Mike Patterson wrote:
>
> ================= Begin forwarded message =================
>
> From: 76627.3473 at compuserve.com (Marianne Hickson)
> To: obol at gaia.ucs.orst.edu (obol)
> Subject: Snowy Owl Bio-Ecology 101 (long)
> Date: Tue, 26 Nov
>
>

> Irruptions are usually attributed to a crash in the lemming (look like large
> meadow mice, but with short tails) population. Others point to huge increases
> in the snowy owl population during peak lemming years; snowy owl clutch sizes
> can boom from 3 up to 10 or 15 when lemming populations explode, with way too
> many left over when prey populations decline in following years. By comparison,
> great horned owls average less than 2 young/yr.
>
> Lemming populations do crash regularly, with accounts varing from every 4 yrs.
> to a range of 4-7, and some reports of an even more dramatic crash every 10-12
> yrs. Such declines reportedly often coincide with low snowfall years, possibly
> reducing breeding activity of lemmings, and also subjecting them to increased
> predation. Additionally, lemmings frequently die off during years of sparse
> snow cover from malnutrition and exposure, especially in genetically weakened
> populations resulting from rapid population buildup. A popular theory is that
> the overcrowding halts breeding for several years at a time.
>
Mike,
I've tried to convince some folks on another list that the irruptions
of the Snowy Owl are more often do to _decline_ in lemming pops not an
increase. Earlier this year we looked at this particular cycle in a
Zoology class at Vanderbilt Universtiy. When the lemmings become
overpopulated the plants that they feed on develolp a specific enzyme
which prevents the lemming from digesting the plant. So, the lemmings
can eat all they want and it just passes on through, so to speak. Thus,
the plant is saved from near annihilation by starving, in effect, its
predator. Naturally this goes right up the food chain to the Snowies and
all of those big clutches, past and present, confront a nearly
insurmountable crisis. So, the young and less dominant owls are forced
by necessity to leave.
Zach
zath at u.washington.edu