corvids are so integral to maintaining forested ecosystems.

The better we can understand the relationship between these brainy birds and the trees they rely on for food, argues University of Nebraska-Lincoln biologist Mario B. Pesendorfer in the journal The Condor, the better we can rely on that relationship when it comes to restoring and preserving those ecosystems.

Take the jays of North America. Florida scrub jays, western scrub jays, and island scrub jays can cache between 5,000 and 6,000 acorns during a good year. Oak forests depend on those years for their continued survival. When researchers compared acorns that had merely fallen to the ground and rolled away and those that had been cached by jays, it was the cached acorns that were more likely to become saplings. Caching is even more important following a forest fire; hidden acorns remain likely to sprout through the burned soil, while those resting above ground would burn right up.

And a study of blue jays found that a single population cached more than 130,000 acorns from just 11 oak trees one year. More than nine of every ten of those acorns were cached in habitats suitable for new oak trees. Some blue jays will even transport those acorns up to four kilometers away, allowing the oak ecosystem to expand.

Or take the nutcrackers of Europe. They spend most of the fall months collecting and hoarding seeds from pine trees. They can transport up to 150 pine seeds at a time in a special pouch in their throats and have been known to fly up to 32 kilometers away before hiding them in small caches of between one and 15 seeds. And because those seeds come from a variety of individual trees, the forests that emerge after they grow are genetically diverse. North American nutcrackers are also responsible for maintaining healthy, genetically diverse populations of trees such as the ponderosa pines that tower over the Rocky Mountains. As climate change alters the birds’ habitats, their caching efforts allow the trees they rely on to migrate. In other words, the nutcrackers facilitate the trees’ climate-change adaptations.

Healthy, diverse oak and pine stands, in turn, provide important resources for a host of other plant and animal species. They provide food and shelter for vertebrates of just about every taxon, as well as millions of invertebrates. They alter the chemistry of soil to facilitate the growth of other plants. They help watersheds their water.

Meanwhile, humans continue to degrade oak and pine ecosystems. The reasons for that damage are diverse: unnatural fire management protocols, for example, or overbrowsing by ungulates, or the introduction of invasive species. “Long-distance seed dispersal and subsequent ecosystem engineering processes could be crucial to the ability of oak and pine ecosystems to respond accordingly,” write the researchers. By ensuring the health of corvid populations, conservationists can in turn ensure the health of whole ecosystems.

In Germany, foresters keep jays on the payroll. Old oaks that provide an abundance of acorns are preserved, while young oaks from elsewhere in the forest are harvested. If seed availability is low in a given year, foresters provide the jays with baskets of acorns and allow nature to take its course. This program allowed jays to contribute to the growth of between 2,000 and 4,000 trees per hectare. Other research has shown that by protecting the jays, forested lands become more profitable.

And in Sweden, the work of Eurasian jays in urban habitats means that humans do not have to be paid to plant acorns. At $2 per acorn, the birds saved $9,400 per hectare each year. Those funds could then be reallocated towards other conservation efforts. Several Mediterranean islands have employed a similar scheme to economically reforest abandoned agricultural fields.

Possibilities for taking advantage of corvid scatter-hoarders clearly abound. Pesendorfer and his colleagues argue that if biologists can better understand the circumstances in which the birds can most efficiently and effectively transport and cache their bounty, conservationists can in turn leverage that information to enhance forest conservation efforts. This would “provide a promising way forward in modern, cost-effective conservation,” they say. – Jason G. Goldman | 24 February 2016

Source: Pesendorfer, M. B., Sillett, T. S., Koenig, W. D., & Morrison, S. A. (2016). Scatter-hoarding corvids as seed dispersers for oaks and pines: A review of a widely distributed mutualism and its utility to habitat restoration. The Condor, 118(2), 215-237. DOI: 10.1650/CONDOR-15-125.1.

Header image: Shutterstock

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