Aim: The abundance-frequency relationship (AFR) is among the most-investigated pattern in biogeography, yet the relative contributions of niche-based processes related to ecological strategies, and of neutral processes related to spatial colonization-extinction dynamics, remains uncertain. Here, we tested the influences of ecological specialization and functional traits on local abundance and regional frequency, to determine the contribution of niche-based processes.

Location: France and the UK.

Taxon: Vascular plants.

Methods: We used two arable weed surveys covering 1544 fields in Western Europe (France, UK), along with functional traits related to resource acquisition, flowering phenology and dispersal. We quantified specialization both to arable habitat and to individual crop types, and performed phylogenetic path analyses to test competing models accounting for direct and indirect relationships between traits, specialization, abundance and frequency. We performed the analyses for all species in each country, as well as for a subset of the most abundant species.

Results: Local abundance of weeds increased with their regional frequency, but the relationship became negative or null when considering only the most abundant weeds. Specialization to arable habitat and to individual crop type either had a similar or opposite effect on regional frequency and local abundance explaining these positive and negative relationships, respectively. Regional frequency was not directly explained by any trait but indirectly by resource requirement traits conferring specialization to the arable habitat. Conversely, high local abundance was directly related to low seed mass, high SLA, early and short flowering.

Main Conclusions: Direct/indirect effects of functional traits on local abundance/regional frequency, respectively, supports a significant role of niche-based processes in AFR. Neutral spillover dynamics could further explain a direct linkage of abundance and frequency. Similar causal paths and consistent influences of traits on specialization and abundance in the two studied regions suggest genericity of these findings.

We analyzed two nationwide surveys of arable weed communities conducted in France: Biovigilance Flore, BVG hereafter (Fried et al., 2008), and in the UK: Farm scale evaluation, FSE hereafter (Heard et al., 2003). The regional frequency of a species was defined as the proportion of sampled fields where the species was present. Local mean abundance was calculated as the average density in sites where the species was present.

We used a fidelity measure to represent ecological specialization to the arable habitat, based on the relative frequency of a species in a reference habitat compared to other habitats (Chytrý et al., 2002). In France, we assessed weed frequency in 5,382 vegetation samples in arable fields (the complete BVG dataset), and in 96,438 samples covering diverse grassland habitats in the DIVGRASS database (Violle et al., 2015). In the UK, we assessed weed frequency in 268 samples in arable fields (the complete FSE dataset), and in 15,756 samples from other habitats excluding arable and horticulture habitats (Carey et al., 2008). We quantified the phi coefficient of association:

phi = ,

with N the total number of samples used (101,820 for France, 16,024 for UK), N_p, the number of samples in arable habitats (5,382 for France and 268 for UK respectively), n, the number of occurrences of the species in the dataset, and n_p the number of occurrences of the species in arable habitats. The phylogeny was derived from a dated and comprehensive megaphylogeny of spermaphytes, by using the function S.PhyloMaker provided by Qian and Jin (2016) with scenario "S3."

The dataset provides three files:

- a csv file with the regional frequency, local abundance and specialization to arable fields of 265 plant species

- a csv file with the trait values for 150 plant species used in the phylogenic path analysis (122 species for the BVG French dataset and 102 sepcies for the FSE UK dataset)

- the phylogeny of the same 150 species