Williamson's sapsucker (Sphyrapicus thyroideus) COSEWIC assessment and status report: chapter 8

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Limiting Factors and Threats

The principal limiting factor to Williamson's Sapsucker populations in Canada appears to be limitations of amount of suitable breeding habitat. In the Okanagan-Greenwood population (89% of the thyroideus Total Population) primary habitat consists of old-growth stands with large veteran Western Larch nesting trees with heartrot, smaller (20-40 cm DBH) Douglas-fir and Western Larch for sap well creation, and older trees with abundant carpenter ants for feeding nestlings. All three habitat components must be present to create suitable habitat. Most (88%) of the Williamson's Sapsucker nests found in this population were associated with stands >170 years old, or with Western Larch trees >170 years old in multi-aged stands (see Habitat section).

The primary habitat threat to these old-growth Western Larch stands is timber harvesting on Crown Land, and land clearing on private land.  Old trees that Williamson's Sapsucker use for nesting and foraging for carpenter ants will not be replaced under current timber management schemes based on approximate 100-year rotations. Even if these individual trees may be replaced by retaining seedtrees or reserved mature trees in clearcuts, these clearcut stands will not be suitable habitat for Williamson's Sapsucker throughout most of the rotation period since adjacent stands containing old-growth trees will still be required to meet most foraging needs.

The trend in habitat over the past 10-14 years, and the next 10 years can be estimated from available data for the Okanagan-Greenwood population. The reduction in suitable habitat from the early 1990’s to 2004 was 23%, i.e. reduced from 19.5% to 15% of the Okanagan-Greenwood population Area of Occupancy (see Habitat Trends section). Over the next 10 years, timber harvesting in this area is expected to proceed at the approximate rate of 1% of the area per year, 80% of which is being directed at the older Western Larch stands occupied by Williamson’s Sapsucker (see Habitat Trends section). In 10 years, this could reduce the suitable habitat from 15% to 7%, or a reduction of 53%. At that rate of timber harvesting, suitable habitat for Williamson's Sapsucker in this Area of Occupancy could be almost completely eliminated within 20 years, or at least reduced to an amount that may not be able to sustain a population, as may have already happened in the Rocky Mountain Trench near Cranbrook.