Category Archives: population dynamics

It’s been a while since I’ve been this proud of a paper

I don’t usually blog about my own papers, except in some rather meta ways, but last week saw the publication of a paper I’m really, really proud of.  And it has some interesting backstory, including its conception right here on Scientist Sees Squirrel.

The paper is called “Site-selection bias and apparent population declines in long-term studies”, and it’s just come out in Conservation Biology.  It started, back in August of 2016, with a post called Why Most Studied Populations Should Decline.  That post made a very simple point about population monitoring, long-term studies, and inferences about population decline.  That point: if ecologists tend to begin long-term studies in places where their study organisms are common (and there are lots of very good reasons why they might), then we should expect long-term studies to frequently show population declines simply as a statistical artifact.  That shouldn’t be controversial – it’s just a manifestation of regression to the mean – but it’s been almost entirely unaddressed in the literature.

A bunch of folks read that blog post.  Some were mortally offended. Continue reading

Why most studied populations should decline

Figure: Time series for two populations, each fluctuating in size. At time zero, I start a long-term study, and can choose either of the two populations (open circles). At some other time, I recensus (closed circles).  Red arrows show net population change.

On any given day it’s hard not to notice another headline about a population in decline.  Amphibians are in decline, songbirds are in decline, bumblebees are in decline, fish stocks are in decline.  Nature is under relentless human pressure, both direct and indirect, and before I proceed to make my point today, I need to be very clear that this pressure is real and severe and I don’t doubt for a moment that it’s driving down population sizes of many, many species.

But there’s a very simple but pervasive statistical problem with the data behind population declines. Continue reading