Plant genotype affects long‐term herbivore population dynamics and extinction: conservation implications

Few studies have linked long‐term herbivore population dynamics with plant genetics. In this study we present evidence that plant genotype and hybridization influence the population dynamics of the poplar bud gall mite, Aceria parapopuli. Using experimental transfers and a five‐year data set on mite abundance on two cottonwood species (Populus fremontii and P. angustifolia) and their naturally occurring hybrids, we demonstrated that hybrid trees exhibiting an F₁ morphology were, on average, extremely susceptible to A. parapopuli. The susceptibility of these hybrids ranged several orders of magnitude and affected the population dynamics of A. parapopuli across the five years of study. Populations grew exponentially on susceptible hybrids in every year, eventually reaching a mean of ∼140 galls per tree. In contrast, populations fluctuated around low densities (0.01–0.87 galls per tree) on their parental host species. Low gall densities on parental trees resulted in high annual extinction rates (mean = 62%) for mite populations on individual parental trees, in contrast to low annual extinction rates (mean = 7%) for mite populations on hybrid trees. We detected significant differences in gall population growth rates (intrinsic rate of increase, r) among hybrid genotypes across four years of study, ranging from r = 0 to r = 1.5, demonstrating that plant genotype influences a fundamental component of population dynamics. We argue that plant genotype should also impact metapopulation dynamics, because plant genotype affected the number of available colonists and directly affected mite extinction rates. If other arthropod species exhibit similar traits, these findings have important conservation implications. Because mite population growth and extinction are so closely tied to rare host genotypes, to conserve such species we must preserve rare host genotypes, which would represent a major challenge to current conservation practices that target species rather than genotypes. Corresponding Editor: R. F. Denno