population declines of top predators ( tps ) , such as sharks , whales , wolves , and raptors , are among some of the most obvious human impacts on biodiversity of the last few centuries [ 3 - 5 ] . recent advances in food web modelling emphasize the role of tps in the maintenance of biodiversity at several trophic levels , with the loss of tps driving secondary extinctions faster than the loss of species at lower trophic levels . besides the immediate risk of tp extinctions , more general implications may be due to the different functions of tps in ecosystems . whereas removal of tps may have relatively little ecosystem impact when the food webs are bottom - up ( or donor ) controlled , much stronger impacts are due to instances in which tps exert strong top - down control on lower trophic levels . where this occurs , declining tp populations may induce a trophic cascade , which in the worst case could result in a permanent regime shift , in the sense that the structure and functioning of the ecosystem could become irreversibly altered [ 8 - 10 ] . this outcome appears to be likely in models of food webs with few strong trophic interactions . yet , in other cases , tps can be involved in reversible transition between alternative ecosystem states ( figure 1 ) . in regime 1 , the consumer is controlled by a top predator ( top - down control ) and the resource is controlled by nutrient availability ( bottom - up ) , whereas in regime 2 , the resource is controlled by the consumer ( top - down ) and the consumer is controlled by resource availability ( bottom - up ) . the model is redrawn from   based on a marine ecosystem with four trophic levels , although this could be applied to ecosystems with a different number of trophic levels , in both terrestrial and marine environments . there has been a long - lasting debate about the role of tps , and the relative impact of bottom - up and top - down forces in ecosystems ( for example ) . fundamentally different environmental contexts ( in particular , aquatic and terrestrial ecosystems ) have been proposed to underlie context dependencies of tp ecosystem functioning . in this report , by insight gained from recent studies , we exemplify how the different approaches favored by terrestrial and aquatic ecologists may affect what we know about the role of tps in ecosystems . the role of behavior has been an important topic in studies focusing on predator - prey interactions and was first investigated in aquatic ecosystems , although much research was biased toward emphasizing risk - aversive prey behavior ( reviewed in ) . however , lima 's plea   to  put back predators in the predator - prey interactions has been realized recently through studies highlighting how tp hunting mode can contribute to trophic cascades . further advances in  behavioral food web ecology are likely to be brought about by studies that simultaneously consider behavioral interactions between predator and prey   as well as between multiple predators . although the role of non - consumptive effects of tps currently appears to be less emphasized for aquatic food webs than for terrestrial food webs , we are not aware of any reasons why they should be less important . technical innovations that make behavioral studies more feasible under water are likely to change this state of affairs . an increasing abundance of smaller predators ( mesopredators ) when tps disappear has been documented in both aquatic and terrestrial ecosystems . for instance , the collapse of cod stocks in the north - western atlantic led to significant changes at several trophic levels , resulting in a transition to an alternative community state . elmhagen and rushton   demonstrated in a terrestrial ecosystem how the decline in tps ( here , wolf and lynx ) favored the increase in abundance of red fox populations , which , in turn , seemed limited by ecosystem productivity . such joint action of bottom - up and top - down forces means that the concept of mesopredator release needs to be expanded toward an ecosystem - based perspective . in a similar way , daskalov et al .   demonstrated how mesopredator populations can switch between top - down and bottom - up control in alternating regimes in the black sea ecosystem . despite clear similarities across ecosystems in the reactions of mesopredator populations to changes in the abundance of tps , the flexible size and stage structure in fish populations exacerbate the propensity for shifting trophic structure and dynamic in aquatic ecosystems , as compared with terrestrial ecosystems . resource pulses ( i.e. infrequent , short - term , and large - magnitude booms in productivity at low trophic levels that cascade upward in the trophic system ) traditionally have been discussed with reference to terrestrial ecosystems . although such resource pulses are among the best examples of bottom - up - dominated ecosystem dynamics , recent research has shown how impacts from mobile tps tracking an asynchronously pulsed resource can dampen pulse amplitude through spatial averaging . moreover , diet - switching of generalist tps is another mechanism driving community dynamics in many pulsed systems , especially through the process of apparent competition between prey sharing the same tp . despite an initial focus on terrestrial food webs , recent reviews highlight that resource pulses do occur in freshwater and marine ecosystems as well , and that comparative analysis may lead to a novel understanding of this area . statistical analyses of time series have played a key role in elucidating changes in tp abundance , the likely causes of these changes ( in particular , the impact of harvesting versus climatic variability ) and finally the strength of trophic interactions . time series of tps have also played a significant role in the development of statistical time series analysis in general ( for example , canadian lynx ) . however , in most terrestrial ecosystems , we usually lack information on trophic level dynamics besides tps ; for example , time series of plant and/or herbivore dynamics are nonexistent ( except for certain pulse systems for which data on insects or seeds do exist ) , whereas studies on marine ecosystems can rely on data from many trophic levels , sometimes from phytoplankton all the way to tps . the lack of information on other trophic levels obviously limits the inference we can draw regarding the importance of top - down versus bottom - up effects . examples of such shifts