Canopy height and competition explain species segregation in wet heathlands

Questions What is the general pattern of species co‐occurrence in managed heathlands? Is the pattern consistent among functional groups? Is it ruled by species competition, or by contrasting environments at a fine scale? Does grazing pressure and herbivore species condition species interactions? Location Erica mackayana wet heaths, Galicia, NW Iberian Peninsula. Methods A null model approach was used to compare species co‐occurrence with generated random matrices from 54 10‐m transects. The C‐score was obtained from the multispecies presence/absence matrix for each transect of shrubs and graminoids recorded at 25‐cm intervals. Differences in canopy height were recorded to assess the importance of the environment compared to inter‐specific competition. Results were linked to different levels of grazing pressure and herbivore species. Results Species segregation was the main pattern for all species, but mainly among graminoid species compared to shrubs. Graminoids showed an even proportion of segregated pairs explained by different canopy heights and competition. These differences were mainly species environmental requirements of canopy height. Levels of grazing pressure enhanced species segregation in graminoids but had no effect on shrubs or the total species set. Conclusions Competition and canopy height affect the E. mackayana heathland composition, but differently for functional groups. A heterogeneous vegetation profile with shrub mats and open gaps created by light grazing promotes species co‐existence within mats and competition in gaps. I suggest this is an optimum structure for the habitat to be targeted through management.