Hello Mad Tom :)
Populations will go through natural cycles like this, mainly due to abiotic
factors that quickly change the environment-in-critical times. Examples
would be big flash storm events right-in-the time of spawning, which sweeps
away eggs or suffocates the eggs with fine particulates, or something like a
major temperature spike during the development phase that greatly decreases
the success of the hatch. When consistent human induced abiotic changes
(slitation, flood pulse, pollution, enrichment) occurr on a watershed...
This is a mechanism by which species are lost from the system. We
permanently interrupt their spawning success and in a few generations, you
have a species that is now rare in the system. The year class success
suffers, and an accumulation of multiple years of poor year class suddenly
looks like there aren't any of that species left.
As well, predation and disease take their normal toll on the remaining
recruiting members of the population, and represent another pressure on year
class success. These biotic factors typically have consistent effects on a
population, until a virulent disease or a superior competitor (either
functionally or predatively) is introduced, which is another way humans have
perfected-in-reducing overall biodiversity.
I think the best annecdote I've seen written concerning this is in
Trautman's Fishes of Ohio, where he talks about his observations on
blennoides in an Ohio stream, and how abiotic factors ultimately delivered
both boom and bust years for the local greensides. So it's funny that you
too observed this in the same species :)
As a side note....
This is also in part why some folks get persnickity about making takes on
imperiled species, even though it may appear that a local population is
robust and very healthy. You never know what is coming up or down in the
recruiting year classes, and we typically take the most robust adults (I am
soooo guilty of this :) because they look the best, and this is just human
nature. The Inner Gollum (tm), so to speak.
When the range of an organism has been pinched down to two or three healthy
populations (thus warranting imperiled status) or the species was already a
rare endemic, a genetic bottleneck has already occurred. You really don't
know what part of the genetic pool you're removing by taking that animal
home. It's probably of smaller importance than I'm making it sound... But
we _don't_ know... And I personally choose to err on the side of caution.
The Muddy Maumee Madness, Toledo, OH
It's never too late to have a happy childhood.
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