A Novel Tool for Making Policy Recommendations Based on PVA: Helping Theory Become Practice

Northern quoll persistence is most sensitive to survivorship of juveniles

Context Projecting the population trajectory of endangered species using models requires conservation practitioners to evaluate how variations in life history parameters may change a population’s viability. This is particularly important for species that occur as fragmented populations and whose densities are naturally low. Simulations may be used to identify conservation actions that have a higher likelihood of reducing a species’ extinction risk. Aims The aim was to apply population viability models to the northern quoll (Dasyurus hallucatus) under alternative scenarios. Methods The current (baseline) northern quoll demographic trajectories were evaluated using field-collected data derived from monitoring programs. The impact of alternative scenarios of mortality (for example, due to increased predation by introduced predators) and population supplementation (either from populations elsewhere or from captive breeding) on the viability of a northern quoll population was then determined. Key results Under current conditions, individual Pilbara populations of northern quoll are projected to persist for over 20 years. However, these populations are sensitive to extinction events. Population growth rate and local extinction risk were most sensitive to changes in juvenile mortality as low as 5% per annum. Increased mortality of the juvenile age cohort above current levels resulted in a projected decline in population size of 22–54%, with a moderate-to-high chance (20–96%) of local extinction within 20 years. Supplementing the population produced a moderate increase in quoll persistence over this time period. Conclusions Populations of northern quolls in the Pilbara, and potentially elsewhere in their range, are highly sensitive to even small perturbations in juvenile mortality rates. The continued persistence of quoll populations in fragmented refuges is characteristic of a species that functions as a dynamic metapopulation in the face of high environmental perturbations. Implications Increased juvenile mortality above current levels – for example through the spread of cane toads or invasion of feral cats – may have serious implications for the persistence of the current network of northern quoll populations and other mammals that exhibit population fragmentation in arid environments. Estimates of survival rates for the juvenile cohort of quolls would improve diagnosis of a species’ population dynamics as well as inform practitioners of key life-history sensitivities.

Using population viability analysis to evaluate management activities for an endangered Hawaiian endemic, the Puaiohi (Myadestes palmeri)

Evolution in the Hawaiian Islands has produced a unique avian assemblage. Unfortunately, many of these bird species are highly endangered or extinct. Despite numerous and increasing threats and great effort aimed at saving endemic birds, we lack basic science necessary for understanding many species of concern. One such species is the critically endangered Puaiohi (Myadestes palmeri), a rare songbird endemic to the island of Kaua'i and the only remaining native thrush on the island. At present, the Puaiohi's breeding population is estimated to be ~500 birds restricted to the Alaka'i Wilderness Preserve. We collected demographic data from 2007-2012 and supplemented it with published sources. Using Vortex, we developed stochastic population models to represent Puaiohi population dynamics under current and potential management scenarios to determine management's potential efficacy in aiding species recovery. Management scenarios modeled included rat control, habitat improvement, general survival facilitation, and provision of nest boxes. The model indicated a decline in abundance with a growth rate (r) of -0.267 under baseline conditions. Female and juvenile survival appeared to be the most influential parameters related to population growth and persistence, so management should focus on increasing female and juvenile Puaiohi survival. Rat control, even at more conservative levels, appeared to be the most effective method of increasing Puaiohi abundance. Our results indicate that practical, attainable management activities can increase Puaiohi and bring the species back from the brink of extinction. Such findings provide an example for other endangered species conservation efforts.

Estimating freshwater turtle mortality rates and population declines following hook ingestion

Freshwater turtle populations are susceptible to declines following small increases in the mortality of adults, making it essential to identify and understand potential threats. Freshwater turtles ingest fish hooks associated with recreational angling, and this is likely a problem because hook ingestion is a source of additive mortality for sea turtles. We used a Bayesian-modeling framework, observed rates of hook ingestion by freshwater turtles, and mortality of sea turtles from hook ingestion to examine the probability that a freshwater turtle in a given population ingests a hook and subsequently dies from it. We used the results of these analyses and previously published life-history data to simulate the effects of hook ingestion on population growth for 3 species of freshwater turtle. In our simulation, the probability that an individual turtle ingests a hook and dies as a result was 1.2-11%. Our simulation results suggest that this rate of mortality from hook ingestion is sufficient to cause population declines. We believe we have identified fish-hook ingestion as a serious yet generally overlooked threat to the viability of freshwater turtle populations.