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

Biodiversity loss has been repeatedly demonstrated to decrease community
resistance to colonisation, 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 to 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, 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.