Cascading effects of induced terrestrial plant defences on aquatic and terrestrial ecosystem function

Abstract

Herbivores induce plants to undergo diverse processes that minimize costs to the plant, such as producing defences to deter herbivory or reallocating limited resources to inaccessible portions of the plant. Yet most plant tissue is consumed by decomposers, not herbivores, and these defensive processes aimed to deter herbivores may alter plant tissue even after detachment from the plant. All consumers value nutrients, but plants also require these nutrients for primary functions and defensive processes. We experimentally simulated herbivory with and without nutrient additions on red alder (Alnus rubra), which supplies the majority of leaf litter for many rivers in western North America. Simulated herbivory induced a defence response with cascading effects: terrestrial herbivores and aquatic decomposers fed less on leaves from stressed trees. This effect was context dependent: leaves from fertilizedonly trees decomposed most rapidly while leaves from fertilized trees receiving the herbivory treatment decomposed least, suggesting plants funnelled a nutritionally valuable resource into enhanced defence. One component of the defence response was a decrease in leaf nitrogen leading to elevated carbon : nitrogen. Aquatic decomposers prefer leaves naturally low in C : N and this altered nutrient profile largely explains the lower rate of aquatic decomposition. Furthermore, terrestrial soil decomposers were unaffected by either treatment but did show a preference for local and nitrogen-rich leaves. Our study illustrates the ecological implications of terrestrial herbivory and these findings demonstrate that the effects of selection caused by terrestrial herbivory in one ecosystem can indirectly shape the structure of other ecosystems through ecological fluxes across boundaries.

Intraspecific leaf chemistry drives locally accelerated ecosystem function in aquatic and terrestrial communities

“…We found ellagitannin, diarylheptanoid and flavonoid variation were all correlated with leaf traits, such as thickness and nutrients that are well-known to affect consumers (Coley 1980, Mattson 1980, Jackrel and Wootton 2015. But our results from both leaf pack and diet experiments suggest ellagitannin content is a greater predictor of ecosystem processes than nutrients (Appendix S2: Table S4), and therefore warrants further study into plant production pathways of these compounds and the ecological implications of relative nutrient vs. defense compound content in red alder and other plants.…”

“…To measure phosphorus we used an ashing, acid-hydrolysis, and phosphomolybdate-blue spectrophotometric protocol (Jackrel and Wootton 2015). The reproducibility was 0.11‰ for 13 C and 0.17‰ for 15 N, and cocoa powder and glutamic acid were used as isotopic controls.…”

Experimental test of water, nutrients, and microclimate on leaf litter mass loss in headwater riparian forests

“…This was not an issue in our experiment as leaves were placed above the bankfull edge of the stream and received water only via a watering can or precipitation. Alder leaves are a common target of herbivorous insects (Jackrel and Wootton 2015), and with a high N content and specific leaf area, alder leaf litter is an easily accessible and nutritious resource for detritivorous invertebrates and microbes (Zukswert and Prescott 2017). Water can promote mass loss by leaching, and in red alder, this can be significant; Zukswert and Prescott (2017) saw a leaching mass loss of about 10% from red alder leaves when placed in a forest within Vancouver, BC.…”

Fear and below-ground food-webs

“…The majority of these inputs are comprised of senesced, uneaten plant-litter, roots, and root exudates (Cebrian, 2004;Pollierer et al, 2007;Pangesti et al, 2013;Jackrel and Wootton, 2015). Thus, it is widely assumed that herbivores primarily affect the way SFWs regulate biogeochemical processes by altering the quantity and quality of plant-derived organics inputs to soils (Bardgett and Wardle, 2010).…”