Globally widespread weeds show preadaptation to non‐analogue climates: a common garden approach

Climatic niche shifts have been documented for plant species introduced into new regions, however the underlying mechanisms are poorly understood. Niche shifts into novel climates are particularly difficult to predict as previous occurrence data cannot reveal whether these novel climates are within species' fundamental niche limits. We have previously shown that three globally widespread species ( Rumex obtusifolius, R. crispus, R. conglomeratus ) have undergone a realised niche shift into both analogue and non‐analogue climates in New Zealand. Here we tested two alternative explanations for this realized niche shift 1) that weeds have adapted since introduction, potentially indicating shifts in their fundamental niches, or 2) that they were preadapted to these climates prior to their arrival. For each species, we collected seeds from the native range (United Kingdom) and their introduced range (New Zealand) and established common gardens in the introduced range under climates either analogous or novel compared to the native range. We compared the relative fitness of individuals from the native and introduced ranges for each species across three critical life stages: germination, survival to reproduction, and fertility. Contrary to expectations, individuals from the native range performed similarly to or better than individuals from the introduced range under all climates. Rumex obtusifolius , performed better under non‐analogue climatic conditions compared to the analogue climate common garden. Our results show little‐to‐no evidence that these species have locally adapted to their environments since introduction, and where differences in performance between individuals from different ranges occurred, individuals from the native range typically outperformed individuals from local populations. We provide empirical evidence that even globally widespread species, as opposed to only range restricted species, undergo realised niche shifts representing previously unmeasured environmental tolerances. We highlight the pressing need for a greater understanding of species' fundamental niches to understand the potential extent of biological invasions.