Inferring local competition intensity from patch size distributions: a test using biological soil crusts

Dryland vegetation is inherently patchy. Th is patchiness goes on to impact ecology, hydrology, and biogeochemistry. Recently, researchers have proposed that dryland vegetation patch sizes follow a power law which is due to local plant facilitation. It is unknown what patch size distribution prevails when competition predominates over facilitation, or if such a pattern could be used to detect competition. We investigated this question in an alternative vegetation type, mosses and lichens of biological soil crusts, which exhibit a smaller scale patch-interpatch confi guration. Th is micro-vegetation is characterized by competition for space. We proposed that multiplicative eff ects of genetics, environment and competition should result in a log-normal patch size distribution. When testing the prevalence of log-normal versus power law patch size distributions, we found that the log-normal was the better distribution in 53% of cases and a reasonable fi t in 83%. In contrast, the power law was better in 39% of cases, and in 8% of instances both distributions fi t equally well. We further hypothesized that the log-normal distribution parameters would be predictably infl uenced by competition strength. Th ere was qualitative agreement between one of the distribution ’ s parameters ( μ ) and a novel intransitive (lacking a ‘ best ’ competitor) competition index, suggesting that as intransitivity increases, patch sizes decrease. Th e correlation of μ with other competition indicators based on spatial segregation of species (the C-score) depended on aridity. In less arid sites, μ was negatively correlated with the C-score (suggesting smaller patches under stronger competition), while positive correlations (suggesting larger patches under stronger competition) were observed at more arid sites. We propose that this is due to an increasing prevalence of competition transitivity as aridity increases. Th ese fi ndings broaden the emerging theory surrounding dryland patch size distributions and, with refi nement, may help us infer cryptic ecological processes from easily observed spatial patterns in the fi eld. Oikos 121: 1914–1922, 2012 doi: 10.1111/j.1600-0706.2012.20192.x No claim to original US government works. Oikos © 2012 Nordic Society Oikos Subject Editor: Wim van der Putten. Accepted 24 January 2012