Do ecological specialization and functional traits explain the abundance–frequency relationship? Arable weeds as a case study

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.