Life cycle dynamics of a key copepod over long time scales: A complex and non-stationary interplay of internal and external driver

Background/Question/Methods

In light of the increasing impacts of climate change and anthropogenic pressures on aquatic and terrestrial animal populations, there is an emerging need of understanding the complex life cycle dynamics of key species and their response to external factors. External physical and biological drivers can have linear, non-linear and non-additive effects on population sizes, which vary in species with complex life cycles between the various life-history stages. To fully understand the long-term dynamics of animal populations it is hence crucial to consider stage-specific effects of external drivers and how these propagate through the life cycle. Within the field of zooplankton ecology, however, long-term studies are often limited to only a few of these processes. Here we provide a novel, integrative study on long-term population dynamics of a key marine ecosystem player explicitly testing for linear, non-linear and non-additive density, climate and food web effects. The study is based on a unique long-term data set of seasonal stage-specific abundance of the important Baltic Sea zooplankton species Pseudocalanus acuspes.

Results/Conclusions

We find that density effects explain generally most of the variability in stage abundances, while hydroclimatic effects are non-linear and strongly stage- and season-specific. For instance, younger stages that reside in the upper water column responded stronger to local thermal conditions while older stages that live in deeper water layers are mainly determined by salinity except in winter. These bottom-up processes, however, are not necessarily stable and can depend on the level of top-down predation pressure. The observed major peak in stage abundances during the 1970s/80s can be consequently explained by a synergistic effect of internal and external forces, in which effects of environmental drivers progress through the life cycle affecting stages in subsequent seasons. Our study demonstrates the complex and non-stationary interplay between external and internal factors regulating long-term animal population dynamics. Hence, we emphasize that integrative, stage-specific modelling studies like ours are important for evaluating the increasing impacts of climate change and anthropogenic pressures on key animal populations.