The effect of supplementation, illegal poaching and inbreeding on the Scandinavian wolf: A population viability analysis. Anna Svahlin, Cristina Gutiérrez, Niki Andersson, Niklas Paulsson Supervisor: Folmer Bokma Ecological Dynamics 15 ECTS
Abstract The wolf population on the Scandinavian peninsula was exterminated in the 1970s and later returned in 1980s with only three individuals from Finland and Russia establishing the population. Due to this, the current population (approximately 230 individuals) suffers from inbreeding and additionally from poaching. In this study we construct a Population Viability Analysis (PVA) to determine how the wolf population will evolve on a long term basis, depending on its inbreeding, human poaching and immigration by simulating different scenarios using VORTEX. We managed to prove that immigration and poaching are vital components when predicting the wolf population persistence. It does not seem that the amount of harmful alleles in the genome affects the population to any great extent. Although VORTEX was not able to account for all the conditions that apply to a wolf community, it still proved to be sufficiently suited for the assigned task.
Introduction In the early nineteenth century there must have been at least 1500 wolves in Sweden, suggesting that the number in the entire Scandinavian peninsula may have been 2000-3000 or even more (Nilsson et al. 2003). In the 1970s, the wolf was officially considered extinct in both Sweden and Norway (Webakken et al. 2001) and then three wolves came from Finland- Russia(1980s). Due to the fact that only three wolves repopulated Scandinavia the current population was subjected to a great bottleneck effect, which results in a low genetic diversity making them more susceptible to inbreeding. This is a problem as it decreases the amount of heterozygosity in the population, allowing for harmful recessive alleles to express themselves, leading to a higher fitness reduction with each passing generation. Without a supplementation of new genes to the population, the level of heterozygosity will decrease; a problem that has shown to manifest itself in wolves as a decrease in litter size and an increase in deformities (Sand et al. 2010). The wolf is territorial and organizes in packs with a dominant alpha-pair. It reaches the reproductive age during its second year and it is not assumed to reproduce after the age of 11 years (Sand et al. 2010). They reproduce annually and the litter size varies between three to eight pups, which have a mortality rate of 30%. Because of the mating monopolization of the alpha-pair, counting the number of packs will grant a more accurate number on the effective population size rather than counting the number of individuals (Aspi et al. 2006). The presence of wolf in Scandinavia is a controversial issue as they interfere with the interest of the many farmers, hunters and reindeer herders in Sweden (See Liberg et al. 2011 a) Human activity causes 80% of the mortality in adult wolves (Sand et al. 2010). Illegal poaching is suspected to account of 50% of adult wolf mortality (Sand et al. 2010, Liberg et al. 2011). The aim of this study is to evaluate how inbreeding and illegal poaching effect the probability of survival of the Scandinavian wolf population and evaluate if, under the current conditions, the population will persist. This will be tested by running simulations with different rates of supplementation, amount of harmful alleles in the population and presence as well as absence of poaching. For calculations of the amount of harmful alleles the concept of Lethal Equivalents (LE) was used. This is a way of measuring the dangers of inbreeding by presenting the cumulative value of the fitness lowering capabilities that the harmful recessive alleles will present in an individual when they are expressed (Miller and Lacy 2005). For example, at LE=3, the potential fitness reduction when all the harmful recessive alleles in an individual are expressed would be 3 (or 300%). Considering the above facts we hypothesize that: Under the current conditions, a constant supplementation is crucial for the viability of the Scandinavian Wolf Population A lower amount of lethal equivalents in the population should result in a lower probability of extinction. Illegal poaching significantly lowers the probability of long-term survival for wolves. Methods Simulations Two sets of simulations with different immigration rates and amount of LE were run for 200 years, 1000 times each, with an initial population number of three individuals. An extinct population was defined as a simulation with only one sex remaining. The values used for LE were 3.0 (Case A) and 6.0 (Case B; as a worst case scenario)(nilsson 2003 and Liberg et al. 2005, respectively). A full set of simulations at LE = 3 but with halved mortality for adults
