Herbivores and macro-detritivores are often considered unimportant to primary production in tropical forests, despite their known importance to plant production in non-tropical ecosystems.  Invertebrate consumers in temperate systems commonly affect primary production through either nutrient-rich frass inputs, or through changes in litter quality and quantity by selective feeding on vegetation and litter.  This study examined how an invertebrate folivore (a walking stick, Lamponius portoricensis) and a detritivore (a snail, Megalomastoma croceum) affect primary production of two understory plants (Piper glabrecens and Miconia prasina) under two different levels of canopy cover in a disturbance-driven forest.  Herbivores were hypothesized to slow down plant growth, through a previously demonstrated decrease in litter decomposition, which was caused by herbivore selective feeding.  Detritivores were hypothesized to increase plant growth, through comminution of litter as detritus feeders, thus allowing greater nutrient availability to plants.  A fully crossed, factorial experiment (herbivore presence/absence x detritivore presence/absence x light gaps/forested sites) began in 2005.  Herbivores and detritivores were stocked at natural abundances inside enclosures in a Puerto Rican rainforest.  In these experimental enclosures, we measured dimensions of individual plants at the beginning of the experiment, and yearly until the end of the experiment.  Aboveground biomass was estimated using regressions calculated from the final harvest in 2008.
Results/Conclusions

Changes in root and leaf moisture, leaf production, and total aboveground production in both plant species were analyzed with ANOVA.  Folivores increased only root moisture, but did not affect plant production.  Conversely, detritivores significantly decreased the number of branches and leaves, wood biomass, and total aboveground biomass.  Detritivore reductions of aboveground biomass were greater in light gaps.  We hypothesize that herbivores reduce root moisture by increasing water loss by feeding on, and subsequently, damaging leaves.  We also hypothesize that detritivore reductions to aboveground biomass are observed because the long-lived, relatively large detritivores sequester nutrients necessary for plant growth, thus reducing nutrient availability.  This nutrient sequestration especially affects plants in light gaps, where light is not limiting to understory plants, but nutrients may be limiting.  This hypothesis is supported by preliminary data suggesting that detritivore presence decreases total soil nitrogen over time.  Given these counterintuitive findings and current unprecedented levels of species declines, this study underscores the need to investigate the impacts of consumer species on ecosystem processes and services, especially in the tropics.