The Enemy’s Enemy Is My Friend: Trophic Cascades and Indirect Predator Effects
For most animals, there are few things as important as reproduction and the survival of their offspring. Likewise, selecting safe nesting sites is an important matter for birds, as it is directly related to the survival of their eggs and chicks, and it would be logical for birds to choose nest sites with a reduced rate of exposure to predators. It may therefore surprise you to know that the nest survival rates of the black-chinned hummingbird, also known as the Archilochus alexandri, are higher when nests are built closer to the nests of the northern goshawk, the Accipiter gentilis, or to those of Cooper’s hawk, the Accipiter cooperii. At first glance, this makes no sense; after all, hawks are one of the most formidable predators in the avian world, and hummingbirds would hardly be expected to come into their proximity, much less become their neighbors. However, predator-prey relationships are rarely so simple, and when a mesopredator — a predator at the middle level of the food chain — is involved, the presence of a hawk may in fact prove to be beneficial for hummingbirds and their nests.
In this case, the Mexican jay, the Aphelocoma wollweberi, is the mesopredator, and while Mexican jays are known to devour hummingbird eggs and chicks, they are also preyed upon by other predators such as hawks. Hummingbirds, however, are too small to serve as a food source for hawks, and face no risk of predation from them. Hawks swoop down from perches in horizontal or descending chases to hunt, meaning that Mexican jays must stay at least as high above ground as hawks in order to avoid predation. This behavior creates a cone-shaped space surrounding active hawk nests which jays generally avoid, creating a relatively safe area for nesting hummingbirds.
As seen in the Mexican jay’s case, high-level predators can cause behavioral or numerical — or both, in some cases — changes in middle-level species, subsequently affecting lower-level species. Such indirect effects are known as trophic cascades, with ‘trophic’ referring to the trophic level (the position an organism occupies in a food chain). For example, in a three-level system consisting of carnivores, herbivores, and plants, carnivores could indirectly benefit plants by killing and therefore reducing the number of herbivores. This is classified as density-mediated indirect interactions, as it is mediated by changing herbivore populations or density. On the other hand, the mere presence of carnivores could also impact herbivore density and behavior. Herbivores wanting to avoid contact may exhibit predator-avoidance behavior such as dispersal or reducing foraging activity to increase vigilance. This is referred to as trait-mediated indirection interactions (TMII), as it is induced by affecting prey traits instead of their actual numbers or density.
In the case of TMII, populations consist of individuals that vary in size, age, and physiological condition even within the same species, and these differences may affect individual choices concerning foraging gains versus the risk of predation. If reducing feeding to avoid exposure to predators results in a complete failure to mature in a small individual, it would be natural for the individual to increase foraging behavior despite the fact that it may increase the risk of predation. An experiment conducted by Ovadia and Schmitz in 2002 using “nursery web” spiders, known scientifically as Pisaurina mira, red-legged grasshoppers, or Melanoplus femurrubrum, and grasses and herbs provided an answer for how this individuality would impact the indirect effect of predators on plants. Individual grasshoppers were sorted into three groups depending on body size; large individuals were those in the upper 15% in size, small individuals were those in the lowest 15%, while average individuals were from the middle portion of the grasshopper samples. The three groups of grasshoppers were then each further divided into two groups where spiders were either present or absent, resulting in a total of six groups. The size of the remaining grasshoppers were measured by the end the end of the experiment. It turned out that the grasshopper belonging to the small group suffered from higher mortality rates but exhibited higher growth rates than the grasshoppers in the middle and large classes. It can be assumed that this higher rate of growth and mortality was achieved by higher foraging activity, which compensated for the lower number and density of the grasshoppers. Therefore, it may be acceptable in some cases not to take individual trait variations into account when studying such indirect effects, although it remains largely unclear due to a shortage of evidence.
There exists an abundance of questions, and in turn, hypotheses, regarding trophic cascades. It is yet uncertain whether DMII’s or TMII’s have a greater impact on ecosystems; many experiments demonstrating TMII’s have been conducted over short periods of time, but experiments drawing comparisons between the effectiveness of respective indirect interactions are yet to be done. Ideas suggest that differences in predator species, size of habitat domains, and prey hunting modes may elicit different antipredator behavior in prey, therefore resulting in variations of the strength or even nature (whether it is DMII or TMII) of trophic cascades. Overall, an understanding of trophic cascades is becoming more and more relevant as they are increasingly applied to the control, management, and conservation of various ecosystems, and I hope to see further efforts to elucidate this phenomenon that once again reminds us of the depth and complexity of nature.