Darwin’s naturalization hypothesis : phylogeny, enemy release and multi-trophic interactions

Abstract

With the increase of human mobility, also the breakdown of biogeographical barriers by voluntarily or involuntarily human-mediated dispersal of species into new regions has increased. The relatedness of an introduced species to the native recipient community it is introduced to may determine the success of the former. This dissertation addresses overall the role of the phylogenetic distance to the recipient community in determining the success of introduced species. The main hypothesis addressed, Darwin’s naturalization hypothesis, is the leading thread through the different chapters. Darwin’s naturalization hypothesis states that an introduced species is more likely to succeed if no closely related species are present in the recipient community, since it would suffer competitive exclusion due to niche overlap with the close relatives. In contrast, the preadaptation hypothesis states that an introduced species is more likely to succeed if closely related species are present in the recipient community, since it would more likely be preadapted to the local environment. These two contrasting hypotheses have been summed up as Darwin’s naturalization conundrum.The overarching question of the role of phylogenetic distance is addressed across different life history stages of the introduced plant, starting from seed germination and seedling survival, following a plant’s growth and development, and finally a plant’s sexual reproduction through pollen transfer, fruit set and seed production. Further, to assess how the role of phylogenetic distance may depend on biotic interactions, several factors are included: disturbance, herbivorous and predatory arthropods, fungal pathogens, and belowground nematodes. To unravel the role of phylogenetic distance, I tested for both linear and non-linear effects. The three chapters of this thesis consist of one thorough analysis of two existing datasets, and of two experimental studies that I conducted myself.In the first study, I addressed Darwin’s naturalization conundrum analysing a total of two datasets from field experiments in Germany and in Switzerland, where alien and native species were introduced either as seeds or as seedlings in native grasslands. The Swiss experiment included a disturbance treatment, while the German experiment included disturbance, fungicide application and herbivore-exclusion treatment. I analysed the role of four community phylogenetic distance measures on seedling emergence and seedling survival: the mean phylogenetic distance to the native community, the weighted mean phylogenetic distance to the native community, the distance to the nearest neighbour in the native community, and the distance to the most abundant neighbour in the native community. Phylogenetic distance affected establishment depending on life stage, with success peaking at intermediate distances for seedling emergence, but decreasing with increasing distances for the seedling survival. This suggests that two excluding forces, competition from close relatives as predicted by Darwin’s naturalization hypothesis and environmental filtering for distant relatives as predicted by the preadaptation hypothesis, act simultaneously, leading to highest success for intermediate relatedness.In the second study, I conducted a mesocosm experiment where I grew six alien target species alone and in competition with one of nine native competitor species. Using enclosure cages, I applied herbivorous arthropods, predatory arthropods and soil nematodes as treatments, to test for effects of multitrophic interactions. I analysed the role of the phylogenetic distance between target and competitor species on target performance, measured as survival, biomass and number of flowers. Phylogenetic distance did not affect any of the performance measures, neither did the trophic interactions treatments. The likely explanation is that during the experiment facilitative effects were predominant, due to the hot temperature during that year.In the third study, I performed a multi-species hand pollination experiment using alien and native species as both pollen donors and pollen recipients. I analysed the role of the phylogenetic distance between donor and recipient species on fruit and seed set. Further factors included were the donor and recipient status (alien or native) as well as trait distance. Phylogenetic distance affected fruit set only for alien recipient, with more distantly related species resulting in lower heterospecific pollen interference. As a new finding, I showed that heterospecific pollen from natives negatively affects alien recipients, which indicates biotic resistance of native communities.To conclude, the relative importance of different biotic factors, including herbivory, fungal pathogens and disturbance, depends on life stage and environmental context. Thus, the resulting patterns of how phylogenetic distance affects performance of an introduced species will vary accordingly, being the net outcome of different mechanism acting simultaneously. For future studies on the role of the phylogenetic distance between introduced species and native communities on performance of the introduced species, one needs to keep in mind the life stage and the environmental context.