Effects of manipulated herbivore inputs on nutrient flux and decomposition in a tropical rainforest in Puerto Rico

Abstract

Forest canopy herbivores are known to increase rates of nutrient fluxes to the forest floor in a number of temperate and boreal forests, but few studies have measured effects of herbivore-enhanced nutrient fluxes in tropical forests. We simulated herbivore-induced fluxes in a tropical rainforest in Puerto Rico by augmenting greenfall (fresh foliage fragments), frassfall (insect feces), and throughfall (precipitation enriched with foliar leachates) in replicated experimental plots on the forest floor. Background rates of greenfall and frassfall were measured monthly using litterfall collectors and augmented by adding 10× greenfall or 10× frassfall to designated plots. Throughfall fluxes of NH(4), NO(3) and PO(4) (but not water) were doubled in treatment plots, based on published rates of fluxes of these nutrients in throughfall. Control plots received only background flux rates for these compounds but the same minimum amount of distilled water. We evaluated treatment effects as changes in flux rates for NO(3), NH(4) and PO(4), measured as decomposition rate of leaf litter in litterbags and as adsorption in ion-exchange resin bags at the litter-soil interface. Frass addition significantly increased NO(3) and NH(4) fluxes, and frass and throughfall additions significantly reduced decay rate, compared to controls. Reduced decay rate suggests that nitrogen flux was sufficient to inhibit microbial decomposition activity. Our treatments represented fluxes expected from low-moderate herbivore outbreaks and demonstrated that herbivores, at these outbreak levels, increase ecosystem-level N and P fluxes by >30% in this tropical rainforest.

Long-term Entomological Research on Canopy Arthropods in a Tropical Rainforest in Puerto Rico

“…Insect outbreaks also create pulses in nutrient fluxes that can alter biotic interactions (Christenson et al 2002, Frost and Hunter 2007, Schowalter et al 2011). Fonte and Schowalter (2005) established replicated inclusion/exclusion cages to measure the effect of herbivory by walkingsticks, Lamponius portoricensis Rehn (Phasmida: Pseudophasmatidae), on understory Piper glabrescens (Miq.)…”

“…Insects represent the bulk of diversity (60-90% of all species) in terrestrial and freshwater ecosystems, have biomass often equal to or greater than that of more conspicuous vertebrates, and are highly responsive to changes in abiotic or host conditions (Schowalter 2016). Both above-and below-ground arthropods have considerable capacity to alter rates and patterns of primary production, decomposition, soil properties, and biogeochemical fluxes (Mattson and Addy 1975, Belovsky and Slade 2000, Schowalter et al 2011. Outbreaks of herbivorous species are among the most dramatic biological processes, often triggered by disturbances Haack 1987, Schowalter 2012).…”

“…Outbreaks of herbivorous species are among the most dramatic biological processes, often triggered by disturbances Haack 1987, Schowalter 2012). Outbreaks are capable of altering ecosystem conditions and climate in ways that may contribute to ecosystem resilience (Chapman et al 2003, Classen et al 2005, Fonte and Schowalter 2005, Frost and Hunter 2007, Schowalter et al 2011, Schowalter 2016, as well as the delivery of ecosystem services (Schowalter 2013(Schowalter , 2016.…”

Manna from heaven: Refuse from an arboreal ant links aboveground and belowground processes in a lowland tropical forest

“…Ants have considerable promise as conduits between canopy and litter; they are voracious consumers that concentrate excreta, carcasses, and other 'trash' into refuse piles in or near their nests, which can create nutrient hot spots that increase nutrient mineralization and modify soil biota communities (Rojas 1989, Wagner 1997, Weiss 2006, Fox-Dobbs et al 2010. Such central place foragers present an opportunity to directly measure connections between above-and below-ground systems even in complex environments like tropical forests where there is a notable gap in our knowledge (Rinker et al 2001, but see Schowalter et al 2011). This is particularly germane given much of the terrestrial primary productivity (Melillo et al 1993) and insect biomass and diversity (Nadkarni 1994) exists in these forests.…”

Long‐term insect herbivory slows soil development in an arid ecosystem

“…Hunter (2001) described multiple pathways by which insect herbivores could alter nutrient cycling including the addition of insect waste products and bodies, altered canopy leachates and litter chemistry, and herbivoredriven shifts in plant and soil communities. While many recent studies have documented the impacts of insect herbivory on ecosystems (e.g., Belovsky and Slade 2000, Frost and Hunter 2004, Madritch et al 2007, Blue et al 2011, Schowalter et al 2011, Zhang et al 2011, it is difficult to assess insect herbivore impacts on soil C and N accumulation in mesic ecosystem soils because the canopies of plants overlap, background pool sizes are high relative to changes in input rates associated with herbivory, and herbivore manipulation experiments may not be sufficiently long to detect a signal. Thus, it remains unclear whether non-outbreak insect herbivores influence larger-scale, longer-term processes such as the development of soil total carbon (C) and nitrogen (N) stocks.…”