When and Where Do Community-Level Trophic Cascades Occur (title) In aquatic syst

Question

When and Where Do Community-Level Trophic Cascades Occur (title)

In aquatic systems, control of plant community structure via trophic cascades has been demonstrated in a variety of habitats, including lakes (Carpenteret al. 1985), rivers (Power 1992) and intertidal (Wootton 1992) and subtidal (Estes & Palmisano 1974) marine systems. In these systems, the standing crop of the plant community is reduced wholesale, and the substrate completely denuded, when predators do not suppress one or a few species of potent grazers. Most of these communities are characterized by simple food webs with little redundancy of consumers, and producers that are single-celled phytoplankton, diatoms, or macroalgae. Because of these factors, Strong (1992) has suggested that top-down control of primary production via trophic cascades may be an idiosyncratic attribute of simple, aquatic systems that are not buffered from run-away consumer effects by multiple predators, and are characterized by weedy, poorly defended primary producers. In general, Strong (1992) suggested that trophic cascades tend to be more important in aquatic vs. terrestrial systems, in simple vs. complex food webs, in homogenous vs. heterogeneous systems, in communities dominated by nonvascular plants (i.e., algae), and in systems where impalatable plants don't replace those that have been overgrazed.

Recent evidence from other systems, however, has suggested that communities dominated by more heavily defended vascular plants are also susceptible to cascading consumer effects. For example, Jackson (1997) has argued that, before humans colonized the Caribbean, large turtle and manatee populations exerted strong, top-down control on seagrasses, which are highly chemically-defended vascular plants, in shallow-water habitats. Consequently, the seagrass beds that have dominated the Caribbean this century may represent recent, human-induced release from consumer control. In terrestrial habitats, others have suggested that large grazing mammals, released from predation, can control vascular plant assemblages in a similar manner (Pace et al. 1999). Examples like these warn that the potential for top-down control of plant community structure may be more pervasive than currently envisioned, especially in those systems thought to be relatively unsusceptible to cascading consumer effects because of plant type and quality (e.g., impalatable, vascular plants, Strong 1992).

From the reading snipit above please answer the following question

Strong 1992 proposed that trophic cascades were likely to be more predominant ecosystems with certain characteristics. What are these characteristics and why are they important?

Explanation / Answer

Most of these communities are characterized by simple food webs with little redundancy of consumers, and producers that are single-celled phytoplankton, diatoms, or macroalgae. Because of these factors, Strong (1992) has suggested that top-down control of primary production via trophic cascades may be an idiosyncratic attribute of simple, aquatic systems that are not buffered from run-away consumer effects by multiple predators, and are characterized by weedy, poorly defended primary producers. In general, Strong (1992) suggested that trophic cascadestend to be more important in aquatic vs. terrestrial systems, in simple vs. complex food webs, in homogenous vs. heterogeneous systems, in communities dominated by nonvascular plants (i.e., algae), and in systems where impalatable plants don't replace those that have been overgrazed.

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