Naturespeak

Mutualism: The Douglas squirrel and west coast conifers

Whistler Naturalists

Now that the remaining Whistler and Brackendale black bears have finally gone off for their winter naps, the only wild resident mammals (some local dogs not included) you are likely to see this time of year are the Douglas squirrels ( Tamiasciurius douglasi). If you haven’t seen them, you are at least likely to hear their familiar warning churring at predators entering their territory, as well as their more pleasant chirping and trilling calls from the coniferous trees of local trails and backyards. To hear the Douglas squirrel call, check out www.bear-tracker.com/dsqrlscold.mpg.

The Douglas squirrel is also known as the chickaree and the pine squirrel. It spends the majority of the year in its summer nest, built of shredded bark, twigs and moss, in the lower levels of a conifer within its 2-3 acre territory. Occasionally, it will take over and rebuild a deserted birds nest. In winter, the Douglas squirrel will move its nest to a hole in a tree, such as one made by a woodpecker. Again, it will line this nest with shredded bark and moss.

The Douglas squirrel is active year round. During very cold spells, like the ones we’ve experienced over the last month, it will take shelter in its tree cavity nest and sleep to conserve heat and energy. But even during these times it will leave its nest for short periods of time to forage for its food middens.

Douglas squirrels feed on the nuts and seeds of all the coniferous (evergreen) trees within its territory. In our area these would be the Western White and Lodgepole pines, the Douglas and Subalpine firs, the Western hemlock, the Sitka spruce and any of the introduced conifers. Other foods eaten by the Douglas squirrel include; flowers, berries, invertebrates, the young of small mammals like mice and small bird eggs. It also eats mushrooms, which are first placed in the crotch of a tree to dry out.

To obtain conifer seeds, its main food source, the Douglas squirrel begins at the bottom of the cone turning it in a regular spiral pattern as it cuts away the scales to expose the seeds. In the spring and summer it will eat the seeds of mature cones; you may have spotted the piles of scales at the bottom of local conifers. But, in mid to late summer, the Douglas squirrel will also harvest green cones in preparation for winter. These cones are buried in moist places in large piles called middens to keep them tender. Sometimes these middens are raided by jays and crows, which have watched and waited while the squirrel worked.

Scientific studies have determined that the Douglas squirrel and the west coast conifers on which it feeds have a mutualistic relationship. Mutualism is a relationship between two organisms in which each gains some benefit. Seeds dropped during the summer by the feeding Douglas squirrel may fall on moist soil and germinate. The cones buried and forgotten over the winter may germinate and grow the following spring. Some studies have indicated that up to 50 per cent of the coniferous trees in certain areas of the west coast may have germinated by these means.

Another aspect of the mutualism between the Douglas squirrel and the west coast conifers occurs in the late spring. While obtaining the seeds of cones, the Douglas squirrel gets the pollen from these cones on itself and then while it feeds on the seeds of other cones, the pollen rubs off and fertilizes them, thus providing the conifers with another means of pollinating their seeds (besides releasing the pollen into the air to be carried by the wind).

These scenarios increase the reproduction rate of the conifer. More conifers result in more habitat and food sources for the Douglas squirrel. Thus both are benefited, benefiting us all.

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