Top predators: Active vs. sit and wait ambush

           Stolzenburg discusses extensively the detrimental effects of the removal of wolves from Yellowstone National Park. Tree saplings such as Aspen and Cottonwood were being consumed at a fatal rate by the over abundance of Elk. Once all wolves were removed from the park in the 1920s, Elk became more and more abundant on a steady diet of tree saplings. It was at this time that populations of these trees species were threatened to the point that they may not recover, all because the top predator (the wolf) was removed.

            The removal of wolves from Yellowstone shows the important role that top predators have on different ecosystem processes. I wanted to find other examples of the roles of top predators in ecosystems and came across a paper by Schmitz (2008) entitled, "Effects of Predator Hunting Mode on Grassland Ecosystem Function". This paper addresses the idea that top predators are essential to ecosystems while also introducing another interesting concept. Schmitz postulates that different modes of hunting of top predators can have differential cascading effects on ecosystem processes.

From Schmitz (2008)

            Schmitz distinguishes two separate modes of hunting; there are active hunters that have wide ranges and there are predators that "sit and wait" and ambush their prey. Active hunters may alter species densities by capturing and consuming, but may not cause chronic behavioral changes in prey. Alternatively, sit and wait predators can cause behavioral responses, or shifts in foraging, in prey because they must continually respond to ambush from predators in a set location. Schmitz hypothesizes that these two distinct predator modes will alter ecosystems in different ways.

            To test his hypothesis Schmitz used grassland ecosystems containing Solidago rugosa , a dominant competitive plant species, along with other herbaceous species (preferred by grasshoppers). Grasshopper species graze upon these plants species while being predated upon by two different predatory spiders; one that actively hunts, and one that sits and waits. Schmitz observed varying ecosystem responses when each type of spider was removed from his treatments.

            Schmitz's findings suggest that the actively hunting spider reduces the numbers of grasshoppers, thus having a positive effect on grasses and S. rugosa. Alternatively, the sit and wait predator may cause a foraging shift in grasshoppers from the preferred nutritious grasses, to the more common S. rugosa which could be considered the "safer" plant to forage. Ultimately, grasshoppers must choose between feeding on preferred grasses and quickly taking refuge from predators, or feeding on S. rugosa (the dominant plant). Overall, this paper introduces an additional variable (different hunting modes) that should be considered when analyzing the role of top predators play in ecosystems.