Hi Wayne (& all),
Sorry for the slow reply. I think your "alternative hypothesis" is a
very good one, and lines up with some of the thoughts that I expressed
in my previous posting about Mark Greenfield's dead Barn Owl.
There used to be a vole biologist on OBOL many years ago (last name
Wolff if I remember right, which is a very good name for a vole
biologist), but last I heard he shifted to a university in Missouri,
about 15 years ago. He's the one who told me about the disease aspect of
the boom-bust population cycle.
For that model, it makes sense that the cycle length would be variable
since it depends on achieving a critical density of voles, not just
time. Also you'd expect that different areas within the region could get
out of phase.
But just to clarify one of my earlier points, I'd like to emphasize that
I was talking about a shift from perennial to annual grasses. Annual
grass-seed fields get plowed every year, so there is always a high
amount of rodent mortality. Annual ryegrass (the main annual grass crop,
though wheat is making a comeback) tends to be an impoverished landscape
compared to perennial ryegrass, fescue, or orchardgrass fields where
rodent populations have a chance to build up over a 3 to 5 year period.
Native prairie restorations are effectively "retired" from the plow. So
rodent populations are limited only by their own natural population
cycles, in combination with their natural predators such as kites and
rough-legged hawks.
I agree fully that it could be good to evaluate the winter raptor survey
data to see if there is synchrony that could be related to vole
population fluctuations. A good proxy for the latter could be sales of
zinc phosphide pellets.
--
Joel Geier
Camp Adair area north of Corvallis
On Sun, 2016-03-20 at 21:18 -0700, whoffman@xxxxxxxx wrote:
Hi -
Rough-legged hawks are more vole specialists than are Red-tails. Many
vole species, including our grassland ones, are strongly cyclical.
When I was at OSU, we were taught that they were typically on a
4-year cycle. I think they are now recognized to be a bit more
variable in cycle length? In any case, this strong and intense cycle
(2+ orders of magnitude between high years and low years) will make it
very difficult to see and interpret habitat based trends in vole
predators.
An alternative hypothesis might be that the low periods of the cycles
get lower in the annual grass fields than in more natural grasslands,
and so the predators are more constrained in those years.
White-tailed Kites are even more specialized on voles (and ecological
equivalents - Hispid Cotton-Rats in the southeast) than are
Rough-legged Hawks. It would be interesting to see how similar
their distributions are in the Willamette Valley, and whether they
expand and contract in synchrony? Are the winter raptor survey data
stored in formats that would allow that to be tested?
Wayne
