Rapid population growth and high management costs have created …
We estimate that as of the close of 2022, there were approximately 91 hippos in the middle Magdalena, and the population is growing at 9.6% per year. At this rate, there will be 230 hippos within a decade (at close of 2032) and over 1000 by 2050. Given the high population growth rate of hippos and the current number of animals, it will require significant investments of money, effort, and time to remove hippos from the wild in Colombia even if aggressive interventions start immediately.
Delaying management actions for a decade will increase minimum costs by a factor of 2.5, and some interventions may become infeasible. If the population control methods we examined here are applied immediately and at large scales, it would require a minimum of 0.5-4.6 million USD to remove hippos from the Magdalena River if cost-minimizing approaches are implemented (Table 1). Instead, if annual effort-minimizing approaches are implemented, the total costs would more than double (1.14–9.96 million USD).
There are significant challenges associated with each management approach, and social and ecological considerations will need to be weighed to determine the best path forward. However, the longer interventions are delayed, the more difficult and expensive they will become. In another decade, minimum costs will increase to 1.3-11.5 million USD, and the logistics of treating the population would become increasingly difficult with an estimated 210 hippos at large by the close of 2031.
Furthermore, action delays that lead to increasing hippo population size and length of time on the landscape could also increase the possibility of natural or human-mediated dispersal into other systems. Male sterilization is the least expensive way to control the hippo population among the management interventions considered here. However, this process entails hippos being captured, anesthetized, transported by helicopter, and surgically operated upon; thus, it is very challenging and can be dangerous for both the people and hippos involved.
Further, male sterilization must be done on many animals every year in order to be effective as a population control strategy. This management intervention also fails to remove hippos from the landscape, and given their long lifespan, would result in hippos remaining in the middle Magdalena for several more decades at least. Therefore, this approach alone may not reduce risks hippos pose to people who use the river for their livelihoods or to native biodiversity in the Magdalena River basin8.
Veterinary-assisted euthanasia is the second most cost-effective method of population control, although it is still relatively expensive given the requirement to capture and anesthetize hippos first. However, it is the most effective way to remove hippos from the landscape. Dart-based contraception was the third most cost-effective method, as the drugs are inexpensive.
However, this method likely must be applied every year and for many years consecutively. Research in other taxa has suggested injection of GonaCon as a contraceptive may promote long-term infertility after several rounds, although this has yet to be demonstrated in hippos23. It would also be challenging to use on hippos outside of captivity, as capture is typically needed to ensure intramuscular injection.
Oral-based contraception, which is frequently used in captive populations, was the most expensive of the options. It also would be very challenging to apply to hippos outside of Hacienda Napoles, suggesting another method would be needed for these individuals, or they would need to be captured and relocated to captivity. Both dart- and oral-based forms of contraception would also leave hippos on the landscape for many years to come, with associated risks to social and ecological systems.
Elasticity analyses suggest that targeting young female hippos is the most effective way to decrease the hippos’ population growth rate; however, management intervention options to target this demographic group through mortality or fecundity control are limited. While theoretically these animals could be preferentially targeted through birth control or veterinary-assisted euthanasia, it is difficult to identify hippo age and sex without capturing the animal first. Additional options include preferential relocation of young females to zoos when possible, and surgical sterilization of females, although female surgical sterilization is even more complex and dangerous than surgical sterilization of males.
We did not explicitly incorporate the spatial structure of the hippo population in our estimates, which may further compound the challenge and cost of management, although our management cost estimates did include higher costs associated with hippos at large outside of Hacienda Napoles. Approximately 53% of the current estimated population lives within Hacienda Napoles, 24% lives in nearby lakes, and 23% lives in the mainstem Magdalena River or much further downstream. It is likely that as hippos spread further into the landscape, the logistical challenges in locating and handling hippos will increase significantly, and some population control approaches, such as feed-based contraceptives, may become nearly impossible to implement24. Other approaches that require animal capture and sedation, such as male sterilization and veterinary-assisted euthanasia, will be much more logistically challenging, particularly as individuals continue to disperse, and these dispersal events will likely continue to seed population growth in the surrounding areas.
Ensuring containment of hippos within Hacienda Napoles to prevent further escapes will improve chances of managing the population. However, containment itself is costly and challenging, in addition to the management costs we estimate here. Furthermore, some people disagree with containment of the hippos, arguing they should be allowed to stay free, suggesting even this management option is not without potential social constraints (Anderson et al.
In prep). It is possible that some other driver could act to decrease population growth, or even lead to a population crash, such as the emergence of a zoonotic epidemic or inbreeding effects from a strong founder effect that reduces genetic diversity in the population. Many invasions follow a boom-bust dynamic25. Hippos in particular can be heavily impacted by anthrax outbreaks and can be sensitive to starvation26. The population in Colombia started with only 4 individuals, only one of which was male, suggesting that the relatively low genetic diversity in the founding population and subsequent inbreeding could lead to reduced genetic fitness27,28, although effects of inbreeding are not always as strong as predicted in wild populations29.
The Colombian hippo population is currently growing at a rate similar to that of healthy, resource-unlimited populations within their native range. Furthermore, the central Magdalena basin lacks the pronounced dry season that often acts to limit population growth in hippos’ native range through periods of drought and associated hippo health declines10. Given the rapid rate of annual population growth and the uncertainty of any future limits on this growth, it is critical that management practices are adopted quickly.
The cost of interventions is extremely high and exceeds the current available funding for hippo management in the Magdalena River. Furthermore, our results are likely conservative because they assume the first and last hippo to be treated are the same cost, whereas search costs for any of the treatment options may increase as the hippo population declines. These costs will continue to increase over time if substantial interventions are delayed.
A 10-year delay will result in 2.5 times increase in minimum costs for each management approach. Culling hippos without the considerations required for veterinary-assisted euthanasia could provide a more cost-effective management approach than those we explored here. However, it is currently not a legal option in Colombia and would likely engender strong objections by some parties18. Additional management resources beyond those that are currently available will need to be mobilized to support introduced hippo management in Colombia, which may in turn lead to diversion of resources from other conservation efforts in the region, to the potential detriment of other flora and fauna in the Magdalena River, a riverine biodiversity hotspot30,31,32,33. International funding could be sought to assist with the hippos’ management, but this would likely divert resources that could otherwise go to bolster research and conservation efforts in the species’ natural range. Social and financial constraints have limited management options for hippos in the Magdalena since the start of their introduction to the wild in the late 1990s-early 2000s.
These constraints allowed the hippo population to begin growing at a rapid rate that has now increased the cost and difficulty of management options. It will now take at least 1-2 million USD to sufficiently decrease hippo population growth, and depending upon what management strategy is selected, there may still be hippos on the landscape for 50-100 years. Increased population size and persistence will likely increase the potential for natural and human-mediated dispersal, so hippos are likely to continue to expand throughout the Magdalena River basin and possibly into other regions as well.
These hippos will continue to influence social and environmental systems during this time, with consequences for goods and services provided for human communities as well as for biodiversity. Social and ecological impacts of this introduction are still nascent but are likely to become much more pronounced with increased population growth. Even if social perceptions about appropriate management approaches shift over the coming years, some management options may become infeasible in the future given the current rapid annual rate of population growth.
This case study illustrates the complex interplay between social, financial, and ecological dynamics that can influence the trajectory of species invasions and suggests early stakeholder communication and outreach is a critical component of invasive species management plans4,34.
Our results highlight the urgent need for sufficient funding to undertake a large-scale effort to control the growth of the Colombian hippo population while it is still feasible.