Trophic cascade
Definition of Trophic cascade:
A trophic cascade is a sequence of indirect effects through several trophic levels, typically initiated by a change in the abundance of a keystone predator.
This is the common definition for Trophic cascade, other definitions can be discussed in the article
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Keystone species
It can happen that a single species, or just a few species, play a crucial role in maintaining the structure of an ecological community and in determining the types and numbers of various other species in the community. Such species are called keystone species. Ecosystems ruled by keystone species are particularly conspicuous in ecosystems where species diversity and the diversity of predator-prey relationships within the food web are modest or low. The decline of keystone species can result in a complete rearrangement of the food web. If the key species is a top predator (also called apex predator), the food web is said to be top-down controlled. The food web rearrangement propagates downward through the food web and is called a trophic cascade.
Marine food webs can also be bottom-up controlled, for example by the limited availability of essential prey such as primary producers or zooplankton. In this case, food-web perturbations can propagate upward when the abundance of this 'keystone prey' declines.
Surma et al. 2025[1] reviewed characteristics of ecosystems governed by top-down and bottom-up control. Characteristics favoring top-down control (bottom-up control) include: high (low) grazing efficiency, simple (complex) food web structure , low (high) prey biodiversity, low (high) omnivory, high (low) predator/prey mobility, low (high) temperature.
Field studies have revealed that top-down control and trophic cascades are fairly frequent in salt marshes, estuaries, seagrass beds, rocky intertidal zones, shallow tropical banks, kelp forests, coralline algal reefs, soft-bottom coastal habitats and continental shelves.
Grazing pressure and prey limitation are generally both present in ecosystems. Top-down and bottom-up processes can therefore occur simultaneously, sometimes even in an alternating pattern[2]. The classification into top-down and bottom-up controlled ecosystems can be useful, but trophic cascades describe only part of the indirect interactions occurring in ecosystems. Numerous non-consumptive mechanisms, such as deterrence and feedbacks, can also produce indirect effects. An example of the latter is the contribution of high trophic species to primary production through recycling and decomposition processes, which in some cases can be significant.
Managers should be aware that human activities can perturb coastal and marine habitats by altering the abundance of key consumers or their prey, with effects that may propagate through several trophic levels.
Regime shift
A trophic cascade can substantially reorganize an ecosystem and, in some cases, contribute to a regime shift—a large and persistent change in ecosystem structure and functioning, see the article Ecological thresholds and regime shifts. A trophic cascade does not necessarily produce a regime shift. Regime shifts through trophic cascades are particularly conspicuous in relatively simple food webs with strong consumer–prey interactions. In more complex food webs, alternative predators, prey and trophic pathways can dampen or obscure cascading effects.[3][1]. Removal of the disturbance and recovery of the keystone species can reverse a trophic cascade, but recovery is not always immediate or complete, as illustrated by the following examples.
The Atlantic cod example
Frank et al. (2005[3]) interpreted the large ecosystem changes following the collapse of predatory groundfish on the eastern Scotian Shelf off Nova Scotia, Canada, during the mid-1980s and early 1990s as a trophic cascade. It is generally believed that overfishing in this period is the primary cause of the collapse of the cod population and the decline of other predatory fishes such as haddock, hake, pollock, cusk, redfish, plaice, yellowtail, flounder and skate. The decline of these tertiary consumers coincided with a sharp increase in their former prey: small pelagic fishes and benthic macroinvertebrates such as snow crab and shrimp. These secondary consumers mainly hunt for large zooplankton, resulting in a shift from large to small zooplankton species. The decline of the large primary consumers allowed for an increase of the phytoplankton abundance, which was also reflected in a decrease in the nitrate concentration, the main limiting nutrient. The great beneficiaries of this ecosystem shift were the seals that were freed from food competition with the predatory fish. Fisheries also benefited, because the shrimp and crab catch provided a better economic yield than the groundfish fishery.
The failure of cod to recover was studied in the southern Gulf of St. Lawrence by Neuenhoff et al. (2019[4]), who identified high natural mortality and predation by grey seals as major mechanisms contributing to continued decline. Food-web feedbacks involving abundant forage fishes have also been proposed as contributing to cod non-recovery. The relative importance of these mechanisms on the eastern Scotian Shelf remains debated.[5]. Reduction of fishing pressure did not result in a recovery of the cod population. A relatively short period of intense overfishing strongly depleted the cod population and was followed by a long-lasting large-scale regime shift.
The otter-kelp example
Another frequently cited example of a trophic cascade involves the sea otter-kelp ecosystem of the rocky reefs of the Aleutian Islands in the western North Pacific. In this ecosystem, the sea otter is the keystone species with sea urchins as its main food source. Sea urchins graze kelp, the abundance of which is inversely related to the abundance of sea urchins. Comparisons of western Aleutian islands with and without sea otters demonstrated that sea-otter decline releases sea urchins from predation, resulting in intense grazing and loss of kelp forests[6]. A similar trophic cascade is inferred to have occurred in the Commander Islands after sea otters were strongly depleted by hunting during the eighteenth century[7]. The recovery of the sea otter after cessation of the hunting subsequently led to the recovery of the kelp forests. In this case the regime shift did not persist after elimination of the disturbance. It was also inferred that the decline of kelp following sea-otter depletion contributed to starvation and ultimately to the extinction of Steller's sea cow, implying that its extinction may not have resulted solely from direct hunting[7].
See also
Wikipedia article Trophic cascade
Related articles
- Overexploitation
- Effects of fisheries on marine biodiversity
- Ecological thresholds and regime shifts
- Resilience and resistance
- Species extinction
- Disturbances, biodiversity changes and ecosystem stability
References
- ↑ 1.0 1.1 1.2 Surma, S., Pakhomov, E.A. and Pitcher, T.J. 2025. Trophic cascades and top-down control: found at sea. Front. Ecol. Evol. 13, 1587171
- ↑ Leroux, S.J. and Loreau, M. 2015. Theoretical perspectives on bottom-up and top-down interactions across ecosystems. In: (Hanley TC, La Pierre KJ, eds.) Trophic Ecology: Bottom-Up and Top-Down Interactions across Aquatic and Terrestrial Systems. Cambridge University Press, pp. 3-27
- ↑ 3.0 3.1 Frank, K.T., Petrie, B., Choi, J.S. and Leggett, W.C. 2005. Trophic Cascades in a Formerly Cod-Dominated Ecosystem. Science 308: 1621-1623
- ↑ Neuenhoff, R.D., Swain, D.P., Cox, S.P., McAllister, M.K., Trites, A.W., Walters, C.J. and Hammill, M.O. 2019. Continued decline of a collapsed population of Atlantic cod (Gadus morhua) due to predation-driven Allee effects. Can. J. Fish. Aquat. Sci. 76: 168–184
- ↑ O'Boyle, R. and Sinclair, M. 2012. Seal–cod interactions on the Eastern Scotian Shelf: Reconsideration of modelling assumptions. Fisheries Research 115–116: 1-13
- ↑ Estes, J.A. and Palmisano, J.F. 1974. Sea otters: their role in structuring nearshore communities. Science 185(4156): 1058–1060
- ↑ 7.0 7.1 Estes, J.A., Burdin, A. and Doak, D.F. 2016. Sea otters, kelp forests, and the extinction of Steller’s sea cow. PNAS 113: 880–885
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