Human-Induced Trophic Cascades along the Fecal Detritus Pathway

Abstract

Human presence and activity in tropical forest is thought to exert top-down regulation over the various ‘green-world’ pathways of plant-based foodwebs. However, these effects have never been explored for the ‘brown-world’ pathways of fecal-detritus webs. The strong effects of humans on tropical game mammals are likely to indirectly influence fecal detritivores (including Scarabaeine dung beetles), with subsequent indirect impacts on detrivore-mediated and plant-facilitating detrital processes. Across a 380-km gradient of human influence in the western Brazilian Amazon, we conducted the first landscape-level assessment of human-induced cascade effects on the fecal detritus pathway, by coupling data on human impact, game mammal and detritivore community structure, and rate measurements of a key detritus process (i.e. dung beetle-mediated secondary seed dispersal). We found evidence that human impact indirectly influences both the diversity and biomass of fecal detritivores, but not detritivore-mediated processes. Cascade strength varied across detritivore groups defined by species' traits. We found smaller-bodied dung beetles were at higher risk of local decline in areas of human presence, and that body size was a better predictor of cascade structure than fecal resource manipulation strategy. Cascade strength was also stronger in upland, unflooded forests, than in seasonally flooded forests. Our results suggest that the impact of human activity in tropical forest on fecal-detritus food web structure is mediated by both species' traits and habitat type. Further research will be required to determine the conditions under which these cascade effects influence fecal-detritus web function.

Do space‐for‐time assessments underestimate the impacts of logging on tropical biodiversity? An Amazonian case study using dung beetles

“…; Nichols et al . ). Moreover, longer sample periods would have increased the probability of attracting dung beetles from outside of the sample units (Silva & Hernández ), and therefore from units with different logging intensities.…”

Do riparian reserves support dung beetle biodiversity and ecosystem services in oil palm‐dominated tropical landscapes?

“…1F), and it is possible that a difference in removal would be seen if dung pats were left out for longer, and a greater proportion of mass was removed. Secondly, it is possible that differences in the communities attending the two bait types diminish our ability to detect correlations between biodiversity and function; Nichols et al (2013b) discuss how dissimilarities between the response of dung beetles communities and dung burial rates to human impact in the Amazon may be an artifact of surveying the community and function at separate times with different baits. However, correlations have previously been detected using different baits within similar forests in Borneo (Slade et al 2011).…”

Co‐declining mammal–dung beetle faunas throughout the Atlantic Forest biome of South America

“…short life cycle, broad distribution, high abundance and rapid responses to environmental alterations) also ensure their effectiveness as ecological indicators Halffter 1997, Bicknell et al 2014). A co-decline in dung beetle species richness often occurs in areas that have experienced environmental degradation, often from large vertebrate depletion (Nichols et al 2009(Nichols et al , 2013b, which is generally associated with habitat loss and/ or overhunting (Barlow et al 2007, Hernández et al 2014. A co-decline in dung beetle species richness often occurs in areas that have experienced environmental degradation, often from large vertebrate depletion (Nichols et al 2009(Nichols et al , 2013b, which is generally associated with habitat loss and/ or overhunting (Barlow et al 2007, Hernández et al 2014.…”