Eva Malecore tells us more about her newly published paper about the effects of phylogenetic distance on seedling emergence…

In his “Origin of Species” of 1859, Charles Darwin expressed the hypothesis that exotic species more closely related to the native community would be more likely to naturalize, since they would share preadaptation to the local environment. This first hypothesis is known as the preadaptation hypothesis; however, what he actually observed, was the opposite. Darwin explained this as follows:

“As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera”

In other words, species more closely related to the native community are less likely to naturalize because they suffer more from competition due to niche-overlap. This second hypothesis is known as Darwin’s naturalization hypothesis. The contradiction of these two long-standing hypotheses is now termed Darwin’s Conundrum.

Figure 1: Darwin’s Conundrum: an introduced species closely related to the native community might be excluded by competitive exclusion, while an introduced species not closely related might be excluded, because it is not preadapted to the environmental conditions of the site of introduction.

With the recent increased interest in invasion biology, Darwin’s Conundrum has drawn more and more attention. Despite several studies addressing this topic so far, no consistent results have emerged: whilst some have shown how the success of introduced plants is directly proportional to the phylogenetic distance to the native community, others have claimed the opposite, and others still find no relationship between introduction success and phylogenetic distance.

The inconsistency may be caused by one or more factors: the use of different spatial and phylogenetic scales, the use of different life-stages, and last but not least, the fact that most tests are based on observational data. Using observational data means making assumptions about a potential pool of introduced species that failed to establish, and about the location and frequency of such introduction.

Figure 2: Field site of one of the datasets used in our study (Müller et al., 2016), where herbivory-exclusion cages were used.

To address these issues, our study used three datasets of experimental introductions of alien and native plant species into semi-natural grassland communities in central Europe. This experimental setup allowed us to assess how phylogenetic distance affects the success of an introduced species.

We found that the phylogenetic distance is important for establishment success, but its role changes according to context and life stage. Most interestingly, we could show how for seed emergence, success peaked at intermediate phylogenetic distance, resulting in a hump-shaped curve (figure 3). This finding suggests that both preadaptation (environmental filtering) and competitive exclusion may act together, suppressing seed emergence at low and high phylogenetic distances.

Figure 3: Seed emergence peaks at intermediate phylogenetic distance, suggesting that both environmental filtering and competitive exclusion may act together, suppressing seed emergence at low and high phylogenetic distance.

Following a dialectic process, we therefore concluded that Darwin´s naturalization hypothesis and the preadaptation hypothesis need not be in conflict: rather, the mechanisms underlying them can operate simultaneously or alternately depending on the life stage and on the environmental conditions of the resident community.

Eva Malecore, University of Konstanz, Germany

Read the full paper online: Non‐linear effects of phylogenetic distance on early‐stage establishment of experimentally introduced plants in grassland communities