emphasize the role of bottom - up processes and , in particular , climatic control . on the other hand , interactions between environmental forcing and harvesting of tps have also been described in marine ecosystems . most of the recent evidence for such shifts comes from aquatic ecosystems , although shifts of vegetation states in terrestrial ecosystems do occur ( such as the biome shift from steppe to tundra   or through desertification ) , yet remain contentious . the role of behavior has been an important topic in studies focusing on predator - prey interactions and was first investigated in aquatic ecosystems , although much research was biased toward emphasizing risk - aversive prey behavior ( reviewed in ) . however , lima 's plea   to  put back predators in the predator - prey interactions has been realized recently through studies highlighting how tp hunting mode can contribute to trophic cascades . further advances in  behavioral food web ecology are likely to be brought about by studies that simultaneously consider behavioral interactions between predator and prey   as well as between multiple predators . although the role of non - consumptive effects of tps currently appears to be less emphasized for aquatic food webs than for terrestrial food webs , we are not aware of any reasons why they should be less important . technical innovations that make behavioral studies more feasible under water are likely to change this state of affairs . an increasing abundance of smaller predators ( mesopredators ) when tps disappear has been documented in both aquatic and terrestrial ecosystems . for instance , the collapse of cod stocks in the north - western atlantic led to significant changes at several trophic levels , resulting in a transition to an alternative community state . elmhagen and rushton   demonstrated in a terrestrial ecosystem how the decline in tps ( here , wolf and lynx ) favored the increase in abundance of red fox populations , which , in turn , seemed limited by ecosystem productivity . such joint action of bottom - up and top - down forces means that the concept of mesopredator release needs to be expanded toward an ecosystem - based perspective . in a similar way , daskalov et al .   demonstrated how mesopredator populations can switch between top - down and bottom - up control in alternating regimes in the black sea ecosystem . despite clear similarities across ecosystems in the reactions of mesopredator populations to changes in the abundance of tps , the flexible size and stage structure in fish populations exacerbate the propensity for shifting trophic structure and dynamic in aquatic ecosystems , as compared with terrestrial ecosystems . resource pulses ( i.e. infrequent , short - term , and large - magnitude booms in productivity at low trophic levels that cascade upward in the trophic system ) traditionally have been discussed with reference to terrestrial ecosystems . although such resource pulses are among the best examples of bottom - up - dominated ecosystem dynamics , recent research has shown how impacts from mobile tps tracking an asynchronously pulsed resource can dampen pulse amplitude through spatial averaging . moreover , diet - switching of generalist tps is another mechanism driving community dynamics in many pulsed systems , especially through the process of apparent competition between prey sharing the same tp . despite an initial focus on terrestrial food webs , recent reviews highlight that resource pulses do occur in freshwater and marine ecosystems as well , and that comparative analysis may lead to a novel understanding of this area . statistical analyses of time series have played a key role in elucidating changes in tp abundance , the likely causes of these changes ( in particular , the impact of harvesting versus climatic variability ) and finally the strength of trophic interactions . time series of tps have also played a significant role in the development of statistical time series analysis in general ( for example , canadian lynx ) . however , in most terrestrial ecosystems , we usually lack information on trophic level dynamics besides tps ; for example , time series of plant and/or herbivore dynamics are nonexistent ( except for certain pulse systems for which data on insects or seeds do exist ) , whereas studies on marine ecosystems can rely on data from many trophic levels , sometimes from phytoplankton all the way to tps . the lack of information on other trophic levels obviously limits the inference we can draw regarding the importance of top - down versus bottom - up effects . examples of such shifts emphasize the role of bottom - up processes and , in particular , climatic control . on the other hand , interactions between environmental forcing and harvesting of tps have also been described in marine ecosystems . most of the recent evidence for such shifts comes from aquatic ecosystems , although shifts of vegetation states in terrestrial ecosystems do occur ( such as the biome shift from steppe to tundra   or through desertification ) , yet remain contentious . as evidence accumulates , the simple distinction between bottom - up versus top - down control gets increasingly blurred , such that the same system can even alternate between these two states ( figure 1 ) . the extent to which marine , freshwater , and terrestrial ecosystems differ with respect to such controls , and which structural parameters determine their strengths , will be reassessed repeatedly as evidence accumulates and valid statistical comparisons accounting for sampling biases are developed . furthermore , we believe that much more can be learned by comparing these ecosystems within the same theoretical framework . indeed , although limnic , marine , and terrestrial ecology are often treated as different disciplines , they contribute to a common theory of food web and ecosystem function . finally , the fact that tps can link different ecosystems ( for example , marine and terrestrial ) through migration or subsidies   testifies for the importance of adopting a broad perspective when studying the role of tps in ecosystems .
to what extent top predators - carnivores at the top of food chains - drive or just respond to ecosystem dynamics is a central , but partially unresolved , question in ecology . in this report 
 , we highlight how different research approaches employed in aquatic and terrestrial ecology may have a bearing on how the role of top predators in ecosystems is perceived .