Do novel interactions with local fauna have reproductive consequences for exotic plants? A case study with thistles, ants, aphids, and pollinators

Abstract

Exotic plants are involved in different interactions with the fauna of the invaded sites, which can facilitate or limit their successful establishment and spread. Here, we evaluated the impact of native aphid-tending ants on the reproductive consequences of the invasive thistle Carduus thoermeri in NW Patagonia, which is frequently infested by aphids. We estimated the number and proportion of viable seeds, seed weight, germination proportion, and mean germination time of thistles in the presence and absence of aphids and ants, and with or without pollinator access. Aphid-infested thistles had 57% less viable seeds and 29% lower seed weight than non-infested thistles. Although ants and aphids had no effect on germination proportion, the mean germination time was ca. 15% faster in seeds from aphid-infested thistles. Our results suggest that the potential indirect effects of aphid-tending ants on thistles (negative effects via pollinator deterrence and positive effects via driving away non-aphid herbivores) are less important than the direct negative effects of aphids. Interestingly, although harboring aphids and ants has negative reproductive consequences for C. thoermeri plants, it could also generate a competitive advantage by giving rise to small and fast germinated seeds. This study illustrates the complexity of novel interactions among exotic plants and native ants, reinforcing the need for more studies to fully understand the potential impact of ant–plant interactions mediated by Hemiptera on the invasion success of plants.