Bhattarai, G. P., L. A. Meyerson and J. T. Cronin. 2017. Geographic variation in apparent competition between native and invasive Phragmites australis. Ecology DOI: 10.1002/ecy.1646.

ABSTRACT

Apparent competition, an indirect species interaction via shared natural enemies, has been shown to play an important role in shaping the structure and dynamics of natural communities. However, its importance in driving species invasion has not been fully explored. Moreover, latitudinal gradients in species diversity and biotic interactions can be expected to create similar spatial variation in the strength of apparent competition between interacting species. We performed replicated field experiments at four sites spanning 7o latitude along the Atlantic coast of the United States to assess the presence and strength of apparent competition between sympatric native and invasive genotypes of Phragmites australis. We also tested the hypothesis that the strength of this interaction declines with increasing latitude. Within each site, native and invasive plants of P. australis were cross-transplanted between co-occurring native and invasive patches in the same marsh habitat and herbivore damage was evaluated by the end of the growing season. For native plants, the incidence of stem-feeding and leaf-mining herbivores was 34% and 221% higher, respectively, when transplanted into invasive than native patches. Total aphids per plant was 296% higher on native plants growing in invasive patches. These data suggest that invasive P. australis has a negative effect on native P. australis via increased herbivory, indicative of apparent competition. In comparison, transplanted invasive plants experienced 341% increase in aphid density because of apparent competition. With the mean effect size 57% higher for native genotype relative to the invasive, apparent competition was asymmetric. Increased herbivory on native plants because of apparent competition was equal in magnitude to the difference in herbivory between invasive and native genotypes. Although the support for the latitudinal gradients in the intensity of indirect interaction was modest, our study shows that the strength of apparent competition/mutualism can vary substantially among the study sites. These results suggest that apparent competition mediated by arthropod-herbivores might be one of the mechanisms enhancing the invasion of introduced genotype of P. australis in Atlantic coast of the US. Invasion success may also vary with latitude, being more likely at southern latitudes where apparent competition is stronger.

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