Cascades in all habitats Community or specieslevel cascades

So much of the discussion of trophic cascades, including their original identification, has been based on aquatic (either marine or freshwater) examples that the question has seriously been asked 'are trophic cascades all wet?' (Strong, 1992). As pointed out by Polis et al. (2000), however, in order to answer this question we should recognize a distinction between community- and species-level cascades (Polis, 1999). In the former, the predators in a community, as a whole, control the abundance of the herbivores, such that the plants, as a whole, are released from control by the herbivores. But in a species-level cascade, increases in a particular predator give rise to decreases in particular herbivores and increases in particular plants, without this affecting the whole community. Thus, Schmitz et al. (2000), in apparent contradiction of the 'all cascades are wet' proposition, reviewed a total of 41 studies in terrestrial habitats demonstrating trophic cascades; but Polis et al. (2000) pointed out that all of these referred only to subsets of the communities of which they were part - that is, they were essentially species-level cascades. Moreover, the measures of plant performance in these studies were typically short term and small scale (for instance, 'leaf damage' as in the lizard-spider-herbivore-seagrape example above) rather than broader scale responses of significance to the whole community, such as plant biomass or productivity.

Polis et al. (2000) proposed, then, that community-level cascades are most likely to occur in systems with the following characteristics: (i) the habitats are relatively discrete and homogeneous; (ii) the prey population dynamics (including those of the primary producers) are uniformly fast relative to those of their consumers; (iii) the common prey tend to be uniformly edible; and (iv) the trophic levels tend to be discrete and species interactions strong, such that the system is dominated by discrete trophic chains.

If this proposition is correct, then community-level cascades are most likely in pelagic communities of lakes and in benthic communities of streams and rocky shores (all 'wet') and perhaps in agricultural communities. These tend to be discrete, relatively simple communities, based on fast-growing plants often dominated by a single taxon (phytoplankton, kelp or an agricultural crop). This is not to say (as the Schmitz et al. (2000) review confirms) that such forces are absent in more diffuse, species-rich systems, but rather that patterns of consumption are so differentiated that their overall effects are buffered. From the point of view of the whole community, such effects may be represented as trophic trickles rather than cascades.

Certainly, the accumulating evidence seems to support a pattern of overt community-level cascades in simple, especially wet, communities, and much more limited cascades embedded within a broader web in more diverse, especially terrestrial, communities. It remains to be seen, however, whether this reflects some underlying realities or simply differences in the practical difficulties of manipulating and studying cascades in different habitats. An attempt to decide whether there are real differences between aquatic and terrestrial food webs was forced to conclude that there is little evidence, either empirical or theoretical, to either support or refute the idea (Chase, 2000).

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