Joakim Hjelm

Trophic cascades promote threshold-like shifts in pelagic marine ecosystems

Fisheries can have a large impact on marine ecosystems, because the effects of removing large predatory fish may cascade down the food web. The implications of these cascading processes on system functioning and resilience remain a source of intense scientific debate. By using field data covering a 30-year period, we show for the Baltic Sea that the underlying mechanisms of trophic cascades produced a shift in ecosystem functioning after the collapse of the top predator cod. We identified an ecological threshold, corresponding to a planktivore abundance of ≈17 × 1010 individuals, that separates 2 ecosystem configurations in which zooplankton dynamics are driven by either hydroclimatic forces or predation pressure. Abundances of the planktivore sprat above the threshold decouple zooplankton dynamics from hydrological circumstances. The current strong regulation by sprat of the feeding resources for larval cod may hinder cod recovery and the return of the ecosystem to a prior state. This calls for the inclusion of a food web perspective in management decisions.

Multi-level trophic cascades in a heavily exploited open marine ecosystem

Anthropogenic disturbances intertwined with climatic changes can have a large impact on the upper trophic levels of marine ecosystems, which may cascade down the food web. So far it has been difficult to demonstrate multi-level trophic cascades in pelagic marine environments. Using field data collected during a 33-year period, we show for the first time a four-level community-wide trophic cascade in the open Baltic Sea. The dramatic reduction of the cod (Gadus morhua) population directly affected its main prey, the zooplanktivorous sprat (Sprattus sprattus), and indirectly the summer biomass of zooplankton and phytoplankton (top-down processes). Bottom-up processes and climate–hydrological forces had a weaker influence on sprat and zooplankton, whereas phytoplankton variation was explained solely by top-down mechanisms. Our results suggest that in order to dampen the occasionally harmful algal blooms of the Baltic, effort should be addressed not only to control anthropogenic nutrient inputs but also to preserve structure and functioning of higher trophic levels.

Interactions among size-structured populations in a whole-lake experiment: size- and scale-dependent processes

Interactions in size-structured populations are characterized by a mixture of predatory and competitive interactions dependent on the size of the individual organism. We analyzed this mixture of size-structured interactions for a species constellation consisting of perch (Perca fluviatilis) and roach (Rutilus rutilus) in a replicated whole lake experiment over 4 yr. Roach are preyed upon by large perch, but at the same time compete with small perch for zooplankton. Predictions regarding the effects of roach on perch performance and resource dynamics in the whole-lake experiment were based on results from previous pond and enclosure experiments carried out over short (months) time periods. Variables measured in the whole-lake experiment included both individual level parameters (diet and growth of perch) as well as population level parameters (mortality, population numbers and size structures of perch, abundance and biomass of zooplankton and benthic macroinvertebrates). The population size of perch ≥ 2 yr old decreased to less than 10% of the pre-treatment levels in treatment lakes after the introduction of roach and remained low over the whole study period. The mortality in perch one-year-old and older increased with size. Larger perch had a higher growth decrease than smaller perch, but also the individual growth rate of young-of-the-year (YOY) perch was lower in treatment lakes than in control lakes. YOY perch were almost excluded during the winter and spring following the roach introduction. The few perch of this year class that survived had the following years a higher growth than the corresponding year class in control lakes. This could be related to an increased availability of predator-sensitive macroinvertebrates in treatment lakes. Perch in treatment lakes fed to a larger extent on macroinvertebrates and less on pelagic zooplankton than perch in control lakes. As expected, zooplankton abundances decreased in treatment lakes the year following the roach introduction, but thereafter zooplankton abundances were higher in treatment lakes due to the absence of perch recruitment in these lakes. No incidence of perch predation on roach was recorded during the study period. We attribute this to the spatial scale dependent escape ability and habitat use of vulnerable size classes of roach. YOY perch had a stronger impact on between-year variation in zooplankton than YOY roach and this can be related to the higher foraging (handling) capacity of the former and the restricted habitat use of the latter. Individual level parameters (diet, growth) had a higher statistical power than population level parameters (abundance, biomass, mortality), and small-scale experiments successfully predicted a higher proportion (81%) of the individual level parameters. However, the lower success in predicting population level responses (38%) was directly connected to a failure to predict individual level parameters (large perch diet and growth). Our results suggest that the small spatial scale was the major limitation on the predictive ability of the small-scale experiments.

Predators with multiple ontogenetic niche shifts have limited potential for population growth and top-down control of their prey

Catastrophic collapses of top predators have revealed trophic cascades and community structuring by top-down control. When populations fail to recover after a collapse, this may indicate alternative stable states in the system. Overfishing has caused several of the most compelling cases of these dynamics, and in particular Atlantic cod stocks exemplify such lack of recovery. Often, competition between prey species and juvenile predators is hypothesized to explain the lack of recovery of predator populations. The predator is then considered to compete with its prey for one resource when small and to subsequently shift to piscivory. Yet predator life history is often more complex than that, including multiple ontogenetic diet shifts. Here we show that no alternative stable states occur when predators in an intermediate life stage feed on an additional resource (exclusive to the predator) before switching to piscivory, because predation and competition between prey and predator do not simultaneously structure community dynamics. We find top-down control by the predator only when there is no feedback from predator foraging on the additional resource. Otherwise, the predator population dynamics are governed by a bottleneck in individual growth occurring in the intermediate life stage. Therefore, additional resources for predators may be beneficial or detrimental for predator population growth and strongly influence the potential for top-down community control.

RELATIONSHIP BETWEEN FISH AND THE NUMBER OF HORNS IN CERATIUM HIRUNDINELLA (DINOPHYCEAE): A FOOD‐WEB‐MEDIATED EFFECT ON ALGAL MORPHOLOGY?1

In a freshwater mesocosm experiment, we explored the potential for direct and indirect effects of roach (Rutilus rutilus) and Eurasian perch (Perca fluviatilis), two planktivorous fishes with different feeding behaviors, on the morphology of Ceratium hirundinella (O. F. Müll.) Dujard., a large dinoflagellate. Three morphs were detected: one with two hypothecal horns, one with a third rudimentary horn, and one with three well‐developed horns. We observed a strong negative relationship between the presence of fish and the proportion of three‐horned cells. The two fishes had strikingly similar effects on C. hirundinella morphology, despite their different capabilities to retain particles of the size of C. hirundinella. This finding suggests that the morphological variation in C. hirundinella was not related to selection by fish. Morphological variations in C. hirundinella could not be explained by fish‐mediated variations in turbidity (i.e., light climate) or by predation pressure by the fish. In contrast, the proportion of three‐horned cells was directly related to the biomass of filter‐feeding cladocerans. This result was unexpected since cladocerans are not considered to consume C. hirundinella and they did not depress C. hirundinella numbers in our experiment. Without excluding other possible mechanisms, we suggest that the third horn might help these dinoflagellates avoid physical contact with the filtering apparatus of the cladocerans and the consequent potential damage caused by these herbivores, which were more abundant in the absence of planktivorous fish.