Population Estimation

Over the years, ecologists have developed a multitude of ways to estimate the abundance of organisms in a given place at a given time. Even the very straightforward method of mark-recapture and the Lincoln-Petersen estimator are well over 100 years old … and still in use today. However, many other sophisticated techniques and models have emerged, particularly in the last 20 or so years. A common feature of nearly all abundance-estimation procedures is the estimation of a detection probability. This is simply the probability that during a survey or sampling period the species (at least one individual) was detected given that it was available to be detected (i.e., it was within the area being surveyed). I’m interested in these various techniques spanning the range of computational complexity from Lincoln-Petersen to N-mixture models.

However, I’m even more interested in how we can take empirically-derived estimates of local abundance, convert these to densities, and then multiply by area to get estimates of population size at scales ranging from small nature preserves to large ecoregions.  This can be a tremendously dangerous (ill-advised) task but it can also be incredibly useful to conservation planning and assessment if done properly.  The keys lie in having (1) good scientific confidence in the estimation of local abundance, (2) reliable knowledge about the area surveyed so as to get accurate density estimates, and (3) additional information (e.g., amount of habitat, population structure, dispersal capacity) so as to extrapolate the density estimate to get an estimate of population size for some area of interest.  This isn’t easy and ecologists do not attempt this (particularly at large spatial scales) as often as they could.  Certainly, any estimate of population size should come with estimates of error (presented in a probability-based framework) and other caveats. Even if the estimates are not super-precise, they can at least serve as ballpark figures for comparison and evaluation. Note that estimating population size is a different task from conducting an analysis of Minimum Viable Population (MVP) size. The latter tells us what we could be aiming for to ensure species persistence and/or what we should try to avoid; the latter tells us what we actually have right now.