Harvest rates

[Kenelm Philip]

John Shuey raised an interesting point:

> Yes, salmon have higher reproductive "potential" than do bears, but only
> in optimal years, when they do better than replace themselves.

> The argument that 99.99% of all salmon die is true from egg to adult
> only - yet applied to adults where breeding success in a stable population
> probably approaches 75%.
                                                                      
> In many ways, this is the invalid argument that we he[ar] over and over
> again with regard to collecting adult insects.  To me the logic (and
> theoretical underpinnings) of the argument seem invalid at best.

I suspect that for many insects, what John refers to as 'optimal years'
are the norm. For example, consider the two cases recently when the numbers
of adult butterflies in the taiga in the Fairbanks area crashed. In one
case the aftereffects of the volcano in the Philippines reduced most species
here to around 10% of their normal levels. In the second case a _major_
outbreak of yellowjackets knocked the numbers down to about 2% of normal.
In the first case, normal numbers were re-established the following summer.
In the second case, it took two summers to recover. Nothing like that
could happen with bears!

	Of course, the mortality in both these cases occurred during early
stages, not as adults. Nonetheless--there were indisputably very few
adults around, and yet the population recovered rapidly.

	 As far as _collecting_ adults goes--note that in many (not all)
cases one tends to collect more males than females. Removing male but-
terflies has little effect, except insofar as it reduces the depth of the
local gene pool, as long as some are left.

	An experiment done back in the 60s showed that the same thing
happened when the number of larvae was _increased_ by a large factor--
two summers later the number of adults in the colony was back on the
pre-existing curve of decline.

	It's true that a paper by Ehrlich indicates that light collecting
pressure on a small, isolated colony may have deleterious effects on that
colony--but not on other such colonies in the region. On the other hand,
last summer I found that _Parnassius eversmanni_ is doing fine on Keno
Hill (YT) after having heard stories of how a party of Japanese collectors
had supposedly almost wiped them out. And my own observations on Murphy
Dome (near Fairbanks) show that at least some isolated populations can
survive decades of light-to-moderate collecting. The logic and theory
may be invalid, but in practice butterflies (in healthy habitats) can
stand collecting. (Which is what one might expect, considering the annual
take of far less resilient organisms like caribou and moose that can
occur up here from a more or less stable population.)

	Things may be different when formerly healthy populations are im-
pacted by habitat destruction. Given good habitat, however, I doubt that
reasonable levels of collecting are a problem for most butterflies or moths.

							Ken Philip
fnkwp at uaf.edu