Non-lethal effects of a native and a non-native piscivorous fish on the interaction between a mesopredator and benthic and pelagic invertebrates

Predator presence may result in non-lethal costs to prey; these costs may be even larger if both predators and prey do not naturally co-occur. We experimentally evaluated the survival of two aquatic invertebrates from the upper Paraná River basin (Chironomus sancticaroli – benthic and Daphnia magna – pelagic) exposed to a native mesopredator fish (Astyanax lacustris) in three non-lethal predation treatments: (i) the absence of piscivorous fish, (ii) the presence of a native piscivorous fish (Hoplias aff. malabaricus), or (iii) the presence of a non-native piscivorous fish (Astronotus crassipinnis). The treatments were crossed with the absence and presence of vegetated habitats. We predicted that the mesopredator might not have adapted foraging strategies to eat in the presence of an unfamiliar predator while invertebrate prey use vegetated habitats for protection. Therefore, we expected that the combination of piscivorous fish presence and vegetated habitat would lead to a classical additive response on invertebrate survival, which will be higher in the presence of non-native piscivorous fish than in the presence of native piscivorous fish. The presence of vegetated habitat and piscivorous fish increased invertebrate survival, but together they did not promote an additive response. Instead, the non-lethal cascade effects of the native piscivorous fish was similar between vegetation treatments, whereas the non-native piscivorous fish led to higher survival of the invertebrates in non-vegetated versus vegetated habitats. Although refuges are understood as elements that enhance predator-prey stability, the effectiveness of submerged macrophyte stands as safe habitats is dependent on some predator traits, such as origin and hunting mode. Our results also indicate that the invasion of A. crassipinnis has the potential to change mesopredator prey selectivity from a diet based on pelagic prey with the native H. aff. malabaricus to the random consumption of pelagic and benthic prey, which may have important consequences for the stability of natural aquatic food webs.

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