Ulrika Candolin, Elina Bertell & Jarkko Kallio
The shrimp Palaemon elegans has recently invaded the Northern Baltic Sea, a heavily disturbed ecosystem. We investigated how the invasion of this species interacts with other disturbances occurring in the sea, in particular with the increased abundance of medium-sized predators (mesopredators). Mesopredators have increased because of the decline of predatory fish such as cod, perch and pike, and because of eutrophication that promotes the recruitment of smaller fish species. We performed a combination of field observations and field and laboratory experiments to investigate the interaction between the shrimp and the most common mesopredator in the area, the threespine stickleback. The result show that the native stickleback is the dominant competitor for resources and causes the invading shrimp to shift its habitat choice towards vegetated habitats and its diet from grazers to algae. This reduces the biomass of algae, which in recent years have become a nuisance in the area.
The stickleback feeds on grazers, which feed on algae, and an increased stickleback population consequently promotes the growth of algal biomass. However, by competing with the shrimp for space and food, the increased abundance of the stickleback forces the invader to increasingly turn to algae for food, which then decrease in abundance.
Thus, a human-induced problem, the growth of mesopredator populations, mitigates another human-induced problem, the increased biomass of filamentous algae, by influencing the behaviour of a non-native, invading species. This demonstrates how human-induced changes in ecological conditions and species abundances can interact in complex ways and result in ecological surprises. It stresses the importance of investigating interactions among human disturbances by showing how a narrow focus on single disturbances may not reveal their ultimate effects.