Effects of Intraspecific and Interspecific Indirect Competition on Foraging Activity in Subtropical Facultative and Obligatory Shredder Organisms

While the recognised influence of resource competition on the structure of aquatic ecosystems is well-established, its effects on allochthonous matter dynamics in streams, particularly regarding interspecific and intraspecific indirect competition, remain understudied and inadequately understood. In this investigation, we explored the impacts of interspecific and intraspecific indirect competition on leaf litter consumption by the shredder organisms Phylloicus spp. and Aegla spp. The effects of indirect competition were evaluated through a microcosm experimental setup, comprised of two controls and three treatments: (i) Phylloicus control competition-free; (ii) Aegla control competition-free; (iii) intraspecific competition within Aegla; (iv) intraspecific competition within Phylloicus; and (v) interspecific competition. In the competition-free controls, a single organism occupied a 2 L microcosm, whereas in indirect competitive pressure treatments, two organisms were introduced into a 4 L microcosm with a 0.05-mm mesh partition. Our findings revealed that interspecific indirect competition had a disproportionately higher impact on Phylloicus, whereas intraspecific indirect competition had a more pronounced effect on Aegla. Phylloicus demonstrated superior shredding efficiency compared to Aegla, possibly due to Aegla's broader feeding range and distinctive leaf litter utilisation strategies. The presence of potential competitors led to reduced consumption rates in Phylloicus, highlighting its sensitivity to competition. Furthermore, Aegla's predatory behaviour and cannibalism likely intensified intraspecific indirect competition within the species. These results underscore the pivotal role of indirect competition in shaping the feeding behaviour of shredder organisms in leaf litter banks, ultimately influencing plant matter fragmentation in aquatic ecosystems.