A diversity world record in a grassland at Oaxaca, Mexico

Background: Plant richness world-records have been reported to occur either in tropical rainforests, or, at smaller spatial scales, in chronically disturbed grasslands. The tight relationship between scale and record richness suggests that there is an unyielding limit to diversity. Hypotheses: If such limit exists, current records should hardly be beaten. Chronic disturbance in grasslands allows richness to approach the limit. Studied species: All vascular plants at the study site. Study site and years of study: A natural, semiarid grassland at Concepcion Buenavista, Oaxaca, Mexico, in 2012 and 2014. Methods: At 21 sites we recorded species richness in randomly sampled 0.1 x 0.1 m quadrats, and measured chronic disturbance caused by different agents. We estimated the maximum richness at each site through maximum likelihood. At one site we searched for high richness quadrats. Results: The study site shares the current world record of plant richness at the 0.1 x 0.1 m scale with 25 species. The estimated maximum diversity at each site decreased with land degradation, but had a maximum at intermediate-high levels of livestock activity. Conclusions: Our results support the idea of a hard limit to species richness, and thus theories of plant coexistence that envisage such limit. Our study site shares attributes with other record-holding grasslands, such as severe resource limitations and chronic disturbance by ungulates, suggesting that these conditions promote high species richness. We hope that, by reporting this diversity record, we also help to change the notion that Mexican grasslands are secondary communities having a negligible biological value.