Detecting spatial heterogeneity in intertidal algal functional groups, grazers and their co-variation among shore levels and sites

Few studies have examined the nature of spatial structure in intertidal systems, especially that of macroalgae and grazers, and it is not clear how consistent spatial heterogeneity is among sites and levels on the shore. Here we studied the consistency of spatial heterogeneity in several algal functional groups and in grazer biomass between two sites and across the shore using a combination of semivariograms and fractal analysis of data collected from transects. Additionally, we tested for consistency in spatial co-variation between algae and grazers using cross-semivariograms. The study was conducted at two sites in South Africa and across a gradient of physiological stress at three levels on the shore (low, mid and high shore). We observed spatial heterogeneity in macroalgal functional groups and in the biomass of grazers at one of the sites and at different levels of environmental harshness (i.e. heights on the shore), while the other site showed consistently random patterns. In terms of alga–grazer co-variation, most transects showed significant co-variation, but with more differences in the sign and the number of lags between sites than among levels on the same shore, suggesting stronger differences in spatial patterns between sites. Also we observed differences in co-variation between macro and mesograzers on the midshore. Variability in spatial patterns among levels on the shore and between sites is not explained by topographic complexity, but may be a product of several biotic factors interacting at small scales (cm–m). Differences in spatial patterns among shore levels and between shores also appear to reflect the indirect effects of an upshore reduction in algal biomass (due to heat and desiccation stress) and human harvesting of grazers at one of the sites, respectively. The results show that spatial heterogeneity of particular organisms (algal groups and grazers) and co-variation between them can emerge at small spatial lags, and reduction in spatial heterogeneity in the intertidal can be induced by a cascading effect driven by disturbance.

► We describe spatial patterns of algae and grazers.
► We tested whether these patterns can include spatial heterogeneity.
► Additionally we examined alga-grazer spatial co-variation.
► Heterogeneity existed at one site and co-variation differed with level and site.
► This reflects shore level stresses and cascading effects of grazer exploitation.