Density-dependence in the declining population of the monarch butterfly

The Eastern monarch butterfly population has significantly declined over the last two decades creating growing concerns around its conservation status. Here, we showed that the overwintering population exhibited a negative density-dependence (i.e. a negative effect on growth rate of the density in the previous year) and that, after accounting for the density effect, the population growth rate tended to decline over time. The negative time effect is probably linked to the host plant (i.e. milkweed) decline in North America. A negative density-dependence was also found in the time series of both egg density per host plant and adult density across North America suggesting the importance of a bottom-up, resource-driven regulation such as host plant limitation and/or of a top-down regulation through generalist natural enemies or diseases. The temporal stability of the density effect indicated that the negative density-dependence and the population decline are likely independent phenomena. One of the most common conclusions of previous research is that environmental stochasticity is the dominant key compounded driver of population dynamics. We showed that density dependence explained 37–50% of the total variation in growth rate in three independent datasets, indicating that several non-exclusive density-related mechanisms can be important in monarch population dynamics.