Enemy pressure exerted on alien and native plants may differ between montane and lowland regions

Abstract

This study analyzed the enemy release hypothesis (ERH) assumptions under different climatic conditions in a lowland region and a mountain region in Poland. A common garden experiment was performed in 2010 and 2011 using 3 native (Impatiens noli-tangere, Solidago virgaurea, and Polygonum bistorta), 2 noninvasive alien (Impatiens walleriana and Impatiens balsamina), and 4 invasive alien (Impatiens glandulifera, Impatiens parviflora, Solidago gigantea, and Reynoutria japonica) species. The species were surveyed for leaf damage and invertebrate pest attacks. In all tests, leaf damage and pests were recorded more frequently in the lowland than in the mountains. The differences were particularly evident in the models for Impatiens and in the leaf damage model for Solidago. The distinction was irrelevant for some species (Polygonaceae); however, these results should be interpreted with caution because the taxonomic relations between the studied Polygonaceae were lower than those between the other tested species. Considering all the tests, the assumptions of the ERH hypothesis were confirmed in 6 cases (29%), whereas there was no confirmation in 15 cases, including 7 cases of nonsignificance and 8 cases of the opposite results. Moreover, the overall result of the comparison between the noninvasive and invasive alien species was the opposite of that predicted by the ERH hypothesis; in more than 2/3 of the cases, invasive alien species were released from the enemy less effectively than were noninvasive ones. In conclusion, the assumptions of the ERH hypothesis do not always hold true, and the invasion success of alien species may depend on several factors, including specific local conditions.