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

Abstract

We used an observed abrupt shift in the dominance pattern of two coexisting copepod species in Mü ggelsee, 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.

Ecology under lake ice

“…; Scharfenberger et al . ), undergoing diapause during summer. Furthermore, a variety of rotifers are well known to proliferate under ice (e.g.…”

“…Even so, as we observed for phytoplankton, it seems likely that many differences between summer and winter zooplankton community composition may be lake-specific, species-specific or better captured by functional trait grouping. Many zooplankton are strict diapausing species that disappear from the water column into sediments during winter (Nilssen & Elgmork 1977;Ventura & Catalan 2005;Larsson & Wathne 2006), but several copepod species in high-latitude lakes of Europe and Canada have been shown to reach peak density in mid-winter (Rigler et al 1974;Rautio et al 2000;Scharfenberger et al 2013), undergoing diapause during summer. Furthermore, a variety of rotifers are well known to proliferate under ice (e.g.…”

Evaluating early-warning indicators of critical transitions in natural aquatic ecosystems

“…The modular nature of lakes also allows for comparisons across different lakes (29). In aquatic systems, various ecological mechanisms have been shown to generate critical transitions between alternative states; the most commonly identified mechanisms include (i) competition between two or more species (2, 28); (ii) trophic cascades through inclusion or exclusion of top predators (16) or parasites (30), resulting in overexploitation traps; and (iii) intraguild predation (IGP) through resource competitors that also prey on one another (31,32). In the present work, we selected 14 state variables of five well-documented case studies of freshwater critical transitions to test whether four commonly used EWIs (AR1, SD, SK, and DR) can be detected reliably in advance of the transition.…”

Environmental Impacts—Lake Ecosystems

“…Abrupt changes within ecosystems are already known under warming trends experienced in the recent past for a number of variables spanning abiotic and biotic components, such as nutrients or algal blooms (Wagner and Adrian 2009a; for review see Adrian et al 2009Adrian et al , 2012. The underlying forces are often unclear, but may involve competition for common resources and the crossing of critical thresholds in the abundance of conspecifics (Scharfenberger et al 2013) or multiple overlapping environmental forces (Huber et al 2008).…”

“…The crossing of critical thresholds may result in abrupt changes in particular elements of an ecosystem Scharfenberger et al 2013) or entire ecosystems-the famous example being the alternative stable states of clear versus turbid lakes (Scheffer and Carpenter 2003). Abrupt changes within ecosystems are already known under warming trends experienced in the recent past for a number of variables spanning abiotic and biotic components, such as nutrients or algal blooms (Wagner and Adrian 2009a; for review see Adrian et al 2009Adrian et al , 2012.…”