Trophic cascades, in which changes in predation affect the biomass of lower trophic levels via indirect effects, vary substantially in strength and incidence. To date, most work to explain this variation has focused on local factors and has ignored the influence of larger regional effects. To study how metacommunity dynamics can mediate local structuring mechanisms and alter the strength of trophic cascades, we constructed mesocosm metacommunities consisting of three pond communities with heterogeneous levels of fish predation and examined how planktonic dispersal rate (5%-140% per week) affected variation in biomass partitioning among planktonic functional groups. Two of the three communities differed continually in the occurrence of fish and supported different but constant environments in a ‘spatial trophic cascade,’ while the third community supported temporally variable fish occurrence in a ‘temporal trophic cascade.’
Results/Conclusions
We find that the presence, but not the magnitude, of dispersal dampens temporal trophic cascades. Dispersal-induced dampening of temporal trophic cascades was directly associated with changes in the composition of planktonic functional groups, and specifically an increase in grazer biomass. In contrast, dispersal had no effect on the strength of spatial trophic cascades due to strong sorting pressures in the communities with constant presence or absence of fish as top predators. The results demonstrate that dispersal can mediate indirect predator effects and may be responsible for a component of unexplained variation in trophic cascade strength. They further suggest that it is the presence of spatial coupling and not the quantitative degree of connectivity that more strongly shapes trophic structure. The findings from this study emphasize the critical role of connectivity in affecting ecosystem properties, and justify a contemporary approach to evaluating trophic structure at the local and metacommunity scale.
