Population viability analysis

Plains zebras (Equus quagga antiquorum) occur in few large, but many small, isolated populations in KwaZulu-Natal. Problems identified in small populations include reduced striping patterns on hind quarters, smaller size, elevated mortality rates and high number of still-births. Inbreeding may be implicated. Population viability analysis (PVA) was conducted with a computer model (VORTEX), and DNA and allozyme analyses were conducted to test the findings of the model. Using standard methods, DNA (PCR-RAPD) and allozyme diversity was assessed in blood samples from 72 plains zebra from four KwaZulu-Natal Nature Conservation Services (KZN-NCS) protected areas: Umfolozi Game Reserve (UGR), Albert Falls (AFNR), Vernon Crookes (VCNR) and Harold Johnson (HJNR) Nature Reserves. Populations of the latter three, small-sized (9-110 individuals) populations were seeded from the same source population (UGR: current population of 2000) during the past 25 years. Information from PCR-RAPD and allozyme analyses were compared with each other as well as to that predicted by population genetic modelling (using VORTEX). Allozyme heterozygosities were consistently high in all populations (12.1-12.9%), with no observable losses associated with reduced population size. On the other hand, percentage loss of : S 0 3 0 5 -1 9 7 8 ( 0 0 ) 0 0 0 9 7 -1 polymorphism (20-39%) calculated from the PCR-RAPD study appeared to be positively correlated with the loss of heterozygosity predicted by population viability analysis (PVA), and negatively correlated with population size. On the basis of the above results, a policy of translocation was advocated for small, intensely managed populations of zebras, whereby a harem should be translocated every five years for a population size of nine (HJNR), while for a population size of 110 (VCNR) translocations should take place every 15 years if heterozygosity is to be maintained at more than 90% within each population over 100 years. #

More than a decade has passed since the catastrophic population decline in Gyps species was reported from South Asia, but much uncertainty remains about quantifying their short-term extinction risk. To estimate the future extinction risk of the white-rumped vulture Gyps bengalensis in Nepal, we conducted counts at 7 nesting colonies between 2002 and 2012. We compared 3 methods of estimating abundance based on count data and calculated mean population growth rates and cumulative probabilities of extinction given the abundance estimates from each method. The first 2 methods of abundance estimation were traditional indices: mean and maximum values of all counts. The third method was a mixture modeling approach that corrected raw counts by a detection parameter. The results of the traditional indices were characterized by high uncertainty levels as reflected in the wide confidence intervals, which limited their capacity to make predictions about the fate of the populations with any confidence. The mixture modeling method provided more reliable results; there was a 51% probability of populations facing quasi-extinction (i.e. ≤20 vultures) in 13 yr and a 99% probability of quasi-extinction in 18 yr. Because the mixture modeling method provided more precise predictions while requiring minimal additional effort, population biologists using count data are encouraged to employ such model-based estimators. The white-rumped vulture populations in Rampur are in danger of disappearing within 2 decades, so conservation efforts should be expedited to prevent the loss of this species.

Reintroductions are commonly used for re-establishing self-sustainable populations in formerly inhabited areas. Reintroductions are expensive, and thus, it is worth performing a thorough demographic analysis of current and likely future population trajectories to guide strategic decisions on release policy. 2. Bearded vultures Gypaetus barbatus were exterminated from the Alps in the late 19th century, mainly due to human persecution. To re-establish them, captive-bred young have been released annually since 1986. Since the first successful breeding in the wild in 1997, the population has increased to 9 pairs in 2006. It is not known, however, for how long releases should be continued to obtain a self-sustaining, viable population. 3. We estimated age-specific survival probabilities with a mark-resighting model and quantified fecundity rates of released individuals. Using the resulting demographic estimates, we built a stochastic population model to estimate population growth rates, and explored the value of continuing to release birds for varying periods into the future. 4. Annual survival probabilities were high (first year of life, 0·88; later years, 0·96); average annual fecundity was 0·6 fledglings per breeding pair. Using the estimated survival probabilities, projected population growth rates would increase with additional years of releases. Yet, the population would grow, even if releases had stopped after 2006. Only if mortality increased by ≥ 50% would the population start to decline. 5. Synthesis and applications. Our population dynamics model provides essential information to optimize decision-making within a major reintroduction programme. From a demographic viewpoint, releases of captive-raised bearded vultures can be ceased in the Alps. The resources freed could be redirected towards a close demographic surveillance of the free-ranging population, with periodic evaluation of its viability and the option to release birds if deemed necessary. Birds available from the captive stock could be used for reintroductions in other areas where the bearded vulture is extinct.

