Although trophic cascades (indirect effects of predators on plants via herbivores) occur in a wide variety of food webs, the magnitudes of their effects are often quite variable. We compared the responses of herbivore and plant communities to predator manipulations in 102 field experiments in six different ecosystems: lentic (lake and pond), marine, and stream benthos, lentic and marine plankton, and terrestrial (grasslands and agricultural fields). Predator effects varied considerably among systems and were strongest in lentic and marine benthos and weakest in marine plankton and terrestrial food webs. Predator effects on herbivores were generally larger and more variable than on plants, suggesting that cascades often become attenuated at the plant–herbivore interface. Top‐down control of plant biomass was stronger in water than on land; however, the differences among the five aquatic food webs were as great as those between wet and dry systems.

We dedicate this paper to the memory of John P. Sutherland, a great scientist, a great human being.Abstract. The usefulness and generality of the keystone species concept has recently been questioned. We investigated variation in interaction strength between the original keystone predator, the seastar Pisaster ochraceus, and its primary prey, mussels (Mytilus ca/ifornianus and M. trossulus). The study was prompted by differences in community structure at two low zone sites along the central Oregon coast, Boiler Bay (BB) and Strawberry Hill (SH). Predators, especially seastars, were larger and more abundant at SH than at BB. Further, sessile animals were more abundant and macrophytes were less abundant at SH. Predators were more abundant at wave-exposed sites at both sites, and at SH, sessile invertebrates were more abundant at the wave-exposed location and sand cover was high at the wave-protected location. To test the hypothesis that variation in predation strength explained some of these differences, we examined the seastar-mussel interaction at locations with high and low wave exposure at both sites. Predation intensity was quantified by determining the survival of mussels in clumps (50 mussels per clump, shell length 4-7 em) transplanted to large plots (18-163 m 2 ) with or without seastars in the low intertidal zone. Predation effects were quantified by determining prey recolonization rates in marked quadrats in the same large plots. Spatial variation in interaction strength was quantified by examining predation at scales of metres (among transplants within plots), lO's of metres (between replicate plots within each exposure at each site), 1 OO's of metres (between wave exposures within locations), and 10 OOO's of metres (between sites). Temporal variation was evaluated by performing the experiments in 1990 and 1991. The relation between prey (mussel) recruitment and growth to differences in community structure was evaluated by quantifying recruitment density in plastic mesh balls (collectors) and growth of individually marked transplanted mussels, respectively, at each site x exposure x tide level combination each month for 4 yr.Predation intensity varied greatly at all spatial scales. At the two largest spatial scales ( 1 O's of kilometres, 1 OO's of metres), differences in both survival of transplanted mussels and prey recolonization depended on variation in seastar abundance with site, wave exposure, prey recruitment and growth, and at SH protected, the extent of sand burial. Variation at the two smallest scales (metres, 10's of metres) was high when seastars were scarce and low when seastars were abundant. Transplanted mussels suffered 100% mortality in 2 wk at wave-exposed SH, but took >52 wk at wave-protected BB. Seastar effects on prey recolonization were detected only at the SH wave-exposed site. Here, where prey recruitment and growth were unusually high, the mussel M. trossulus invaded and dominated space within 9 mo. After 14 mo, whelks, which increased in both size and abundance in the absence o...

The interaction of climate and the timing of low tides along the West Coast of the United States creates a complex mosaic of thermal environments, in which northern sites can be more thermally stressful than southern sites. Thus, climate change may not lead to a poleward shift in the distribution of intertidal organisms, as has been proposed, but instead will likely cause localized extinctions at a series of "hot spots." Patterns of exposure to extreme climatic conditions are temporally variable, and tidal predictions suggest that in the next 3 to 5 years "hot spots" are likely to appear at several northern sites.

Trophic cascades have been documented in a diversity of ecological systems and can be important in determining biomass distribution within a community. To date, the literature on trophic cascades has focused on whether and in which systems cascades occur. Many biological (e.g., productivity : biomass ratios) and methodological (e.g., experiment size or duration) factors vary with the ecosystem in which data were collected, but ecosystem type, per se, does not provide mechanistic insights into factors controlling cascade strength.Here, we tested various hypotheses about why trophic cascades occur and what determines their magnitude using data from 114 studies that measured the indirect trophic effects of predators on plant community biomass in seven aquatic and terrestrial ecosystems. Using meta-analysis, we examined the relationship between the indirect effect of predator manipulation on plants and 18 biological and methodological factors quantified from these studies. We found, in contrast to predictions, that high system productivity and low species diversity do not consistently generate larger trophic cascades. A combination of herbivore and predator metabolic factors and predator taxonomy (vertebrate vs. invertebrate) explained 31% of the variation in cascade strength among all 114 studies. Within systems, 18% of the variation in cascade strength was explained with similar predator and herbivore characteristics. Within and across all systems, the strongest cascades occurred in association with invertebrate herbivores and endothermic vertebrate predators. These associations may result from a combination of true biological differences among species with different physiological requirements and bias among organisms studied in different systems. Thus, although cascade strength can be described by biological characteristics of predators and herbivores, future research on indirect trophic effects must further examine biological and methodological differences among studies and systems.

Organisms eating each other are only one of many types of well documented and important interactions among species. Other such types include habitat modification, predator interference and facilitation. However, ecological network research has been typically limited to either pure food webs or to networks of only a few (<3) interaction types. The great diversity of non-trophic interactions observed in nature has been poorly addressed by ecologists and largely excluded from network theory. Herein, we propose a conceptual framework that organises this diversity into three main functional classes defined by how they modify specific parameters in a dynamic food web model. This approach provides a path forward for incorporating non-trophic interactions in traditional food web models and offers a new perspective on tackling ecological complexity that should stimulate both theoretical and empirical approaches to understanding the patterns and dynamics of diverse species interactions in nature.