Local Variation in Shredder Distribution can Explain their Oversight in Tropical Streams

Abstract

Stream shredders play an important role in the breakdown of allochthonous leaf litter-a well-known, key process in temperate headwater streams. In contrast, it has been suggested that litter breakdown in tropical streams is driven by microorganisms, shredders being scarce or absent. We propose that shredders have been overlooked in some tropical streams for two reasons: (1) assuming that tropical shredders belong to the same taxa as temperate ones, without determining the diet of tropical litter fauna; and (2) the small spatial scale of most tropical stream studies, which do not account for intra-and inter-site comparisons. We explored shredder abundance and species richness in six streams in each of two tropical regions, the Australian wet tropics (AWT) and Panama (PAN), finding 734 individuals of 12 shredder species in AWT and 391 individuals of 16 species in PAN. Shredder species richness was positively related to altitude in AWT, but not in PAN. Shredder contribution to total leaf breakdown in the field was 24 ± 3 SE percent in AWT and negligible in PAN, but this was probably due to the unsuccessful colonization of experimental cages by PAN shredders. In the laboratory, shredder contribution to total leaf breakdown was higher than in the field (35% ± 2 SE in AWT and 64% ± 3 SE in PAN) and varied with leaf decomposabiliry. Our results support earlier indications that shredders are not scarce or functionally unimportant in the tropics, and suggest that their contribution to litter processing should be determined along altitudinal gradients.Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp.

Shredders are abundant and species-rich in tropical continental-island low-order streams: Gorgona Island, Tropical Eastern Pacific, Colombia

“…Mean abundance, richness and mean biomass of FSG were not significantly correlated with any environmental variable ( Cheshire et al, 2005;Camacho et al, 2009;Tomanova et al, 2006;Ríos, Encalada & Prat, 2009;Boyero et al, 2011a;. Our results provide further evidence against the hypothesis of a generalized poverty and scarcity of shredder macroinvertebrates in the tropics, supporting recent arguments by Cheshire et al (2005), Camacho et al (2009) and Ríos et al (2009).…”

“…In the Tropics, in general, such studies are still insufficient because shredder insect composition and the life histories of each taxon are poorly known (e.g. Dudgeon, 1994;Ramírez & Pringle, 1998;Tomanova, Goitia & Helešic, 2006;Yule et al, 2009;Cheshire, Boyero & Pearson, 2005;Li & Dudgeon, 2008;Camacho, Boyero, Cornejo, Ibáñez & Pearson, 2009;Chará-Serna, Chará, Zúñiga, Pedraza & Giraldo, 2010;Boyero et al, 2011a;. In one of the few studies conducted in tropical island streams, (Yule, 1996a;1996b) found in Bougainville Island (70-80% shaded), 10 shredder taxa (out of 48 species), including 2 crustaceans and 8 insects.…”

“…Quests for shredding macroinvertebrates in tropical streams have been poorly replicated, a major drawback for identifying the actual patterns of richness, due to the great stream to stream variability (Camacho et al, 2009). Thus, it is necessary to replicate samples among streams and habitats at a regional scale.…”

Relationships of shredders, leaf processing and organic matter along a canopy cover gradient in tropical streams

“…A growing body of evidence emphasizes the importance of algal carbon in tropical food webs of forested small streams (Salas and Dudgeon, 2001;Mantel et al, 2004;Brito et al, 2006;Li and Dudgeon, 2008;Lau et al, 2009;Neres-Lima et al, 2016;Brett et al, 2017) and it has been claimed that the contribution of macroinvertebrate shredders to leaf breakdown is small in certain tropical running waters due their scarcity (Dudgeon and Wu, 1999;Dobson et al, 2002;Gonçalves et al, 2006bGonçalves et al, , 2007Ardón and Pringle, 2008;Alvim et al, 2015). On the other hand, there is contrary evidence of high diversity and abundance of macroinvertebrate shredders, high secondary production and importance on leaf breakdown (Cheshire et al, 2005;Camacho et al, 2009;Yule et al, 2009;Encalada et al, 2010;Masese et al, 2014;Tonin et al, 2014;Andrade et al, 2017;Neres-Lima et al, 2017).…”

“…Methodological factors could underestimate shredder importance in tropical running waters: benthic macroinvertebrates are frequently assigned to functional feeding groups according to classifications proposed for temperate organisms (Camacho et al, 2009). Thus, certain organisms, abundant in tropical running waters and not usually classified as shredders, like Leptophlebiidae, could be involved in leaf processing (Bello and Cabrera, 2001;Andrade et al, 2017).…”