The relative roles of top-down (consumer-driven) and bottom-up (resource-driven) forcing in exploited marine ecosystems has been a much-debated topic in recent decades. Examples from diverse marine systems of exploitation-induced, top-down trophic forcing have led to a general view that human-induced predator perturbations can disrupt entire marine food webs, yet other studies reporting no such evidence provide a counterpoint. Though evidence continues to emerge, an unresolved debate still exists regarding both the relative roles of top-down vs. bottom-up forcing in marine systems as well as the capacity of human exploitation to instigate top-down, community-level effects. Using time-series data for 104 reef communities spanning tropical to temperate Australia from 1992-2013, we explored the relationships among long-term trophic group population density trends, latitude, and exploitation status over a wide geographical range.
Results/Conclusions
We found that trophic control tended to be more bottom-up driven in tropical systems and more top-down driven in temperate systems. Further, alternating long-term population trends across multiple trophic levels (a method of identifying trophic cascades), presumably due to top-down trophic forcing, occurred in roughly fifteen percent of locations where the prerequisite significant predator trends occurred. Such trends were significantly more likely to occur at locations with increasing densities of predators over time. Within these locations, we found a marked latitudinal gradient in the prevalence oflong-term,alternating trophic group trends, from rare in the tropics (<5% of cases) to relatively common in temperate areas (~45%). Lastly, the strongest trends in predator and algal density occurred in older reserves; however, exploitation status did not affect the overall chance of alternating long-term trophic group trends occurring. These data suggest that both the type and degree of trophic forcing is related to one or more covariates of latitude, and that ecosystem resiliency to top-down control does not universally vary as a function of exploitation level.