On the relationship between abiotic stress and co‐occurrence patterns: an assessment at the community level using soil lichen communities and multiple stress gradients

Abstract

The stress‐gradient hypothesis (SGH) predicts a shift from predominant competition to facilitation as abiotic stress increases. Most empirical tests of the SGH have evaluated the interactions between a single or a few pairs of species, have not considered the effects of multiple stress factors, and have not explored these interactions at nested spatial scales. We sampled 63 0.25‐m2 plots, each subdivided into 100 5×5 cm and 25 10×10 cm sampling squares, in a semi‐arid Mediterranean environment to evaluate how co‐occurrence patterns among biological soil crusts (BSC)‐forming lichens changed along natural stress gradients driven by water and nutrient (N, P, K) availability. According to the SGH, we tested the hypothesis that the fine‐scale spatial arrangement of BSC‐forming lichens should shift from prevailing interspecific segregation to aggregation as abiotic stress increases. Co‐occurrence patterns ranged from significant species segregation to aggregation at the two spatial scales studied. When using the 5×5 cm grid, more plots showed significant species segregation than aggregation. At this sampling scale, co‐occurrence increased as water and nutrient availability decreased and increased, respectively. Small increases in soil pH promoted species co‐occurrence. Interspecific segregation was promoted as the cover of highly competitive species, such as Diploschistes diacapsis, increased. No significant relationships between co‐occurrence and the surrogates of abiotic stress were observed when data was arranged in a 10×10 cm grid. Our co‐occurrence analyses partially supported predictions from the SGH, albeit the results obtained were dependent on the type of abiotic stress and the spatial scale considered. They show the difficulties of predicting how co‐occurrence patterns change along complex stress gradients, and highlight the need of incorporating the effects of abiotic stress promoted by different resources, such as water and nutrients, into the conceptual framework of the SGH.

Do biotic interactions modulate ecosystem functioning along stress gradients? Insights from semi-arid plant and biological soil crust communities

“…Callaway 2007). The results presented from Studies 1 and 4 are new re-analyses of previously published data (Maestre et al , 2009bMaestre & Escudero 2009). …”

Microbial Nursery Production of High-Quality Biological Soil Crust Biomass for Restoration of Degraded Dryland Soils

“…If we consider a matrix with taxa in rows, sites in columns, and abundances as entries, we maintained the richness of each row (i.e., row sums are fixed) and we set abundances among columns to be equiprobable (i.e., all sites had the same average number of entries) to build the null model (78). This fixed rows-equiprobable columns null distribution model retains taxa frequencies, i.e., rare taxa remain rare and common taxa remain common (79). The use of this null distribution model is recommended because it has a low probability of type I errors (78).…”

Lichen species dominance and the resulting photosynthetic behavior of Sonoran Desert soil crust types (Baja California, Mexico)

“…Few studies have dealt with small scale pattern formation of crusts. However, one study revealed a scale-dependent positive correlation between abiotic stress such as water and nutrient availability and co-occurrence of BSC-lichens (Maestre et al 2009). Büdel et al (2009) distinguished between 7 major crust types in their study on South African BSCs.…”