(simulating the absence of poaching; Liberg et al. 2011 b) were run to test for the need of supplementation under these conditions (Case C) (see Table 1 for a summary on cases). Within these three cases, four variations in supplementation to the population were made; ranging from 1.3 wolves supplemented every year but with a stop at 30 years, every year, every other year and every 5 th year (Scenario 1,2,3 and 4 respectively; see Table 2). From this point on, each simulation will be called by according to case and scenario (i.e case B, Scenario 1 will be B1). Table 1. Shows a summary on the amount of lethal equivalents and the rate of mortality in adult wolves accounted to illegal poaching in every case. Case LE Illegal Poaching accounts for % adult mortality A 3.0 50 B 6.0 50 C 3.0 0 Table 2. Shows the different supplementation rates for each scenario. Scenario Supplementation 1 Every year (stops at year 30) 2 Every year 3 Every 2 years 4 Every 5 years The Model All simulations were done using VORTEX PVA Software 9.99b (Miller and Lacy 2005) as it provides a discrete, stochastic, individual-based, model that has been proven to be reliable on previous occasions (Brook et al. 2000). The simulations are run on a year-to-year basis with discrete reproduction and mortality. While this approach sacrifices a certain flexibility within the model (i.e. changes in management over several years) a lot of information can be easily included even if the authors themselves possess a limited amount of knowledge on the field of programming (a skill that is more or less required when writing complex models of your own). VORTEX provides an individual based simulation based on a large set of parameters that decide the different probabilities that are being calculated as discrete, sequential events, for further information, see Miller and Lacy (2005). Inbreeding is an issue of great concern when constructing a PVA for the Scandinavian wolves and is modelled as a reduction in juvenile fitness based on the amount of LE i.e. increased inbreeding will lead to decreased pup survival. Due to the low amount of wolves currently occupying the Scandinavian peninsula, no density dependence was assumed when running the model. Data All the data that was required to conduct the PVA was not to be found in the format demanded by VORTEX; so certain transformations need to be performed. This is a list of transformations and conclusions that were made. Per cent of females breeding: VORTEX can not account for the fact that wolves arrange in packs. This problem was circumvented in the following way: the average number of wolves in a Scandinavian pack is 6 individuals (Sand et al. 2010) and out of these it is assumed that only the alpha pair mates. Under the assumption that the gender-ratio is 1:1; one out of three (33%) females breed at any given time.
Reproductive rates: The probability of a reproductive female giving birth for a given year is 70% with a mean amount of 5 pups and a maximum of 8 (Sand et al. 2010) according to a binomial distribution as the amount of pups that are given birth to has to be an integer. This distribution had to be calculated by hand. Based on the work of Sand et al. (2010) the supplementation to the Scandinavian wolf population for each year was found to be 1.3 individuals/year on average. As the initial population that colonized the Scandinavian peninsula were three individuals, our simulations start at this amount of wolves as well. This provided a realistic model that also included the bottleneck effect. Results Our result showed In the first scenario, that without immigrating wolves, independent of lethal equivalents,none of the 1000 populations in case A1 and B1 survived for 200 years. Most of them went extinct after 50-60 years (20-30 years after the supplementation stops. Since the real population have sustained for 30 years that would be the same thing as if we stopped supplementation today (see Fig. 1 and Fig. 2 respectively). When we ran case C1 it was clear that without illegal poaching all of the simulated population was able to survive even without immigrating wolves. In the second scenario (A2, B2 and C2) supplementation was added every year and none of the 1000 populations that were simulated went extinct (see Fig. 1 and Fig. 2) in any of the cases.,meaning that 1,3 immigrants each year is enough to keep all simulated popuation surviving all 200 years. Fig. 1 Shows the different mean probability of extinction at a certain year between a population supplemented every year (green line) and a population supplemented every year with a stop after 30 years(blue line). (LE=3.0, cases A2 and A1 respectively)
Fig. 2 Shows the different mean probability of extinction at a certain year between a population supplemented every year (green line) and a population supplemented every year with a stop after 30 years(blue line). (LE=6.0, cases B2 and B1 respectively) When immigrants is added every second year there s no discernable differences between having different lethal equivalents( i.e. case A3 and B3)(The scenario with supplementation every second year showed no discernable difference between case A and B) (Fig.3 and 4 respectively). Even though some simulated populations survive the 200 years of simulation most of them became extinct already in the first years. Although the populations that manage to survive the first 20 or 30 years have a quite good predicted chance of survival indicating the scenario for the real wolf population (i.e. the Swedish wolf population has persisted for more than 30 years). If we compare this with adding supplementation every fifth year(case A4 and B4) the fluctuation in the results get even larger indicating that the outcome of this scenario gets even more uncertain( i.e. it is not safe to assume that the wolf population will not become extinct). Although comparing immigrants every second and fifth years only shoves that there is only a delays in time of extinction. That means that both scenarios with supplementation every second and fifth year are not safe because of the uncertain outcome for the wolf population. When the population get illegal poaching release (case C) they re not suffering from any limitation in growth other than their own carrying capacity. This means that if the wolf population do not suffer from illegal poaching even a scenario with supplementation just every fifth year also is a safe scenario.
