Intraspecific leaf chemistry variation drives local adaptation in aquatic and terrestrial communities

Background/Question/Methods

Intraspecific variation can have large-scale ecosystem consequences.  Cross-ecosystem fluxes can intertwine otherwise disparate food webs, but the effects of within species diversity 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 of the Olympic Peninsula of Washington, 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.  Here we ask whether leaf chemistry drives this local adaptation pattern by studying spatial structuring of leaf defensive chemistry and carrying out field experiments using artificial diets prepared with red alder leaf extracts.

Results/Conclusions

Red alder trees growing in geographic proximity shared more similar leaf chemistry than more distant neighbors.  Leaf chemistry data were sufficient to correctly predict the specific site on the specific river that each tree was growing in nature.  We also found this variation in chemistry drives local adaptation to individual trees both within and across an ecosystem boundary.  Aquatic communities preferred diets containing local red alder leaf extracts than diets containing extracts from red alder growing along a different river.  Adaptation was also evident at the within-river scale: aquatic communities consumed more diet prepared from trees growing upstream from a site in the same river compared to trees from downstream in that same river.  These results show that aquatic communities adapt locally to the chemical composition of alders that regularly supply resource subsidies: upstream leaves wash downstream to that location and represent a natural food resource, but downstream leaves wash further downstream and represented a novel food resource.  We found similar local preference patterns in riparian soils at the between and within-river scales.  Taken together, these results suggest that selection for defensive traits can cascade across ecosystem boundaries to impact ecosystems that are divorced from the actions of co-evolutionary feedback.