Ecological and evolutionary processes in natural populations are largely influenced by the population’s stage-structure. Commonly, different classes have different competitive abilities, e.g., due to differences in body size, suggesting that inter-class competition may be important and largely asymmetric. However, experimental evidence states that inter-class competition, which is important, is rare and restricted to marine fish. Here, we manipulated the adult density in six semi-natural populations of the European common lizard, Zootoca vivipara, while holding juvenile density constant. Adult density affected juveniles, but not adults, in line with inter-class competition. High adult density led to lower juvenile survival and growth before hibernation. In contrast, juvenile survival after hibernation was higher in populations with high adult density, pointing to relaxed inter-class competition. As a result, annual survival was not affected by adult density, showing that differences in pre- and post-hibernation survival balanced each other out. The intensity of inter-class competition affected reproduction, performance, and body size in juveniles. Path analyses unravelled direct treatment effects on early growth (pre-hibernation) and no direct treatment effects on the parameters measured after hibernation. This points to allometry of treatment-induced differences in early growth, and it suggests that inter-class competition mainly affects the early growth of the competitively inferior class and thereby their future performance and reproduction. These results are in contrast with previous findings and, together with results in marine fish, suggest that the strength and direction of density dependence may depend on the degree of inter-class competition, and thus on the availability of resources used by the competing classes.
Density-dependence Experimental populations Inter-class competition Intra-class competition Population dynamics
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We thank Cristina Romero-Diaz and Itziar López Zandueta for field assistance and two anonymous reviewers for their constructive comments. The capture and handling of lizards was conducted under the licenses provided by the Gobierno de Navarra. K.H. is a postdoctoral fellow of F.W.O-V1.
Author contribution statement
LMS-J and PSF conceived and designed the study. LMS-J, MP-A, KH, and MCB conducted the experiment. LMS-J, MP-A, and PSF analysed the data. LMS-J and PSF wrote the manuscript. All authors critically revised the manuscript.
Compliance with ethical standards
The work was supported by the Spanish Ministry of Education and Science (CGL2005-01187, CGL2008-01522) and the Swiss National Science Foundation (PPOOP3_128375) to P. S. F.
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable institutional and national guidelines for the care and use of animals were followed.
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