Population Viability Analysis

Building better models to understand extinction risk

Population viability analysis (PVA) is a powerful conservation tool, but one remaining unapproachable for many species because of the amount of data required to build useful models. This is especially true for species occurring in multiple populations, each of which requires a separate PVA.

With NASA and other partner funding, Trout Unlimited and a host of collaborators developed an entirely new class of PVA models. These models draw on available field data and information about the environment to estimate extinction risk across multiple trout populations simultaneously.  Because of this simultaneous estimation, they draw on information from well-sampled populations to inform estimation for poorly- or even un-sampled populations.

We focused primarily on threatened Lahontan cutthroat trout to improve decision making for this imperiled species. Working with management agencies, we collated all field data and built a new database, which is now being housed and maintained by the Nevada Department of Wildlife. Our primary models are now published along with a third application focused on evaluating Lahontan cutthroat trout management needs and opportunities. As part of this latter effort we built a Decision Support Tool, the Lahontan Cutthroat Trout Simulator, which allows users to explore full-model results and covariate influences in a Google Earth-enabled framework. This tool can be used by species managers and conservation practitioners to understand which populations are most at-risk, and evaluate the benefit, in terms of reduced extinction risk, of management actions such as removing non-native trout (a primary threat to Lahontan cutthroat). It also allows one to estimate the likelihood of success when reintroducing Lahontan cutthroat to historical waters. Collectively, these models and support tools help move us toward better conservation for trout and any species with multiple isolated populations.


This paper applies the above Multiple Population Viability Analysis (MPVA) models to evaluate conservation needs for the threatened Lahontan cutthroat trout. We estimated extinction risk across 84 LCT populations based on all available data from 1985 to 2016. Our results confirmed that agency-designated conservation populations tended to have lower extinction probabilities than other populations, meaning this designation is a valid characterization of relative recovery potential. But this wasn’t always the case. We found that six conservation populations currently prioritized for recovery planning have a 70% or greater probability of extinction by 2045, and 11 more are at or above 20% risk of extinction. Our models also predicted relatively low (<10%) extinction risk for several non-conservation populations. For these cases, which would not have been identified without MPVA, agencies may want to consider re-prioritization of conservation status and management needs. MPVA verified that removing non-native trout is an effective management action, which was not surprising – but because the effect wasn’t the same for every population, it provided useful information aboutwhich populations would benefit most from these efforts. Similarly, reintroduction success differed across populations, so results can point agencies to the most productive waters to consider for establishing new populations.”

Here we built an earlier model (Wenger et al. 2017) into a tiered framework containing two other newly-developed models that allow us to handle important statistical issues like incomplete detection of fish in the field and various types of sampling bias. Collectively, these models let us process information from the ground up and retain information on all sources of error, appropriately rolling field information into estimates of population size, which are then projected into the future to predict extinction risk. We applied a full suite of environmental variables including flow, stream temperature, riparian vegetation condition and non-native species to understand factors that influence population behavior across the historical range of Lahontan cutthroat trout.

This paper describes the foundational model behind our Population Viability Project. We detail a new class of PVA model that, for the first time, allows for the estimation of extinction risk across multiple populations simultaneously. Our novel approach incorporates field data field and information about the environment, drawing on well-sampled populations to inform estimates for poorly- or even unsampled populations. Though applied here to a threatened inland trout (Lahontan cutthroat trout) the approach is applicable to any species with multiple isolated populations and makes PVA applicable to a wider range of species than ever before.