Patterns of density-dependence have been described for many species, but variation in patterns of density-dependence across multiple years has seldom been experimentally tested. Negative density-dependence is assumed to be the pattern for most species, but component Allee effects (positive density-dependence) have been identified in some species. Naturally abundant and social species that have undergone severe population declines may be most likely to show component Allee effects. June sucker (Chasmistes lirous) is a formerly abundant species that has undergone massive population decline, and it is now listed as an endangered species. We tested for variation in patterns of density-dependence in young June sucker across years, and effects of simulated vegetation on density dependence. We measured growth and survival responses to low, medium, and high densities in an experimental framework over four years and responses to simulated vegetation structure in two years. We used a combined analysis of all four experiments to assess variation in patterns of density-dependence among years. Experiments were run in two drought years with declining water levels during the growing season, and two wet years with stable water levels.
Results/Conclusions
Overall, patterns of density-dependence varied dramatically among years ranging from strong negative density-dependence to strong Allee effects in both growth and survival. Allee effects were most prominent in the survival response and during years of declining water levels. June sucker exhibit Allee effects at low densities in both growth and survival during the critical period of first year growth. In many temperate freshwater fishes, size at the end of the first growing season is a critical determinant of overwinter success. Variation in density-dependence was greater than anticipated and these data suggest that characterization of density-dependence from single years will not adequately represent population dynamics. Vegetation had neutral or negative effects that were not consistent across densities. Recovery efforts should consider annual variation in density-dependence and plan stocking efforts accordingly.
