Not Giving Into the (Selection) Pressure
A common measure of prey immune function is not constrained by the cascading effects of predators (2021) Hasik et al., Evolutionary Ecology. https://doi.org/10.1007/s10682-021-10124-x
Image Credit: Adam Hasik, Image Cropped
The immune function is a critical component of an organism’s ability to defend itself from parasites and disease. Without it, we would be in much worse shape when we got sick. Despite this usefulness, the immune function is costly to use as organisms have to consume enough food to have the energy needed to mount an immune response. This is easier said than done, however, and there are often many factors that come into play when it comes to acquiring energy.
For example, I previously broke down another study showing that predators limit the strength of competition among prey animals. This can happen through a few paths (see diagram below). First predators will eat prey animals and reduce their population size (a direct effect), limiting how many prey animals there are to compete for food. Second, some prey animals will reduce their activity in the presence of predators (an indirect effect), and because they reduce their activity they are not able to consume as much food. Finally, natural selection occurs when predators eat the more active prey, and only the less active individuals are left. This combines the direct and indirect effects, and all of these pathways means that on the whole there is less competition for resources among the prey. If there is less competition, then there is more food, and prey animals should have better immune functions. Using the same animals from that previous study, my colleagues and I asked if these three effects of predators affected the immune function of prey animals.
What We Did
For more detail on the study itself, please see the previous post. In short, we tested damselflies in the lab to quantify how the direct effects of predators on reducing prey numbers, the indirect effects on activity rates, and the combined effects via natural selection affected damselfly growth rates. Any reduction in growth rate is evidence for competition among the damselflies. We first tested this in the lab to separate these various effects, and we then repeated the experiment in the field in six different lakes that varied in predator (fish) density and prey density. We found that fish predators did indeed reduce the strength of competition among damselflies.
For this study, we were interested in how these same effects of predators affected damselfly immune function, specifically the phenoloxidase (PO) cascade. The PO cascade is a measure of innate immune function (see Did You Know), and if the effects of predators on prey competition reduce PO then that may leave damselflies more vulnerable to attack from parasites. To measure if predators affected PO, we collected damselflies from both the lab and field experiments and measured how much PO each damselfly had. If PO increased as competition among damselflies decreased, this would be evidence for a link between the effects of predators and damselfly immune function, and by extension a link between predators and parasites.
Did You Know: Innate vs. adaptive immune function
Innate immune function is a non-specific immune defense, meaning it is more general in nature and not geared toward a specific parasite. The PO cascade is a great example of innate immune function, because it works by sealing any foreign body in melanin, killing the threat and protecting the host. Adaptive immune function, on the other hand, is a specific immune response geared toward a threat that the body has previously detected. An example of that is the vaccine against Covid-19, which works by giving your body an inert “piece” of the virus to build a defense against. That way, if you come into contact with the actual virus, your immune function is now ready to fight it off.
What We Found
There were no effects of predators on PO, neither direct effects via changes in damselfly number, indirect effects via changes in activity levels, nor via the combined effects of natural selection. This was true for both the lab experiment and field experiment. However, in the field we found a relationship between prey density and PO, meaning that PO does indeed depend on how much food is available for the damselflies, but competition was not strong enough to limit how much PO they made.
Though we sought to link the effects of predators to prey immune defenses, we only tested one aspect of immune function. Immune defenses are multi-faceted, and PO is not the only weapon in the damselfly arsenal to defend themselves from a parasite. Previous studies have shown that different aspects of immune function can either respond the same to a given stimulus, or they may have the opposite response! So in the future it will be important to test for relationships between predators and other aspects of immune function.
This study is a classic example of “negative results”, and by that I mean that we did not find what we expected to find. Negative results are (unfortunately) sometimes seen as uninteresting, but our negative results in this study are anything but! We showed that the effects of predators do not limit a critical component of immune defense, despite those same effects having been shown to have effects on competition and growth in the same individuals. This means that not only are growth and immune function apparently disconnected from one another in this system, but so are predators and parasites. I cannot wait until future studies are able to illuminate more about these various relationships.
Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is frantically preparing for a move to the other side of the world. 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.