Apparent competition

Copepods and krill are main zooplankton species in the Barents Sea and constitute around 80 % of the diet of capelin, the dominant plankton-eating fish in the region. In a recent study (Stige et al. 2018) we analysed statistically time-series of biomass of copepods, biomass of krill and abundance-at-age and size-at-age of capelin. We found that krill biomass associates positively with the survival of capelin, so that high krill biomass leads to an increase in capelin biomass. High capelin biomass, in turn, is associated with low biomass of copepods – most likely because the copepods are eaten by the capelin. This results in apparent competition between krill and copepods, whereby high biomass of krill leads to low biomass of copepods with a delay of 1−2 years, mediated by the shared predator, capelin.

What about the feedback from capelin on krill?

If high krill biomass increases the survival and biomass of capelin, also the predation pressure from capelin on krill should start to increase? Yes, and such time-lagged negative feedback effects are exactly what drives classical predator-prey cycles as formalized in the Lotka-Volterra model. But in our case, we found that the negative feedback from capelin on the krill was too weak to cause predator-prey oscillations. If, for example, good krill recruitment caused the krill biomass to double in a given year, the increased predation from capelin the coming years would not be enough to cause krill biomass to drop to a level below what it would have been without such exceptionally good recruitment. The “boost” in krill biomass would simply gradually subside over the coming years. We also found that bottom-up effects by copepods on capelin biomass appeared to be too weak to cause oscillations. The capelin population in the Barents Sea has collapsed approximately every 10 years since the 1980s, with associated increases in zooplankton biomasses. Our results suggest that these fluctuations are not a result of predator-prey cycles between zooplankton and capelin.

How did we arrive at these findings?

Several studies have analysed top-down effects by capelin on copepods and krill in the Barents Sea, and there have also been studies investigating bottom-up effects. The novelty of our study compared to previous studies on copepods, krill and capelin in the Barents Sea was that we used an approach that allowed us to quantify bottom-up and top-down effects jointly in one statistical model. We thereby found the parameter values that best described the time-series dynamics as a whole.

What are the implications of the findings?

While competition among species for shared prey is commonly studied, there is less attention to apparent competition mediated by shared predators. This is despite many studies reporting strong top-down effects in several marine ecosystems, suggesting a potential for apparent competition. Apparent competition may therefore be more common than appears from the literature. The analysis approach we used here can be useful for identifying and quantifying such effects also in other systems. The identification of such indirect ecosystem effects is important, for example, for understanding and anticipating effects of climate change. In the Barents Sea, we hypothesize that future warming may impact high-latitude copepods negatively not only through direct effects, but also through apparent competition with the more boreal krill species in the region – which are expected to benefit from high temperatures.

Reference:

Stige, L.C., Kvile, K., Bogstad, B., Langangen, Ø. (2018). Predator-prey interactions cause apparent competition between marine zooplankton groups. Ecology. DOI: 10.1002/ecy.2126