Human-caused mortality of wildlife is a pervasive threat to biodiversity. Assessing the population-level impact of fisheries bycatch and other human-caused mortality of wildlife has typically relied upon deterministic methods. However, population declines are often accelerated by stochastic factors that are not accounted for in such conventional methods. Building upon the widely applied Potential Biological Removal (PBR) equation, we introduce a new population modeling approach for estimating sustainable limits to human-caused mortality, and applied this to a case study of bottlenose dolphins impacted by capture in an Australian demersal otter trawl fishery. Our approach, termed 'Sustainable Anthropogenic Mortality in Stochastic Environments' (SAMSE), incorporates environmental and demographic stochasticity, including the dependency of offspring on their mothers. The SAMSE-limit indicates the maximum number of individuals that can be removed without causing negative stochastic population growth. We calculated a PBR of 16.2 dolphins per year using the best abundance estimate available. In contrast, the SAMSE model indicated that only 2.3 to 8.0 dolphins could be removed annually without causing a population decline in a stochastic environment. The results suggest that reported bycatch rates are unsustainable in the long-term, unless reproductive rates were consistently higher than average. The difference between the deterministic PBR calculation and SAMSE mortality limits illustrates that deterministic approaches may underestimate the true impact of human-caused mortality This article is protected by copyright. All rights reserved. of wildlife. This highlights the importance of integrating stochasticity when evaluating the impact of bycatch or other human-caused mortality on wildlife, such as hunting, lethal control measures or wind turbine collisions. Although population viability analysis (PVA) has previously been used to evaluate the impact of human-caused mortality, SAMSE represents a novel methodological PVA framework that incorporates stochasticity for estimating acceptable levels of human-caused mortality. It offers a broadly applicable, stochastic addition to the demographic toolbox to evaluate the impact of human-caused mortality on wildlife.

Small populations with restricted geographic ranges such as rhinoceros (Rhinoceros unicornis) are prone to extinction
due to anthropogenic factors. The identification of factors underpinning the survival of such species is of critical
importance for population persistence. We used VORTEX population viability analysis (PVA) to assess rhino population
viability in Nepal. We simulated deterministic single-population models under different scenarios to assess viability of
two distinct rhino populations in Nepal: a source population in Chitwan National Park and an augmented population in
Bardia National Park. The impacts of poaching on the populations and the potential for rhino translocation from one
population to another were assessed under the PVA framework. Population and demographic data were obtained from
censuses and from published literature. The model output suggested that the Chitwan population is stable and capable of
supplying at least 10 rhinos every 3 years for translocation provided poaching is restricted (≤15 animals per 3 years).
However, the Bardia population is more vulnerable and unable to persist without supplementation even at the lowest
poaching rate (2 animals per year). Supplementation of at least 10 animals every 3 years for 30 years is crucial for establishing
a viable population of rhinos in Bardia. This level of supplementation can withstand the poaching rate of ≤2 animals
per year. Our study demonstrates that poaching is the major factor determining rhino population viability in Nepal.
The supplementation of the Bardia rhino population with animals from the Chitwan population and increased effort to
reduce poaching are expected to enhance the viability of rhino populations in Nepal.

Lepanthes caritensis is a small epiphytic orchid endemic to Puerto Rico. Although this species is very rare and demographic studies show that its numbers are declining, it is not protected under the United States Federal Endangered Species Act (ESA). Furthermore, questions remain regarding its distribution, population size and ecological interactions, including host-specificity, which could influence its likelihood of extinction. The primary objective of this study was to document the overall population size and distribution of this orchid to determine whether it warrants federal listing. To this end, extensive surveys were conducted to document the current distribution and population size of L. caritensis. These data were used to classify the species’ rarity status according to criteria outlined by Rabinowitz, NatureServe and the IUCN. Results of these surveys indicate that the overall population of L. caritensis is larger and more widely distributed than previously thought. In addition, the species is not host-specific outside its originally described range, and is found on at least 4 different host species. Nevertheless, this species is indeed rare: it meets the criteria for the NatureServe category ‘critically imperiled’ and the IUCN category ‘Critically Endangered’. Consequently, the most important conclusion of this study is that this species warrants legal protection under the ESA, particularly if its population continues to decline.