Ecological interactions among benthic communities are crucial for shaping marine ecosystems. Understanding these interactions is essential for predicting how ecosystems will respond to environmental changes, invasive species, and conservation management. However, determining the prevalence of species interactions at the community scale is challenging. To overcome this challenge, we employ tools from social network analysis, specifically exponential random graph modeling (ERGM). Our approach explores the relationships among animal and plant organisms within their neighborhoods. Inspired by companion planting in agriculture, we use spatiotemporal co-occurrence as a measure of mixed species interaction. In other words, the variety of community interactions based on co-occurrence defines what we call "co-occurrence social diversity." Our objective is to use ERGM to quantify the proportion of interactions at both the simple paired level and the more complex triangle level, enabling us to measure and compare co-occurrence social diversity. Applying our approach to the Spanish coastal zone across 8 sites, 5 depths, and sunlit/shaded aspects, we discover that 80% of sessile communities, consisting of over a hundred species, exhibit co-occurrence social diversity, with 5% of species consistently forming associations with other species. These organism-level interactions likely have a significant impact on the overall character of the site.

a)      Study Sites

The field data selection coincided with a program to quantify changes in the sessile community along a depth gradient in multiple locations of two distant geographic areas. The Spanish islands of Illes Medes and L'illa de Benidorm are located in the Mediterranean Sea and are nature reserves. L'illa de Benidorm is located about 3.5km from the coast, near Benidorm, and has been part of the Sierra Helada Natural Park since 2005. Illes Medes is an archipelago of seven islands about 0.85 km from L'Estartit. It has been protected since 1983 and became a National Protected Natural Park in 2010. These two locations were selected due to their proximity to the coast, ease of sampling, rocky bottoms, and because they are island enclaves close to the coast.

b)     Sampling design

During May and June 2010, 6 dive sites in Illes Medes and 2 dive sites in L'illa de Benidorm  were selected. For each site, except Garbí and Gros due to a lack of suitable substrate, a photophilous aspect was paired with a sciaphilous aspect. To capture the impact of depth on the community assemblages, each site was surveyed  at different depths along the rocky reef wall at 2, 4, 7, 14, and 23 m deep. The sciaphilous aspects could not be located for the 2 sites in L'illa de Benidorm and only one set of transects at various depth profiles were photographed.  Based on the continuous line transect method, photo quadrats using a camera (Sony Cyber-shot Vario-Tessar 10.1 Mpx with a constant 45cm distance measurement frame) covered a 5m long transect for each depth profile. Approximately 25 overlapping images were captured per transect.  Later back in the laboratory, we identified every species (where possible) directly next to the photographed transect tape for each centimeter division so that 500 data points were recorded per transect. The final dataset contains 17 sites with five depth transects, resulting in 85 sets of benthic data.