Team:Manchester/PopulationDynamics

Introduction

As part of our research into the ethical aspects of our project, we have modelled the future population of the Sumatran orangutan in a variety of situations; including with and without our project being implemented, and reflecting the uncertainty found in the economical research we compiled as part of this project as to how much we can replace traditional methods of palm oil production.

This involved the use of the program Vortex: “a Monte Carlo simulation of the effects of deterministic forces as well as demographic, environmental, and genetic stochastic events on wild population”^[1]. Using this, we were able to run 200 iterations of each scenario, giving us a series of results based on the probability of a number of chance events, including catastrophes (eg. fires, landslides), inbreeding, litter frequencies and sex. This lead to the production of a variety of outcomes based on the occurrences of these chance events, the majority of which adhere to a general trend around a mean.

We decided to base our model on the Sumatran orangutan due to their smaller population size. Restricted to the island of Sumatra in Western Indonesia, this species is at threat due to mass deforestation, a large amount of which occurs as a direct result of the palm oil industry. As the majority of the remaining orangutans are concentrated in a relatively small area in the north of the island, we treated this as a single population. We also chose Sumatra as it is home to a number of other endangered animals, included the critically endangered Sumatran tiger and Sumatran rhino, which are facing the same threats, and potentially the same future, as the Sumatran orangutan.

The aim of this aspect of our modelling ventures is to validate the need for our project, and set an overall deadline as to when our project must be put into action in order to save the Sumatran orangutan.

Method

The building of the baseline for our orangutan model used methods previously outlined in the paper Orangutan Population Biology, Life History, and Conservation^[2], and we adapted this to represent current orangutan population levels and deforestation levels, as well as the effects of our project.

Average age of first reproduction has been established to be 15 years for female Sumatran orangutan and 25 years for male Sumatran orangutans. No menopause has been recorded in the Sumatran orangutan, and therefore Sumatran orangutans were presumed to continue to produce litters until towards the end of the orangutan's lifespan, at 50 years of age (orangutans in the wild are thought to potentially live up to several decades, but the oldest recorded lifespan is 55 years)^[3].

Density-dependant effects on reproduction were modelled using the same growth curve found in Lacy (2009), where , with P0 =18.2, Pk = 11.1, A = 1, B=2 and N = initial population size.
We modelled the effect of deforestation using time variable carrying capacity: where K is carrying capacity and P is the present population.

Results

Using the methods described previously, we were able to model a variety of scenarios and monitor the population sizes of the orangutan throughout them.

We began by establishing when we can expect to see an extinction of the orangutan based on deforestation data collected previously as part of our bioethics research. This data put the extinction timeline for the Sumatran orangutan to be around 43 - 45 years. All models ran to extinction. This is a slightly earlier estimation than previous explorations into the potential extinction of the orangutan, which is likely to be because of an increase in the rate of deforestation over the past few years, fuelled by palm oil and biofuel targets, although most models show dangerously low population levels in 40 years time^[2].

Figure 1: Predicted extinction of orangutans, based on deforestation data analysed on our Ethics pages (2.36% annual loss of rainforest coverage). 200 simulations are shown

Using this simulation, estimates of the orangutan population at set time periods in the future were created. Then, a variety of scenarios made possible by the implementation of our project were created.

To begin with, we modelled the effect that a complete end to further deforestation would have on the orangutan population, which is a potentially achievable outcome of our project. This time frame for this occurring is difficult to predict, so we began by looking at what we could expect if the implementation of our project takes a further 40 years.

Our best case scenarios from the previous model suggested that in 40 years time we will be expecting Sumatran orangutan population to be around 650 individuals. Therefore, we modelled a population of orangutans at this number for another 100 years, presuming that deforestation had stopped, but that the previous land that had been used for the production of palm oil is not being reconverted to land viable for orangutan use (for example, the land being too drained of nutrients to support rainforest, being converted to the production of another crop, or even continuing to be used for palm oil production, in order to support global demand which may never be able to be abolished - more information on these likely scenarios is available on our economics and bioethics pages, found here).

