The dramatic reorganization of ecosystems and losses of biodiversity that frequently follow the loss of top predators have provided ecologists the insight into that top predators shape and maintain healthy ecosystems around them in marine, aquatic and terrestrial environments and that their effects have the potential to be harnessed to manipulate ecological processes and species abundances for the benefit of biodiversity conservation (Soul´ e et al., 2003). Top predators often have positive effects on biological diversity owing to their key functional roles in regulating trophic cascades and other ecological processes. Top-order predators typically exert top–down control on ecosystems through their direct predatory and competitive interactions with herbivores and smaller predators (Frank et al., 2005). The disruption of these interactions can have dramatic effects on lower trophic groups, and result in declines in small species of animal prey (Crooks & Soul´ e, 1999) and the depletion of plant species diversity and biomass (Estes & Duggins,1995; Terborghet al., 2001). Besides, the loss of top-order predators has been identified as a key factor contributing to continuing species extinctions and the global biodiversity-loss crisis (Duffy, 2003; Ray, 2005).
Recent studies from diverse systems show that predators influence prey populations and communities by inflicting mortality on prey (Direct predation) and inducing costly antipredator behavior by their prey (Risk effect: changes in prey specie resulting from behavioral responses to the risk of predation.)
Researchers of marine systems involving large-bodied predators often implicitly assume that trophic cascades occur via direct predation (so-called lethal effects) on mesoconsumers (predators or herbivores in mid-trophic levels). Using this framework, the effects of predation could be fully quantified based on the diets, metabolic rates and abundances of predators and data on prey population dynamics 18. Declines in top predator abundance should release mesoconsumers from predation and indirectly increase the mortality rate of resource species (a species that is eaten by mesoconsumers) 5,19. The loss of top predators is thus predicted to cause numerical increases in mesoconsumers and declines in resource species.
Beyond direct mortality, however, predators also strongly affect prey behaviors, such as foraging 20,21. A large number of studies show that organisms can reduce predation risk through behavioral mechanisms. Here, unlike indirect effects of predator-inflicted mortality, whereby lower densities of mesoconsumers increase the density of resource species, risk effects might mediate predator effects on resource species without influencing equilibrium population densities of mesoconsumers. This can occur when predation risk alters the intensity and spatiotemporal pattern of mesoconsumer exploitation of resource species without suppressing mesoconsumer populations (i.e. they are not limited by bottom-up forces). Thus, the numerical responses of mesoconsumers and resource species to top predator declines are the sum of direct predation (density-mediated) and risk (behaviorally mediated) effects 28. The relative importance of either mechanism varies from case to case, but a growing body of evidence suggests that behaviorally mediated indirect effects can be surprisingly strong.
Hence, widespread declines of large predators across the world’s oceans are expected to strongly influence smaller-bodied mesoconsumers and the species that are eaten by mesoconsumers. Long-line surveys in the tropical Pacific documented up to 10-fold declines in catch rates of 12 large pelagic predators (tunas, billfishes and sharks) from 1950 to 2000 coincided with 10- to 100-fold increases in catches of pelagic stingrays (Dasyatis violacea) and other smallbodied mesoconsumers over the same timeframe (Ward, P. and Myers, R.A. 2005).
This demonstrates that declines in top predators can impact several trophic levels and affect other fisheries. Although the consequences of top predator removal can vary across communities 12,13, an increasing number of studies are detecting large-scale cascading effects.
In case of doing a comparison, the average magnitude of direct effects of top carnivores on herbivores and of indirect effects of top carnivores on plants, when measured as log response ratios, were equal to or stronger than those found in aquatic systems (Brett and Goldman 1996)
Although trophic cascades might be transitory, trophic interactions can also be strong and might stabilize systems in an ‘alternate state’. An example is the otter–urchin–kelp interaction of coastal North America (Estes, J.A. and Duggins, D.O., 1995). Otters stabilize a system of abundant kelp forests by reducing urchin grazing. Removal of otters shifts the system to urchin dominance with substantial reductions in kelp coverage and productivity. Thus, trophic cascades can induce dramatic shifts in both the appearance and properties of ecosystems. However, the contrast of these states can be profound. These phenomena represent an important class of nonlinear ecological interactions. Understanding these interactions remains a challenge to the prediction of ecological dynamics and to the management of ecosystems
Their loss has been identified as a major factor contributing to the decline of biodiversity in both aquatic and terrestrial systems (Letnic et al., 2012).
Hence, understanding predator function is of fundamental importance for biodiversity conservation and pest control at regional, national and even continental scales as it considers the ecological role that top predators have in maintaining functional and biodiverse ecosystems.