Abstract
A central goal of population ecology is to identify the factors that regulate population growth. Monarch butterflies (Danaus plexippus) in eastern North America re-colonize the breeding range over several generations that result in population densities that vary across space and time during the breeding season. We used laboratory experiments to measure the strength of density-dependent intraspecific competition on egg laying rate and larval survival and then applied our results to density estimates of wild monarch populations to model the strength of density dependence during the breeding season. Egg laying rates did not change with density but larvae at high densities were smaller, had lower survival, and weighed less as adults compared to lower densities. Using mean larval densities from field surveys resulted in conservative estimates of density-dependent population reduction that varied between breeding regions and different phases of the breeding season. Our results suggest the highest levels of population reduction due to density-dependent intraspecific competition occur early in the breeding season in the southern portion of the breeding range. However, we also found that the strength of density dependence could be almost five times higher depending on how many life-stages were used as part of field estimates. Our study is the first to link experimental results of a density-dependent reduction in vital rates to observed monarch densities in the wild and show that the effects of density dependent competition in monarchs varies across space and time, providing valuable information for developing robust, year-round population models in this migratory organism.
Highlights
A central tenet of population ecology is to identify the factors that regulate population growth [1,2,3]
Applying Experimental Results to Natural Densities We applied the proportional reduction in survival caused by density dependence found in our experiment to natural larval densities of monarchs recorded between 1997 and 2006 throughout the monarch range in eastern North America [27]
Applying Experimental Results to Natural Densities Using the mean larval densities of monarch butterflies from eastern North America presented by Lindsey et al [15], we applied the results of our density-dependent survival function to estimate the strength of density dependence across space and time
Summary
A central tenet of population ecology is to identify the factors that regulate population growth [1,2,3]. Eastern populations undertake a long-distance migration and re-colonize the breeding distribution over successive generations [11,12,13] and population density varies within seasons [10,14,15], among years [10,16,17], and between different regions on the breeding grounds [10,15,16]. If density-dependent effects operate on the breeding grounds quantifying how variation in density influences growth, survival or reproductive rates could be used to predict changes in population growth rates that are spatially and temporally dependent
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