Hunting and Evolution
Hunting alters viral transmission and evolution in a large carnivore (2022) Fountain-Jones et al., Nature Ecology & Evolution, https://doi.org/10.1038/s41559-021-01635-5
It’s no secret than humans have had an enormous impact on the native wildlife of our planet, and we have looked into many of these complicated relationships and effects before on Ecology for the Masses. One common interaction is that of hunting, whereby humans hunt and kill an animal for recreation and/or food. Regardless of your feelings on hunting, such removal of animals can be an issue in systems where there is density-dependent transmission, meaning the more animals there are, the more likely there is to be parasite transmission within the populations of these animals. Reducing animal populations via hunting can either decrease, have no effect on, or even increase density-dependent transmission.
These changes in transmission dynamics (and subsequent changes in infection patterns) will have effects on the evolution of the parasites infecting these animals, making it easier for researchers to detect if (and how much) transmission is occuring. To investigate these patterns, today’s authors studied data on feline immunodeficiency virus (FIV) and its puma (Puma concolor) hosts. FIV is mostly benign and infects its hosts for life, though puma hosts can become infected with different strains of FIV. The goal of today’s study was to understand how hunting affects transmission dynamics of FIV within populations of puma that are hunted.
Did You Know: Transmission Dynamics
Given that we are all currently living within a pandemic caused by a viral outbreak, it’s likely that you may have heard the term “transmission dynamics” before. Transmission dynamics are simply the ways in which a virus/disease spreads from one individual to another within a given population. By studying these dynamics, we can track where and when an outbreak started, as well as identify so-called super-spreaders, or those individuals that are responsible for more transmission events to uninfected individuals than the average infected individual.
What They Did
The authors compared transmission dynamics between two puma populations separated by ~500km. One population has experienced varying levels of hunting pressure (hereafter the treatment region), and one population that has experienced stable management conditions (hereafter the control region). Using data on viral infections among the infected pumas, they reconstructed a transmission tree, which would estimate which puma infected another puma, and when.
Next, the authors tested for an effect of puma population size on the population growth rate of FIV. They compared the population size of FIV to the population sizes of male and female puma using correlational analyses. Unfortunately, the authors could not conduct these analyses in the control region (see Problems?).
What They Found
Though the treatment and control regions were of a similar size, the authors found that the population growth rate of FIV was over twice as high in the treatment area. There was also a burst in transmission within the treatment area during the period when hunting ceased, which was likely the result of transmission of males that became numerically dominant in the area. Indeed, the probability of a male in the treatment area being involved in a transmission event was twice that of a female.
Comparing the population growth rates of FIV to the population sizes of male and female puma revealed even more evidence for males driving transmission dynamics in this system. Specifically, FIV population growth rate increased with the population size of male puma, but there was no relationship with the population size of the females. Interestingly, this only occurred in the treatment area, likely due to the changes in male population size. That is, males were hunted more than females, and when hunting stopped males increased in number, which increased competition among males for females. Such competition results in aggressive interactions among males, which transmits FIV.
As I said above, the authors did not investigate the relationship between FIV population growth rates and puma population size in the control area. This happened because the authors did not have comparable data on puma population size in the control area (which could mean that they did not have as many samples or the puma population size wasn’t measured the same way for the same amount of time). These limitations are part of data collection, especially when working with a large carnivore like the puma. While I cannot fault the authors for this, I do feel that it limits the conclusions of this study, as we only know part of the story.
This study is important for two main reasons. First, the methods used can be utilized in other host-parasite systems to better understand how management practices affect disease dynamics within the managed population, which is incredibly important for populations of animals facing threats from disease, such as the Florida panther and Tasmanian devil. Secondly, the methods are also useful for understanding the evolutionary relationships between and histories of free-ranging host species and their parasites.
Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who has never seen a puma in real life (and that’s not a bad thing). You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.