Realistic species loss has little effect on local resource depletion and competitive pressure in a temperate wet meadow

Abstract Biodiversity loss has been repeatedly demonstrated to decrease community resistance to colonization, increasing their sensitivity to invasive species. Based on these results, Elton's hypothesis proposed that biodiversity loss reduces the competitive ability of plant communities by emptying ecological niches and, thus, increasing the availability of unused resources. Yet, direct evidence for the effect of diversity loss on resource use and the consequences for the community competitive ability remains scarce, especially under natural conditions. We created a species richness gradient (1–27 species on average) simulating a realistic species loss through the long‐term (6 years) removal of rare and subordinate species in a species‐rich oligotrophic wet meadow. To assess the effects of species richness on the competitive ability of communities, we transplanted phytometers of two species (the grass Holcus lanatus and the forb Plantago lanceolata ) into plots varying in species richness and into gaps where plant competition was eliminated. We then compared phytometers' performance (relative growth rate) and functional traits responses (vegetative height, leaf area, leaf nitrogen and phosphorus content, specific leaf area and leaf dry matter content). We also tested the effect of targeted species richness on local light interception, soil nutrient content, soil water content, and soil surface temperature and whether they could reliably explain phytometers' performance and trait responses. Both phytometer species showed a much lower relative growth rate in the vegetation than in gaps demonstrating strong responses to plant competition. However, the effects of species richness were surprisingly weak as all plant communities ranging from monocultures of a dominant species to the richest communities (27 species on average) exerted a similar competitive pressure. Surprisingly, almost none of our measures of local resource depletion explained trait or growth rate responses. Synthesis. Phytometers effectively quantified competitive pressure exerted by natural plant communities. However, the realistic loss of rare and subordinate species had only limited effects on community competitive ability, and these effects were not explained by changes in local resource depletion. This highlights the complexity of diversity–competition relationships and cautions against inferring invasion resistance from species richness alone.