Relation of Minimum Viable Population Size to Biology, Time Frame, and Objective

Abstract

Shoemaker et al.’s (2013) estimate of the minimum viable population size (MVPS) of the bog turtle (Glyptemys muhlenbergii) is 3 orders of magnitude below population sizes typically viewed as minimally viable (Reed et al. 2003; Trail et al. 2007). Even Flather et al. (2011), who question the utility of MVPS for conservation planning, concede that long-term persistent populations will require thousands of individuals. Rather than conclude their result is incorrect, Shoemaker at al. suggest that MVPS has been overestimated for long-lived species. Although we agree in principle that small populations, especially of long-lived species, have conservation value, we do not agree that Shoemaker et al.’s result provides evidence that small populations are viable. This position is inconsistent with existing empirical evidence of persistence of small populations (Simberloff & Gibbons 2004; Fagan et al. 2005; Fagan & Holmes 2006). We suggest that Shoemaker et al.’s estimate is a gross underestimate, emanating from their definition of viability, which is too narrow, not biologically meaningful, and ignores factors such as demographic and environmental stochasticity, loss of genetic variability, and catastrophes. Their conclusion that populations of long-lived species can be orders of magnitude smaller than currently believed does a disservice to species conservation and raises the longterm discussion in conservation biology about whether we should be focusing on the minimum for management. Shoemaker et al. used too short a time frame, 100 years. Frankham and Brook (2004) and O’Grady et al. (2008) identify the importance of setting the proper time frame for conservation questions and point out that generation length is often the appropriate scale. It can be shown easily that even a strongly declining population can be viable over the short time frame of 10 generations ( 100 years for long-lived species) and that the