Meet the mice that make the forest

It’s easy to look at a forest and think it’s inevitable: that the trees came into being through a majestic procession of seasons, seeds and soil, and will grow back as long as environmental conditions allow. .

Hidden in plain sight are the creatures whose work makes the forest possible – the multitudes of micro-organisms and invertebrates involved in maintaining this soil, and the animals tasked with delivering seeds too heavy to be carried by the wind to the places where they will germinate.

If one is interested in the future of a forest – which tree species will thrive and which will decline, or whether those threatened by a rapidly changing climate will successfully migrate to newly hospitable lands – one should look to these seed-dispersing animals.

“All the oaks that try to move north are trying to follow the habitable range,” said Ivy Yen, a biologist at the University of Maine, who could be found in the late afternoon in the Penobscot Experimental Forest in nearby Milford. , arranging acorns on a tray for mice and voles to find.

“The only way they’re going to move with the changing temperatures is with the animals,” Ms Yen said of the trees. “Will personality affect this? Will there be people more likely to help? »

Ms. Yen is a doctoral student in the lab of Alessio Mortelliti, a wildlife ecologist who arrived in Maine nearly a decade ago with a particular interest: how seed dispersal intersected with the study emerging animal personality.

Although researchers have previously studied how animals move seeds across landscapes, the possible role of their personalities has remained largely unexplored. The Penobscot Experimental Forest, with its 1,800 acres of closely monitored woodlands managed using various forestry techniques, provided a landscape-scale setting for exploring this question.

Every summer for the past seven years, Dr. Mortelliti’s students have trapped deer mice and southern red-backed voles in their study plots – about 2,000 animals in all – and subjected them to tests that measure where they fall on a spectrum between bold and shy. Before being released, each is tagged with a microchip, much like those used to identify lost animals.

The beacons trigger sensors, like the one Mrs. Yen had mounted above her tray of acorns. Each acorn was painted with bands of color to indicate its species: red oak, bur oak, black oak, white oak, swamp white oak, scarlet oak, pine oak, willow oak. Red oak is already abundant in the region, but the other species have only recently arrived or are expected to as rising temperatures push their ranges north.

Whether these trees succeed in this slow-motion migration — and eventually grace new landscapes with their lofty presence, sequestering carbon, providing shelter and feeding wildlife — will depend on the countless encounters between a mouse or vole and an acorn.

Does the animal take the nut? If so, is the nut eaten quickly or saved for later? Where does the animal hide it? How often do they not return, either because they forget the location or – as often happens to bite-sized creatures in a forest full of hungry predators – because they are eaten first , thus giving the acorn a chance to germinate?

“People see that a forest is regenerating,” Dr. Mortelliti said. “But what people don’t see is that the forest regenerates as a result of the decisions of small mammals.”

In Dr. Mortelliti’s study sites, each of these encounters is documented. When a mouse or vole approaches a tray laden with acorns; a sensor reads their microchip, identifying the animal; a motion-activated camera captures the moment, recording which nut they took. During this season, Ms. Yen said, she would produce over 1,800 acorns.

On that autumn night, Ms. Yen set out five trays, each about 100 feet apart. Around each, she sprinkled a non-toxic fluorescent powder that temporarily adhered to visitors’ feet. When she returned before dawn, armed with an ultraviolet flashlight under which the powder glowed, small constellations of footprints circled each tray and moved away into the darkness.

People don’t realize how many mice and voles there are, Ms Yen said. She estimated that for every 13 steps she took on her way to the site, she would come across a mouse or a vole – not out in the open but hidden under a leaf or cozy in a grass-lined burrow. In the light of twinkling stars and a toenail-shaped moon, the rodents had done their quiet work. Every acorn was gone.

One by one, Ms. Yen followed each lead. Tiny footsteps gleamed under his flashlight, skirting mossy mounds and under fallen branches and up tree trunks, then back down. Mouse walking is the opposite of walking as the crow flies: some trails have run out of steam, the powder has run out. Others ended in a cache – a hollow under a root, a rotting stump, a hole dug in the ground and carefully covered. Ms. Yen scored the final points with small orange flags.

Some acorns, stored for the coming winter, were intact. Others had been eaten, but from the painted shell fragments, Ms. Yen identified the species. With the help of Elizabeth Pellecer Rivera, a graduate research assistant, she took notes on each. Sensor data and camera recordings would later show that much of the collection was done by a particularly industrious deer mouse known to researchers as 982091062973077, a 13-gram male trapped in late September and revealed by tests as being quite timid, albeit with a cautious exploratory streak.

At the end of the season, Ms. Yen, Dr. Mortelliti, and two graduate students, Maisie Merz and Brigit Humphreys, will analyze all of this data and look for patterns.

Perhaps certain personality types will prove more likely than others to select certain oaks. It may take a particularly daring rodent to hoist up a huge bur oak acorn and then stagger under its weight, vulnerable to predators, until it finds a hiding place. Perhaps the shy mice will be more likely to secrete them in the places best suited to sprouting an overlooked nut.

The findings will join a cortege of studies that have emerged from the experience over the past few years, most of them led by Allison Brehm, Dr Mortelliti’s first doctoral student and the person who taught Ms Yen how follow.

In a 2019 study in Ecology Letters, which Dr. Mortelliti described as a “proof of concept,” researchers showed that the personality of small mammals influences their choice of seeds. Earlier this year, the team described how some deer mice, depending on their personality, were more likely than others to hide nuts from red oak, white pine, and American beech in ways that promote germination.

In turn, rodent personality-specific foraging strategies changed when predators were present, the researchers showed in a 2021 Oikos paper.

And land use modifies these dynamics. For example, the 2019 study found that in areas that had been logged years earlier, small mammals tended to be bold. A study the following year found that a more natural forest, with a mixture of habitats rather than the uniformity favored by most commercial logging, contained a greater diversity of personalities.

“This diversity of personality types is maintained in populations because it’s a good thing, just as genetic diversity is a good thing,” Dr. Brehm said.

Rafał Zwolak, an ecologist at Adam Mickiewicz University in Poland who studies seed dispersal and animal personality, called the research “absolutely groundbreaking”.

“I hope their work will inspire researchers in other labs, working in other ecological systems, to focus on this topic,” he said.

Asked to define the practical implications of his research, Dr Mortelliti said: “Preserving a diversity of personalities. There is no ideal personality; on the contrary, different individuals perform different roles. Depending on the circumstances – drought, natural disturbances, fluctuating predator populations – different personality types can emerge. These nuanced dynamics don’t preclude logging, Dr. Brehm said, but they do argue for caution.

“If you have to manage a landscape, you don’t want to manage it the same way,” she said. “You want to handle different parts differently so you have a heterogeneous landscape.” Techniques can be used to maintain a variety of tree species, ages and sizes, trying to mimic what would occur naturally.

Much remains to be studied, Dr. Mortelliti noted. The measures of shyness and boldness are not the whole animal personality; they are simply relatively well characterized and easy to measure in the field. Oaks aside, hundreds of other plant species change range, each following its own animal-induced trajectory.

As Ms. Yen finished her work, night gave way to twilight before dawn. A blue jay called; chirped a red squirrel. Both are seed dispersers with personalities that can affect their contributions to the forest. The same could be said of bears, foxes, crows, turtles and even ants – a whole yet unexplored menagerie affecting not only plants but even fungi.

“I only watch two species at night,” Ms. Yen said. “It’s a very small snapshot of what’s going on.” A full picture may not emerge for decades, but the outlines are already clear: it takes a lot of personalities to raise a forest.

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