Tuesday, November 17, 2009

Middle Midden Managers

It might be easy to miss the red squirrel's role in the big scheme of things. But it is persistence that makes her one of the main "movers and shakers" in Montana's coniferous forests. Her eight-ounce presence impacts everything from bears to birds, and even the forest itself.

Red squirrels (Tamiasciurus hudsonicus) are solitary except during breeding. Each adult defends a territory that contains two critical features; several grass nests built on branches or in tree hollows, and one or more food caches called "middens."

Chattering red squirrel at feeding site (c) John Ashley

Mother squirrels normally give birth once a year to 3-4 helpless, hairless young. The young won't even venture from the nest during their first month, and they'll nurse for more than two months, but they'll reach adult size in just four months.

The youngsters need to grow fast, because they must establish their own territory before winter arrives; only one in four will survive until spring. At most, they only live 6-7 years, so the key to survival lies buried in those traditional midden sites.

Makeshift red squirrel midden with several hundred Douglas fir cones in one corner of the neighbor's tool shed (c) John AshleyRed squirrels specialize in harvesting and eating the seeds that are stored in the cones of spruce, fir and pine trees. They may cut and cache two-thirds of the yearly seed crop produced in an area. In a poor cone-production year, one red squirrel territory may only grow enough cones to provide half the annual energy needed by that squirrel.

In captive experiments, red squirrels needed to eat 125 (new) to 175 (year-old) spruce cones per day to survive, when no other food was provided. Wild squirrels are not too picky and they'll also eat mushrooms, fruit, nuts, insects, buds and bark. But the conifer cone seeds are their first choice.

So in the fall, these hard-working squirrels cut down and cache the extra cones that they will eat all winter. Lots of cones. In one study, between 12,000 and 16,000 per year, per squirrel. Each of their middens might contain many hundreds of cones. If extra cones accrue each year, the middens will eventually store enough food to get the squirrel through one or maybe even two winters in years of bad cone crops. These mega-middens can measure several feet across.

And bigger middens mean fatter bears.

Black and grizzly bears regularly raid the red squirrel middens, especially if they are stocked with the cones of white bark pines. These pine nuts are almost one-half fat, and the bears will eat them almost exclusively in the fall of a good crop year (September - November), and again the following spring (March - May). Black bears sometimes climb the trees to break off cone-bearing branches, but black and grizzly bears get almost all of their white bark pine nuts by digging up red squirrel middens.

Grizzly bears also eat the nuts from limber pine cones along the eastern front of Montana's Rocky Mountains, but not in Yellowstone. In one study area, the Yellowstone wildfires in 1988 burned about one-half of the white bark pine stands. Afterwards, there were about one-third fewer red squirrel middens, the middens were soon only half as big, and the number of bear excavations fell by about one-half in the years that followed.

That's a big impact. But these little red squirrels impact more than just bears.

Pine martens in Yellowstone were twice as likely to use "subnivean" (under the snow) areas that included red squirrel middens. In nearby Wyoming, the most important selection variable for marten den locations was the presence of middens.

Up here in western Montana, red squirrels were identified as the second-most common prey item for Canada lynx during winter (although weight-wise, snowshoe hares accounted for 96% of lynx diet).

Red squirrels are also a regular menu item for some forest raptors, like Great-horned Owls and Northern Goshawks. One Goshawk researcher found red squirrel bones in more than half of the pellets regurgitated by the hawks. But when it comes to songbirds, it's the red squirrels who are doing the dining.

Red squirrels can eat a lot of the eggs and chicks in the nests of smaller birds. In lodgepole pine forests, songbird species that nest high up in the tree canopy are two to three times more abundant in stands that don't have red squirrels. Birds that nest in tree cavities don't seem to be impacted.

Sleepy red squirrel warming up in the morning sun (c) John AshleyThese busy squirrels also impact individual trees, and even the forest itself.

Because some seed caches are never eaten, red squirrels are a key tree planter and seed disperser. They also harvest mushrooms, drying them up in tree limbs (in the wind) to save for eating in winter.

Many of these mushrooms are the fruits of mycorrhizal fungi that grow around the trees' roots and help the trees absorb water and minerals. A mushroom hanging out in the branches becomes an over-achiever, spreading its spores much farther than it could at ground level.

In forest types that burn frequently, like lodgepole pine, the little red squirrels can alter post-fire seedling rates, which in turn can impact the course of forest succession. Lodgepole pines produce two kinds of cones, regular and "serotinous." Serotinous cones are covered in resin and do not release their seeds until the resin is melted by fire.

This is a great fire adaptation, until a seed predator moves in and eats most of the seeds that are stored in the canopy, in those serotinous cones. Lodgepole pine stands in the Rockies that don't have red squirrels consistently bear nearly 100% serotinous cones. But the stands with squirrels average less than 50% serotinous cones.

Forests and forest birds, pine trees and pine martens, black bears and grizzly bears. The hard-working red squirrel impacts each of these natives. Separately, each interaction might be easy to miss. But a wider view lets us see how the persistence of this pint-sized squirrel fits into the bigger picture -- one pine cone at a time.

Behind the lens: This red squirrel (above) strategically positioned itself on a branch that was awash in a morning sunbeam -- an energy-effecient way to warm up in the morning, if your hands are too small to hold a cup of coffee. Nikon D700, 500mm f4 lens on monopod.