Figure 2: Predicted Sumatran orangutan population levels, if deforestation in Indonesia was halted after 40 years.

We can see from these simulations that orangutans at this small a population are unable to recover from the effect that palm oil has had, or even reach carrying capacity in the land which has been left to them. This is likely to be due to a mixture of the sparse population density of the orangutan leading to lower frequencies of mating, and inbreeding of the remaining population. In addition, there are possibilities that the population of the orangutans could drop to dangerously low levels, below 200 individuals, where a chance event such as flooding and landslides could erase the remainder of the population, meaning that we could still see an extinction of the Sumatran orangutan within the next 140 years.

We also ran simulations for if certain events occurred in 30 years, rather than 40 years, time. As our simulations for this time period showed more variance, we modelled both a best case (2520 individuals remaining) and worst case (1664 individuals remaining) scenario.

Figure 3: Predicted Sumatran orangutan population levels, if deforestation in Indonesia was halted after 30 years and populations at this points were at the worst case scenario level indicated by previous simulations (1664 individuals)

Figure 4: Predicted Sumatran orangutan population levels, if deforestation in Indonesia was halted after 30 years and populations at this points were at the best case scenario level indicated by previous simulations (2520 individuals)

From Figure 3 and 4, we can see that in most cases, orangutan population levels remain at a viable level, although in certain instances, struggles to maintain a population at carrying capacity. However, it would seem that if we were able to implement our project within a 30 year time frame, the orangutan may be able to maintain a healthy population size.

However, due to the giant demand for palm oil demonstrated during our economic investigations, it is unlikely that we will ever be able to entirely replace the palm oil industry. Therefore, we also modelled the orangutan populations at this critical 30 year point if we were able to lower annual rainforest loss by a half.

Figure 5: Predicted extinction of Sumatran orangutan, if annual loss of rainforest coverage could be halved (1.18%) in 30 years, with an initial population size of 2520 individuals

We can see that lowering the rate of deforestation has a massive impact on the population of the orangutan, with an estimated extinction date of 105 to 118 years as opposed to 45.

Discussion

We can see that a decline in the rate of traditional methods of palm oil production would have a massive impact on the survival on the Sumatran orangutan. Our estimates suggest that the tipping point is around 30 years from now - if we have not implemented a method to reduce traditional methods of palm oil by this time, we can expect to see an extinction of the Sumatran orangutan. It is important to note that the actual date could be earlier than this, as detrimental effects to the population such as hunting has not be modelled.

At these levels, programs such as rehabilitation of captive orangutans could have a dramatic impact on the survival rate of the Sumatran orangutan. This could be one of the most feasible methods of rescuing the orangutan population. Captive orangutans might not be competitive enough to survive in the wild though, and may not have developed resistance to diseases found in the wild, which would be detrimental to their survival when released. However, thousands of wild orangutans are currently in sanctuaries throughout the world and introducing these back into the wild would dramatically increase this species’ chance of survival.

For future work, we would like to investigate the effect that reformation of the land previously used for the palm oil industry would have in the event that our project would be able to stop deforestation completely.

Conclusion

With these population models built based on the current rainforest conditions, we can tell that the Sumatran orangutan would definitely become extinct in the near future if we don’t do something to restore their numbers. The only way to bring the Sumatran orangutan back from the brink of extinction is to end deforestation. Perhaps unsurprisingly, the time at which deforestation is halted makes a massive difference to survival rates, and the earlier we do so the larger the orangutan population would be. It follows that a larger population of orangutan would improve chances of future population growth and maintenance of a viable population size.

This imminent extinction does not only affect the orangutan of course, countless species of endangered flora and fauna could be saved at the same time. Deforestation is currently the only way to make more room for palm oil plantations but it is a man-made disaster which, as demonstrated in these models, disturbs the ecosystem and speeds up the extinction of many species. But with such a high demand of palm oil in the market, deforestation is not going to be overcome unless the naturally occurring palm oil can be replaced by an alternative, perhaps a synthetic biology-derived one. Thus, by the implementation of our project, the introduction of synthetic palm oil, we believe that these helpless animals and their habitats can be saved in the most effective way.

References