Association behavior between sand tiger sharks and round scad is driven by mesopredators

In marine systems, behaviorally-mediated indirect interactions between prey, mesopredators, and higher trophic-level, large predators are less commonly investigated than other ecologic interactions, likely because of inherent difficulties associated with making observations. Underwater videos (n = 216) from SharkCam, a camera installation sited beneath Frying Pan Tower, a decommissioned light house and platform, on a natural, hard bottom site approximately 50 km off Cape Fear, North Carolina, were used to investigate association behavior of round scad Decapterus punctatus around sand tiger sharks Carcharias taurus. Videos containing sand tiger sharks were analyzed for the simultaneous presence of round scad, and six species of scad mesopredators, with scad-shark interactions assigned to one of three categories of association: no visible interaction, loosely associated, or tightly associated. The likelihood of scad being loosely or tightly associated with sharks was significantly higher in the presence of scad mesopredators, suggesting that sharks provide a predation refuge for scad. This behaviorally-mediated indirect interaction has important implications for trophic energy transfer and mesopredator control on hard bottoms, as scad are one of the most abundant planktivorous fish on hard bottoms in the western Atlantic Ocean. Although we were not able to provide statistical evidence that sand tiger sharks also benefit from this association behavior, we have clear video evidence that round scad association conceals and attracts mesopredators, enhancing predation opportunities for sand tiger sharks. These interactions potentially yield additional trophic consequences to this unique association and highlight the value of exploring behaviorally-mediated interactions in marine communities.

Low predation intensity on the stalked crinoid Democrinus sp. (Echinodermata) in Roatán, Honduras reveals deep water as likely predation refuge

Predation has been hypothesized to play a key role in the evolutionary and ecological history of crinoids. Whereas evidence of predation on crinoids in the form of injuries can be common, quantifying predation intensity, which is critical for properly evaluating such hypotheses, has proven challenging. Here, we used a longitudinal approach to quantify predation intensity on the extant, deep-water, stalked crinoid, Democrinus sp. The quantitative estimates are based on data collected from a manned submersible during expeditions conducted over a 3-yr span. These results indicate that this deep-water crinoid is subject to much lower predation intensity than are crinoids living in shallow water, consistent with (1) an inverse relationship between predation intensity and depth, and (2) the hypothesis that for stalked crinoids, which are unable to handle high predation intensity, deep water is a refugium.

Top predators suppress rather than facilitate plants in a trait-mediated tri-trophic cascade

Classical ecological theory states that in tri-trophic systems, predators indirectly facilitate plants by reducing herbivore densities through consumption, while more recent work has revealed that predators can generate the same positive effect on plants non-consumptively by inducing changes in herbivore traits (e.g. feeding rates). Based on observations in US salt marshes dominated by vast monocultures of cordgrass, we hypothesized that sit-and-wait substrate-dwelling predators (crabs) could actually strengthen per capita impacts of potent grazers (snails), by non-consumptively inducing a vertical habitat shift of snails to their predation refuge high on canopy leaves that are vulnerable to grazing. A two-month field experiment supported this hypothesis, revealing that predators non-consumptively increased the mean climbing height of snails on grasses, increased grazing damage per leaf and ultimately suppressed cordgrass biomass, relative to controls. While seemingly counterintuitive, our results can be explained by (i) the vulnerability of refuge resources to grazing, and (ii) universal traits that drove the vertical habitat shift—i.e. relative habitat domains of predator and prey, and the hunting mode of the top predator. These results underline the fact that not only should we continue to incorporate non-consumptive effects into our understanding of top-down predator impacts, but we should do so in a spatially explicit manner.

Effect of predation on male mating behaviour in a unisexual-bisexual mating system

Mate choice for conspecifics is beneficial when closely related species live in sympatry, but mate choice can be costly in the presence of predators. Male sailfin mollies are sexually parasitized by gynogenetic Amazon mollies. Amazon mollies must mate with male sailfin mollies to initiate embryogenesis, but inheritance is maternal. We tested if male sailfin molly mate choice for conspecific females is affected by predation risk. Male mate choice was tested in one of four treatments: (1) predation/no refuge, (2) predation/refuge, (3) no predation/refuge and (4) no predation/no refuge. Predation consisted of dipping the beak of a great blue heron decoy in the aquarium prior to a mating trial. Refuge was provided by java-moss. For each trial the number of mating attempts toward each female was recorded. There was a significant interaction between predation and refuge on strength of preference (SOP) for conspecific females. The highest SOP was in the no predator/no refuge treatment, and the lowest SOP was in the predator/no refuge treatment. These results suggest that the cost of predation is higher than the cost of mating with heterospecifics, and that the presence of a refuge may reduce this cost. This could explain the continued maintenance of Amazon mollies.