Trophic Cascades

Daniel S Gruner, University of Maryland, College Park, Maryland, USA

Published online: March 2013

DOI: 10.1002/9780470015902.a0003183.pub2

Trophic cascades in ecological communities are defined as the propagation of indirect effects between nonadjacent trophic levels in a food chain or food web. Typically, cascades are driven by predation from the top-down, with altered herbivore densities mediating the ultimate effects on the biomass of primary producers. Predator traits and nonconsumptive changes in the behaviour of prey also can propagate cascading indirect effects, and variation in nutrients or energy at the base of food webs may mediate cascades from the bottom-up. Significant debate has revolved around the relative dynamical strength of top-down versus bottom-up forces and over the importance of trophic cascades in different aquatic and terrestrial ecosystems. Polarisation has eased with the increased availability of experimental and comparative case studies, meta-analytic syntheses of these data and development of theory. The basic conceptual construct of trophic cascades has been applied to classical biological control of agricultural pests, biomanipulation of lakes and streams and the conservation and restoration of apex predators for fisheries and wildlife management.

Key Concepts:

  • Limited nutrient availability and high C:N and C:P stoichiometry can constrain biomass turnover and the strength of trophic cascades.
  • Many lake and stream ecosystems support four trophic levels, with piscivorous fish ultimately suppressing phytoplankton in a consumptive interaction chain.
  • Trophic cascades can occur via direct consumption and density reductions of prey and through behavioural changes in prey foraging from predation risk.
  • Fisheries intensification has selectively reduced top predators and large-bodied species and reduced the length of oceanic food chains.
  • Reintroduction of top predators, such as wolves, to the American west has facilitated recruitment of woody shrubs and trees and reduced stream erosion.
  • Long-term studies are critical to testing for and identifying trophic cascades on landscape and regional spatial scales.

Keywords: food webs; herbivory; indirect effects; nutrient cycling; omnivory; predation; primary productiony; top-down control; bottom-up forces; trophic interactions

Figure 1. (a) Trophic cascade in a linear food chain where the top predator (T) has a positive indirect effect (dashed arrow) on the resource (R) through its direct negative effect (solid arrow) on the consumer (C). (b) With a change from a linear food chain to a slightly more complex food web, the number of indirect effects (dashed arrows) increases substantially. In this case, an increase in top predator 1 will lead to a decrease in consumer 1, but compensation at the same level will occur through an increase in consumer 2. The change in total biomass of consumers may thereby be smaller than that in the case of a linear food chain.
Figure 2. Predictions from the mathematical model of the ecosystem exploitation hypothesis (EEH) (Oksanen et al., 1981) for density of plant resources (R), herbivorous consumers (C) and top predators (T) along a gradient of extrinsic potential productivity. Reproduced with permission from Borer and Gruner (2009).
Figure 3. Estimated differences in pathways of carbon flows and pools between aquatic and terrestrial ecosystems (Reproduced with permission from Shurin et al., 2006). Thickness of the arrows (flows) and the area of the boxes (pools) are proportional to relative magnitudes, and sizes are scaled as log units to cover four orders of magnitude in difference. The C's indicate consumption terms (i.e. CH is consumption by herbivores). Ovals and arrows in grey indicate unknown quantities. © The Royal Society.
Figure 4. Schematic figure showing generalised differences in biomass relationships between a terrestrial and an aquatic system. The high turnover rates of phytoplankton and zooplankton, and lower mass-specific metabolic rates of poikilothermic predaceous fish, mean that the biomass of primary carnivores can be higher in many aquatic systems.
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Related Sub-Topics:

Herbivory, Predation and Parasitism | Food Webs | Energy Flow and Stoichiometry
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Article Title: Trophic Cascades
 
How to Cite close
Gruner, Daniel S(Mar 2013) Trophic Cascades. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003183.pub2]