Fig. 3. Shows the different mean probability of extinction at a certain year between two simulations with increased supplementation rates (LE=3.0). Green line represents a simulation with a supplementation every five years (case A4) whereas the blue line represents a simulation with a supplementation every two years (case A3) Fig. 4 Shows the different mean probability of extinction at a certain year between two simulations with increased supplementation rates (LE=6.0). Green line represents a simulation with a supplementation every five years (case B4) whereas the blue line represents a simulation with a supplementation every two years (case B3) Finally when supplementation is added every fifth year there is an even higher uncertainty in both case A and B. There is a wider range of extinctions occurring for the first years but also more frequently in the following years (i.e. larger fluctuations in result) (Fig. 3 and 4). It is clear from our results that the number of lethal equivalents was not as important (i.e the result did not differ between scenarios with 3 or 6 lethal equivalents) as the number of
immigrating wolves. They play a much more important role in our results to sustain a viable wolf population. Our results also showed that the only safe scenario was the scenario with 1,3 immigrants each year. Already in the second year scenario there were many simulated populations that went extinct even though if they survived the first years they were very likely to sustain over time. Our result also showed that when illegal poaching was removed the population was able to sustain under all simulated conditions (i.e with or without supplementation) indicating that this also play an important role for a viable wolf population that can sustain over time. Discussion The results showed a clear importance of wolf immigration to maintain a viable population. The incorporation of new genes from outbred immigrants in the breeding population is needed in order to guarantee the long-term survival of the Scandinavian wolf. To maintain this supplementation it is important to manage wolf protection in order to ensure that certain amount of wolves would be able to pass through Scandinavia and mate with the existing population. In addition, the results showed that there is no need to have a great amount of supplementation to sustain the Scandinavian wolves; just a few immigrant wolves could lead to population persistence. We were unable to discern any difference between treatments with different LE (Fig. 1-4). This could be due to the current level of inbreeding; as the population already suffers heavily inbreeding even a low number of LE will result in a severe lowering of fitness unless new genes are supplemented to the population. With illegal poaching Without illegal poaching Fig. 5 Shows the influence of illegal poaching on the probability of extinction comparing two simulations with (blue line) and without (green line) illegal poaching (cases A4 and C4 respectively) (LE=3.0, supplementation every 5 years). On the other hand, it is also important to note the influence of illegal poaching in the population viability. As seen in the results and Fig. 5, without any illegal poaching the wolf population would persist even if the supplementation takes place every 5 years, thus it should be important to improve the penalty of wolf killing and manage some actions to reduce its causes i.e. better fences to keep the wolves out of pens, but also increased compensation for hurt/killed cattle.
VORTEX is a worldwide software that provides several models and simulations, but using this kind of pre-written model there are some limitations: there is no possibility to determine parameters that in some cases could be of importance. In this model it was impossible to determine the gender of the migrating wolves for each year while it is known that most of them are males (Swedish Environmental Protection Agency, 2009). There were no option to change some parameters over time, for example we could not study how wolf population would be on the long term when supplementation changes over time. Also, the results showed a clear difference between different scenarios (interpreted from standard error bars) but adding these figures to the report was impractical as the graphs would contain too much information. VORTEX is not fitted for modelling behaviours such as alpha-mating as in the natural environment wolf alpha-males mate with other females beside the alpha when there is a high supply of prey. The problem with modelling was circumvented by applying a high percentage of mate monopolization from the male perspective. In conclusion, inbreeding in the Swedish wolf population can be decisive for its viability and thus, it is necessary to consider those factors that are able to reduce inbreeding by securing a constant flow of new genes to the population. In addition, the reduction of illegal poaching could make a crucial difference to the population on the long term (see Fig. 5) as it plays an important part in keeping the wolves at a threatened state. Acknowledgements We would like to thank Folmer Bokma for supervising this project and helping us with any and all problems. We extend our greatest gratifications to Cris s dad for helping us calculate the binomial distribution of wolf reproduction. Last, but not least, we would like to thank Bob Lacy for developing such as an amazing software as VORTEX that let us prod some serious buttocks in the world of PVAs.
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