Interactions among density, climate and food web effects determine long-term life cycle dynamics of a key copepod

Increasing pressure on animal populations through climate change and anthro- pogenic exploitation fuel the need to understand complex life cycle dynamics of key ecosystem species and their responses to external factors. Here, we provide a novel, integrative study on the long-term population dynamics of Pseudocalanus acuspes , a key species in the Baltic Sea, explic- itly considering its distinct life-history stages, and testing for linear, non-linear, and non-additive climate and food web effects. Based on a unique data set of stage-specific abundance covering almost 5 decades of sampling (1960 to 2008, with 1408 samples), we use generalized additive mod- eling (GAM) and its respective non-additive threshold (TGAM) formulation to test for (1) density effects on subsequent life-history stages within the internal life cycle, (2) the effect of exogenous bottom-up (i.e. hydro-climatic) and top-down (i.e. predation) pressures, and (3) changes between bottom-up and top-down regulation. We show that linear density effects are always present, explaining a high proportion of interannual variability, while effects of external pressures are non- linear or non-additive and strongly stage- and season-specific. In general, younger stages of P. acuspes are more affected by atmospheric winter conditions and water temperature, whereas older stages are influenced by conditions of deepwater salinity and predation pressure. These bot- tom-up processes, however, are not necessarily stable, and can depend on the level of top-down predation pressure. Our study demonstrates the complex and non-stationary interplay between internal and external factors regulating long-term animal population dynamics.

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