The trophic cascade, whereby predatory animals control the abundance of herbivores, thereby influencing the biomass of plants, is a central concept in ecology, especially for aquatic systems. Shurin et al. have performed a comparative meta-analysis of experimental manipulations of predator numbers in six different ecosystems, using biomass of plants and biomass or density of herbivores as the units of measure. There was 10-fold variation among the systems in the effects of predators on herbivore density--greatest in lakes and oceans and least in streams and terrestrial systems. The stronger effects of predators in lakes and oceans suggests that human effects on plant production and ecosystem processes, already severe through exploitation and bycatch of predators, will be greater in aquatic systems than on land.
In a separate study, Schmid-Araya et al. survey a range of different stream food webs and find that the number of actual connections compared to possible connections between species (the "connectance" of the food web) decreases with increasing numbers of species. The connectance values were lower than in other aquatic systems, again suggesting a lesser role for top-down control in streams. -- AMS
Ecol. Lett. 5, 785 (2002); J. Anim. Ecol. 71, 1056 (2002).
