Effects of separate and combined predation by the invasive mud crab (Rhithropanopeus harrisii) and the native fish (Perca fluviatilis) on the marine isopod (Idotea balthica) in the northern Baltic Sea

Abstract

Introductions of novel predators have caused dramatic declines and extinctions of native prey species worldwide. This also concerns the littoral community of the northern Baltic Sea, where the functionally novel predatory mud crab Rhithropanopeus harrisii has caused a rapid reduction in the abundance and diversity of the native key herbivores. Predator invasion success may be attributed to prey naivete, or the absence of predator recognition and appropriate anti-predator behaviour of native prey due to the lack of coevolution. There is a knowledge gap regarding the combined effect of invasive and native predators on prey. If the combined mortality effect is synergistic, it could lead to drastic declines in native prey populations. In this thesis, I have examined the effects of separate and combined predation by the invasive mud crab and the native perch Perca fluviatilis on the mortality of the marine herbivorous isopod Idotea balthica. I also observed the anti-predator behaviour of the isopod to evaluate its naivete. For this experiment, I collected all the animals from the Archipelago Sea. I conducted a predation experiment in the laboratory and compared mortality rates of the isopods preyed on by mud crabs only, perch only, and both the predator species together. Based on a survival analysis, both the separate and combined predation by the two predators caused a significant mortality on the isopods. However, the two predators together consumed less isopods than was expected based on how much they consumed separately. This suggests that there was an antagonistic interaction between the predators, which led to a reduction in their combined effect. As the two predators were never observed to interfere each other directly, the reduced prey mortality is possibly due to a trait-mediated indirect interaction, involving a behavioural modification between the predators and the prey. These results have important community- and ecosystem-level implications. Although the mud crab is an efficient predator that may benefit from the possible naivete of the isopods, when both predators co-occur, they cause lower mortality on the isopods. Thus, the perch and other similar native predators may have a key role in the ecosystem of the northern Baltic Sea, as they may attenuate the adverse impact of the mud crab. To understand the ecological impacts of introduced predators in the native communities, we need more research that considers the multiple trophic levels involved in predator-prey dynamics and the key role of prey naivete in predator introductions.