Since the inception of invasion science, numerous hypotheses have been proposed to shed light on why certain communities harbour a greater abundance or richness of invasive species in comparison to others and why not all non-native species become invasive. In this Hypothesis Description paper, as proposed by Mietchen et al. (2024), we introduce one of the oldest hypotheses in the field, which was posed by Charles Darwin in The Origin of Species (Darwin 1859) and that is now known as “Darwin’s naturalisation hypothesis”. In his iconic book, Darwin wrote: “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” (Darwin 1859; p. 60). Since Darwin’s first formulation, the naturalisation hypothesis has been referred to as “Darwin’s theory” (Rejmánek 1996) and “Darwin’s invasion hypothesis” (Schaefer et al. 2011), while Daehler (2001) was, to our knowledge, the first publication to refer to it as “Darwin’s naturalisation hypothesis”. Darwin’s naturalisation hypothesis offers an explanation as to why some communities are invaded more than others and, thereby, provides the basis for predicting biotic resistance, namely a community's ability to reduce or prevent invasions.

The hypothesis draws from classical niche theory and poses that non-native species are unable to establish and increase in abundance if the species in the recipient community already have a high niche overlap with the non-native species (MacDougall et al. 2009), resulting in intense competition or shared enemies (Cadotte et al. 2018). Community resistance, as described by these mechanisms, acts as the opposite of invasion success, limiting the establishment and spread of non-native species. Using observations from the flora of the US, Darwin initially hypothesised that closely-related species would compete more intensely than species that are less closely related to each other, based on the assumption that relatedness serves as a proxy for niche overlap (Daehler 2001). Daehler (2001) slightly changed Darwin’s formulation to a more succinct version, proposing that there will be a reduced probability of naturalisation by non-native species with native congeners. Duncan and Williams (2002) added to the definition by mentioning that resistance from the recipient community could result from competition, but also from herbivore pressure and/or pathogen attacks. Furthermore, Jeschke and Erhard (2018) formulated the hypothesis in terms of the richness of closely related species in the recipient community. They posited that invasion success of non-native species would be higher in areas with fewer closely-related species than in areas that are rich in closely-related species.

Although the formulation of the hypothesis has not changed significantly over time, the emphasis has been set on competitive interactions and, recently, more advanced tools to measure relatedness have led to defining non-native/native species relatedness in terms of phylogenetic distances (e.g. phylogenetic nearest neighbour distance) instead of broad taxonomic classification categories. In this line, Park et al. (2020) expanded the definition of Darwin’s naturalisation hypothesis, stating that lack of competitive exclusion resulting from phylogenetic distinctness would facilitate the establishment of non-native species. The mechanism described by Park et al. (2020) builds on previous work exploring the relationship between phylogenetic distances and ecological interactions (e.g. Cavender‐Bares et al. (2009)). This formulation is more specific and emphasizes invasion success rather than the resistance of the native community, but it only refers to the establishment phase of the invasion process. It also restricts biotic resistance to being the result of competitive interactions, overlooking potential antagonistic effects of pre-adapted herbivores and pathogens that the non-native species might encounter (Duncan and Williams 2002).

Indeed, Schaefer et al. (2011) proposed renaming the hypothesis to “Darwin’s invasion hypothesis”, arguing that Darwin’s focus on competition indicates that he had later stages of the invasion process in mind. Since this suggestion was not widely adopted in literature, our contribution retains the original name as a reference for future studies testing the hypothesis. This also has the advantage of preventing further confusion with another contrasting hypothesis attributed to Darwin: the pre-adaptation hypothesis. Under this idea, Darwin also suggested that close relatedness to the native community could benefit non-native species through shared adaptations to the environment (Darwin 1859). These opposing hypotheses about the role of phylogenetic distance in invasion success are collectively referred to as “Darwin’s naturalisation conundrum” (Diez et al. 2008). For the sake of completeness and to avoid confusion, we include alternative names (see section 'Synonyms') and identify related or opposing hypotheses to Darwin’s naturalisation hypothesis, thereby also providing an overview of this hypothesis’ conceptual context (see section 'Related hypotheses').

Following Heger et al. (2024), we propose formalising the definition of Darwin’s naturalisation hypothesis in a structured format of ‘subject – relation - object’ to clarify the various interpretations and facilitate computational applications. Specifically, we propose the following formal definition: “High phylogenetic distance between non-native species and the recipient community increases invasion success”. Here, we build on the increasingly available and comprehensive phylogenetic data across several taxa, expanding the scope of the hypothesis beyond the establishment phase. With this contribution, we compiled a list of early definitions (section 'Hypothesis definitions') and proposed citable, formalised versions for clarity and accessibility (section 'Formalised representation of hypothesis variants'). Therefore, this work builds on current mapping efforts for the landscape of hypotheses in the field of invasion biology (Jeschke et al. 2021).

Hypothesis name

Synonyms

Acronym

Identifiers

Domains that make use of this hypothesis

Reviews, meta-analyses and large-scale analyses

Related hypotheses

Table 1

Table 2

This contribution proposed a unified and concise definition of Darwin’s naturalisation hypothesis building on previous formulations (Table 1) and following a structured format that can be replicated for other hypotheses in the field of invasion biology and beyond. We also provide formalised representations of previous formulations of this hypothesis (Table 2). Such an effort is necessary to provide a base map, avoiding contradicting or inaccurate interpretations, thereby guiding the development of future empirical and theoretical studies. For instance, clear formulations can aid applications such as the design of communities resistant to invasions during ecological restoration using phylogenetic information (e.g. Yannelli (2021)). Further and as previously stated by Heger et al. (2024), structuring hypothesis formulations in this way improves machine readability, which is particularly important for AI applications. We also hope that other experts will build on our effort by updating this contribution in the future or following the format used in this series to contribute with descriptions of other hypotheses.

This publication was financially supported by the Deutsche Forschungsgemeinschaft (HE 5893/8-1), the VolkswagenStiftung (97 863) and the Alexander von Humboldt Stiftung through the Feodor Lynen Fellowship granted to FAY.