Threshold-driven shifts in two copepod species: Testing ecological theory with observational data

Ulrike Scharfenberger, Aldoushy Mahdy and Rita Adrian

Limnol. Oceanogr., 58(2), 2013, 741-752 | DOI: 10.4319/lo.2013.58.2.0741

ABSTRACT: We used an observed abrupt shift in the dominance pattern of two coexisting copepod species in Müggelsee, a shallow eutrophic lake in Germany, to investigate mechanisms leading to this shift, by embedding our findings into the framework of intraguild predation theory and theoretical scenarios of threshold-driven regime shifts. We proposed that the abrupt increase in Cyclops kolensis, changing its status from a rare to the dominant species as available algal prey declined in the lake, was due to its superior exploitative competition for commonly consumed algal prey. However, C. kolensis was only able to thrive in the emerging low food niche when abundances of competing larger Cyclops vicinus, a predator of C. kolensis juveniles, fell below a critical threshold. This is consistent with the “state threshold scenario” of regime shift theory, for which a response variable exhibits an abrupt shift, here C. kolensis, after the driver (C. vicinus) crosses a threshold. We confirmed the nonlinear relationship between the two copepod species by excluding potentially matching abrupt changes in other abiotic and biotic driving variables, and successfully classifying C. kolensis abundance probability on the basis of C. vicinus abundances using logistic generalized linear modeling. C. vicinus decline followed the “driver threshold scenario” of regime shift theory, whereby an abrupt change in a driver (cryptophytes) causes a sudden shift in a response variable (C. vicinus). We illustrated how observational data on plankton communities match predictions derived from ecological theory.

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