Cascading effects of intraspecific variation in plant subsidies on aquatic and terrestrial ecosystem function

Background/Question/Methods

Cross-ecosystem fluxes can intertwine otherwise disparate food webs, but the effects of biodiversity within species on fluxes across ecosystems boundaries is not known. Fresh leaves, which vary in traits such as defensive compounds against terrestrial herbivores, drop off trees and enter streams, providing a vital resource for riverine organisms.  Previously, we found substantial variation among individual red alders in leaf decomposition rates in rivers, and showed that locally derived leaves decompose faster than leaves introduced from trees growing along other riparian zones.  These results indicated that aquatic decomposer communities have locally adapted to the specific trees supplying the riparian subsidies at a particularly fine scale.  To explore the basis of individual variation in decomposition rate, we studied the effects of induced plant defenses and nutrients on terrestrial herbivore feeding, and on aquatic and terrestrial leaf decomposition through a combination of experimental simulation of herbivory on red alder and measurement of phenotypic leaf traits.

Results/Conclusions

The experimental herbivory treatment on red alder trees effectively dissuaded terrestrial insects from feeding on adjacent undamaged leaves.  The effects of the induced defense carried over into aquatic ecosystems, where aquatic decomposers fed 60% faster on leaves from controls compared to trees undergoing simulated herbivory.  The defense response included a two-fold sharper decline in leaf nitrogen levels compared to controls.  The defense response to herbivory appears more complex than nitrogen reduction alone, because treatment effects remained after statistically factoring out nitrogen content.  This same herbivory treatment had no effect on terrestrial soil decomposers even though these decomposers also showed strong preference for nitrogen rich leaves.  These finding illustrate how phenotypic or genetic diversity and the effects of selection in one ecosystem can indirectly shape the structure of other ecosystems through ecological fluxes across boundaries.