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Louis T. Bubrig and Amanda K. Gibson: Read the article
Bubrig et al. modeled host-parasite interactions where the host population experienced recurring bottlenecks: turns out that bottlenecks can hugely benefit hosts in their quest to rid themselves of parasites!
When studying how different species interact with each other, it is straightforward to picture that they exist at an equilibrium. For our purposes, we consider equilibrium as an expected numerical balance between each interacting species within an ecosystem. In an equilibrium model, when numbers depart from this balance, they eventually return to it. Equilibrium gives a tidy scenario where you can measure how species will react to a changing climate because they will undoubtedly settle back into a familiar numerical balance. The diversity of our world’s ecosystems provides us with a variety of species with different life histories, some of which go against this idea of a balanced ecosystem. In light of this fact, how can we understand and model species interactions that reject equilibrium?
Prevalent exceptions to the trend of returning to equilibrium are species that experience boom-bust life cycles. These species may have a booming increase in population followed by a bust, a decline in their population size. That bust could be caused by a disturbance to the population, including disease, natural disaster, or lack of resources. Imagine a hypothetical species named species A. At the start of a year, there are plenty of resources allowing species A to grow to a population of 1,000. After six months, resources are dwindling, and the lack of available food causes the population to decrease to 100. This demonstrates one instance of a boom-bust. Imagine that after six months, resources are plentiful again. This becomes a boom-bust cycle, as species A can then rapidly increase again by taking up all the resources, followed by a population bust as their resources become scarcer. The word disturbance seems inherently negative, but many species seek out disturbances. These species exploit unstable environments and temporary resources. To understand the spread of disease and interaction changes in these species that thrive within boom-bust cycles, studies need to focus on non-equilibrium dynamics.
A common species interaction is between a host and its parasite. Bubrig and Gibson set out to create a mathematical model to predict parasite prevalence for host species that experience boom-bust life cycles. For this model, a parasite is an organism that can be transmitted between members of the host species, and parasite prevalence is the percentage of hosts that are infected. A decrease in the host population was referred to as a bottleneck. Parasite prevalence was assumed to be 100% when no bottleneck occurred. To observe the effect of host bottleneck occurrences on parasite prevalence, Bubrig and Gibson modeled recurring bottlenecks, bottlenecks at an increased frequency, and bottlenecks at an increased severity.
To determine what could happen when multiple bottlenecks happened consecutively, they set the model to perform recurring bottlenecks that were kept at a consistent frequency and severity. After repeated bottlenecks, parasites lost the ability to fully infect the host population and were kept at a maximum value that was lower than the initial 100%. The reason for this is that after a bottleneck event, the hosts reproduce faster than the parasites can spread throughout the uninfected portion of the host population. The hosts now have more space to reproduce, and parasites have a further distance between hosts to travel.
With the understanding that recurring bottlenecks give the host an advantage, the next step is to see if increases in frequency and severity of the bottlenecks may exaggerate this effect. For this portion of the model, parasite prevalence may become so low that parasite extinction can occur. First, a higher frequency means more bottlenecks occurring in a shorter period. As frequency increased, parasite prevalence had an increasingly difficult time recovering, as host birth rate was higher than parasite transmission. In this situation, parasite extinction became slightly more likely. Next, higher severity means that each bottleneck reduces the population to a smaller value. As the host population decreased to lower and lower values, the parasite prevalence decreased as well. Parasite extinction became significantly more likely, as removing more individuals from the population increases the chances of eliminating all the infected individuals.
This paper holds open the door for further research on non-equilibrium modeling. This model should encourage the collection of experimental data on specific host species that experience boom-bust cycles. Bubrig and Gibson were transparent about some of the areas where their paper can be expanded upon, and future researchers should walk through that open door. While a straightforward problem may seem ideal to solve, complexities are what allow the Earth to be so essential, productive, and beautiful. Understanding these complexities will benefit our knowledge of non-equilibrium host-parasite dynamics, but to similar importance, will allow us to better protect these delicate ecosystems.
Mel Grillo is a second-year PhD student in the Bolnick Eco-Evo-Immuno Dynamics Lab at the University of Connecticut. Their research focuses on analyzing untargeted metabolomics datasets to understand within and among population differences in threespine stickleback (Gasterosteus aculeatus). Mel believes that decoding the metabolome will greatly aid our understanding of phenotypes. Beyond their research, Mel plays the piano and watches too much reality television.
