Abstract
Identifying the relative importance of predation and resources in population dynamics has a long tradition in ecology, while interactions between them have been studied less intensively. In order to disentangle the effects of predation by juvenile fish, algal resource availability and their interactive effects on zooplankton population dynamics, we conducted an enclosure experiment where zooplankton were exposed to a gradient of predation of roach (Rutilus rutilus) at different algal concentrations. We show that zooplankton populations collapse under high predation pressure irrespective of resource availability, confirming that juvenile fish are able to severely reduce zooplankton prey when occurring in high densities. At lower predation pressure, however, the effect of predation depended on algal resource availability since high algal resource supply buffered against predation. Hence, we suggest that interactions between mass-hatching of fish, and the strong fluctuations in algal resources in spring have the potential to regulate zooplankton population dynamics. In a broader perspective, increasing spring temperatures due to global warming will most likely affect the timing of these processes and have consequences for the spring and summer zooplankton dynamics.
Highlights
It is commonly accepted that both bottom-up and topdown forces can simultaneously affect on ecological communities [1] and the relative strength of each of them has been evaluated in numerous studies (e.g. [2,3,4])
The importance of 0+ fish predation is still subject of controversy and while some studies show a clear connection between juvenile fish abundance and zooplankton decline [18,20,21], others do not (e.g. [22] )
Residuals from the analysis were not significantly different from normal distributions (KolmogorovSmirnov Z,0.683, p.0.740). Effects of both resource level and fish abundance were specified in the univariate tests of between-subjects effects of significant MANOVA factors. They revealed that total cladoceran densities depended on resource level (F1,27 = 14.659, p = 0.001) and fish density (F4,27 = 15.752, p,0.001), and that cladoceran densities differed between sampling occasions (F3,27 = 7.525, p = 0.001)
Summary
It is commonly accepted that both bottom-up and topdown forces can simultaneously affect on ecological communities [1] and the relative strength of each of them has been evaluated in numerous studies (e.g. [2,3,4]). It is commonly accepted that both bottom-up and topdown forces can simultaneously affect on ecological communities [1] and the relative strength of each of them has been evaluated in numerous studies On the other hand, during late spring predation by newly hatched fish (0+ fish) on zooplankton is high, which may be an alternative explanation to the dramatic crash of the zooplankton community (reviewed by [17,18]). Low reproduction rates and high mortality due to starvation have been suggested to cause the commonly observed rapid zooplankton decline in spring, whereas fish predation should only account for a minute proportion of the zooplankton mortality [15]. Notwithstanding, fish predation may be important later during the season and explain the failure of the zooplankton population to recover from the spring population collapse
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