Defining reintroduction success using IUCN criteria for threatened species: a demographic assessment

Abstract

Despite recent efforts to develop the science of reintroduction biology, there is still no general and broadly accepted definition of reintroduction success. We investigate this issue based on the postulates (1) that successful reintroduction programs should produce viable populations and (2) that reliable assessments of ultimate success require that populations have reached their regulation phase. We assessed if the viability of these reintroduced populations could be evaluated using the same criteria as for remnant populations, such as the Internation Union for Conservation of Nature (IUCN) Red List criteria. Using modeling, we projected the viabilities of theoretical populations with various life history and environmental characteristics and we tested whether population sizes (criterion D of the IUCN) and other potential predictors are relevant proxies of the risk of extinction (criterion E of the IUCN) in the case of remnant populations with an unknown past history and in the case of reintroduced populations that have reached their carrying capacity. We found that, as for remnant populations, population size can be used as a relevant indicator (although subject to considerable uncertainty) of the viability of reintroduced populations. However, the results demonstrate the importance of the reintroduction failure filter, that is, the fact that the reintroduced populations that have successfully reached their carrying capacity are those with the highest and more stable growth rates, especially if populations have been reintroduced with a few individuals. As a consequence, the general relationship between the current size of a population and its projected viability will, most likely, differ considerably between remnant and reintroduced populations. Overall, our results demonstrate that there are no theoretical limitations on the application of some of the criteria widely used for remnant populations to define reintroduction success, although these criteria are very conservative for reintroduced populations and might be rescaled to account for the demographic filter that early extinction constitutes for these populations.

Alternative perspectives on reintroduction success

“…By designing reintroductions as experiments, we have the opportunity to learn, even if the end result is a failure (Maschinski & Haskins, 2012). In summary, I disagree with Robert et al (2015) in their suggestion that we need to have a standardized definition of reintroduction success, not because it lacks advantages, but because I find its acquisition near impossible given the high degree of variability in species needs when it comes to reintroduction success criteria. We need to not set ourselves up for failure by standardizing criteria and definitions as suggested by Robert et al (2015) that may not be achievable.…”

“…In summary, I disagree with Robert et al (2015) in their suggestion that we need to have a standardized definition of reintroduction success, not because it lacks advantages, but because I find its acquisition near impossible given the high degree of variability in species needs when it comes to reintroduction success criteria. We need to not set ourselves up for failure by standardizing criteria and definitions as suggested by Robert et al (2015) that may not be achievable. Reintroduction outcomes truly exist on a continuum, at one point in time appearing successful, but perhaps ending in failure a few years or decades down the road, as Monks et al (2012) illustrated.…”

“…And indeed for some species, reintroduction may be the only remaining option for survival (Maschinski et al, 2011). Robert et al (2015) discuss an important concept in the science and practice of reintroduction, that is, the definition of reintroduction success. In agreement with the authors, I feel that it is crucial to examine all factors that contribute to reintroduction success in order to properly assess the outcome, yet I question whether or not we need a standardized definition.…”

“…Comparing reintroduced populations to remnant populations is important because the latter can provide baseline data that are critical for interpreting whether the reintroduction was successful or not (Monks et al, 2012). On the other hand, Robert et al (2015) illustrate some statistical biases in models based on demographic factors between the two types of populations. Another important difference between remnant and reintroduced populations is the existing genetic variation within the population and the opportunity for maintaining genetic variation through interactions with metapopulations.…”

“…Thus, reintroduced populations have a large number of unknown variables, in terms of a successful reintroduction outcome, based on environmental factors alone. In light of these uncertainties, the system of using criteria derived from remnant populations as the basis for determining success of reintroduced populations provides a platform from which we can learn and make improvements but these comparisons should be used with caution as Robert et al (2015) suggest.…”