Critical assessment and ramifications of a purported marine trophic cascade

Abstract

When identifying potential trophic cascades, it is important to clearly establish the trophic linkages between predators and prey with respect to temporal abundance, demographics, distribution, and diet. In the northwest Atlantic Ocean, the depletion of large coastal sharks was thought to trigger a trophic cascade whereby predation release resulted in increased cownose ray abundance, which then caused increased predation on and subsequent collapse of commercial bivalve stocks. These claims were used to justify the development of a predator-control fishery for cownose rays, the “Save the Bay, Eat a Ray” fishery, to reduce predation on commercial bivalves. A reexamination of data suggests declines in large coastal sharks did not coincide with purported rapid increases in cownose ray abundance. Likewise, the increase in cownose ray abundance did not coincide with declines in commercial bivalves. The lack of temporal correlations coupled with published diet data suggest the purported trophic cascade is lacking the empirical linkages required of a trophic cascade. Furthermore, the life history parameters of cownose rays suggest they have low reproductive potential and their populations are incapable of rapid increases. Hypothesized trophic cascades should be closely scrutinized as spurious conclusions may negatively influence conservation and management decisions.

Trophodynamics as a Tool for Understanding Coral Reef Ecosystems

“…Sufficient information of reef ecology such as spatiotemporal abundance, distribution, habitat associations, environmental inputs, diet, and life history of a species is necessary to inform ecological roles and function (Figure 2). While recent examples of poor management decisions have created a need for standardizing approaches in the field of trophodynamics (Grubbs et al, 2016), studies are still omitting key concepts before implementing applied research. We have highlighted such studies in relation to popular methods, but also identified recent research that is incorporating and combining new methods to account for some of the challenges faced within the field.…”

“…Even if assumed cascading events are observed such as phase shifts, finding the cause of such patterns at large spatial scales is challenging (Dulvy et al, 2004). Attention should also be paid to temporal gradients over which cascades occur, considering life-history and growth capabilities of populations as well as lagged effects from perturbations that can occur over extended time periods (Dulvy et al, 2004;Grubbs et al, 2016). The difficulty in linking trophic levels to the same event that categorizes a trophic cascade is difficult to support empirically, particularly when most reefs are already in a degraded state from external stressors.…”

“…Given the lack of examples of trophic cascades in coral reef habitats, it is necessary to ensure an appropriate framework is used to interpret data for these ecosystems. While applied research is a necessary step for improved conservation, misinterpretation of ecological roles can lead to poor conservation and management outcomes (Grubbs et al, 2016).…”

Towards a balanced presentation and objective interpretation of acoustic and trawl survey data, with specific reference to the eastern Scotian Shelf

“…There is a growing list of studies that question the viability of the trophic-cascade hypothesis in the marine environment (e.g., Grubbs et al 2016;Sinclair et al 2015) and those that refute the arguments of Frank et al, providing evidence against top-down forcing by cod on adult snow crab (Chabot et al 2008;Émond et al 2015), showing the lack of evidence for pelagic fish affecting the recovery of cod (Swain and Mohn 2012), presenting evidence for bottom-up control of phytoplankton by herbivorous zooplankton (Head and Pepin 2010;Head and Sameoto 2007), and even contesting the methodology used to detect the ESS trophic cascade (Pershing et al 2015). We are unaware of any studies providing independent support of the PO hypothesis, although many studies have relied on its conclusions.…”

“…Regardless of the causes of the collapse, the lack of recovery of east coast cod populations to their former numbers despite over 25 years of low fishing effort and moratoria has been well documented (Bundy and Fanning 2005;Smedbol and Wroblewski 2002;Swain and Mohn 2012) and its causes and consequences extensively debated in terms of regime shifts, top-down versus bottom-up trophic dynamics, and more recently "oscillatory runaway consumption dynamics of the forage fish complex" (Frank et al 2005(Frank et al , 2011Greene 2013;Greene et al 2012;Greene and Pershing 2007;Grubbs et al 2016;Pershing et al 2015;Sinclair et al 2015).…”