Alpine cushion plants have species–specific effects on microhabitat and community structure in the tropical Andes

QUESTION: Species–specific interactions can connect particular species to others, which has important implications for species interdependence and co‐existence. However, species‐specific effects of ecosystem engineers remain little explored, particularly in the alpine tropics. We investigated the effects of two cushion plants with subtle differences in morphological traits in a tropical Andean ecosystem. We asked whether these foundation species differed in their effects on local abiotic factors, species richness and the structure of plant assemblages growing within them. LOCATION: La Culata National Park, northern tropical Andes, Venezuela. METHODS: We determined shallow soil temperature, soil organic matter and water content within two co‐existing cushion species, the larger and more compact Azorella julianii and the smaller and denser Arenaria musciformis, and in adjacent open areas. We also recorded the total number of plant species growing within both cushion species, and in paired open areas at three sites. We then compared local species richness, total plant abundance and the abundance of each species within cushions and outside using the relative interaction index (RII), and community dissimilarity patterns using NMDS ordination. We also compared species accumulation curves across sites between the cushions and open areas. RESULTS: Both cushions had similar positive effects on soil organic matter, but soil water content was increased only with Azorella. Both cushions had a positive effect on local species richness and total plant abundance. In addition, both species significantly modified species abundance patterns, showing a positive association with several species, many of them grasses. However, facilitative effects on other species were generally stronger for Azorella than Arenaria. Moreover, community structure of plants growing within the two cushion plants differed. The exotic invader Rumex acetosella increased in abundance within Azorella but not Arenaria. CONCLUSIONS: We found evidence for species‐specific effects of ecosystem engineers in the alpine tropics; two cushion species with subtly contrasting traits had different effects on microhabitat conditions and plant assemblages. This could in turn increase abiotic and biotic heterogeneity and promote species co‐existence and emphasizes interdependence among species in alpine plant communities.