Crab Herbivory Regulates Plant Facilitative and Competitive Processes in Argentinean Marshes

Abstract

Interactions among plants have been hypothesized to be context dependent, shifting between facilitative and competitive in response to variation in physical and biological stresses. This hypothesis has been supported by studies of the importance of positive and negative interactions along abiotic stress gradients (e.g., salinity, desiccation), but few studies have tested how variation in biotic stresses can mediate the nature and strength of plant interactions. We examined the hypothesis that herbivory regulates the strength of competitive and facilitative interactions during succession in Argentinean marshes dominated by Spartina densiflora and Sarcocornia perennis. Spartina densiflora is preferred by the dominant herbivore in the system, the crab Chasmagnathus granulatus. We experimentally manipulated crab herbivory, plant structure, and shade, and we found that, when herbivory was low in the spring and summer, competitive interactions between plants were dominant, but in the fall, when herbivory was highest, facilitative interactions dominated, and Spartina densiflora survival was completely dependent upon association with Sarcocornia perennis. Moreover, experimental removal of Sarcocornia perennis across recently disturbed tidal flats revealed that, while Sarcocornia perennis positively affected small Spartina densiflora patches by decreasing herbivory, as patch size increases and they can withstand the impact of herbivory, competitive interactions predominated and Spartina densiflora ultimately outcompeted Sarcocornia perennis. These results show that herbivory can mediate the balance between facilitative and competitive processes in vascular plant communities and that the strength of consumer regulation of interactions can vary seasonally and with patch size.

Facilitation shifts paradigms and can amplify coastal restoration efforts

“…Clumping may reduce porewater salinity at other sites, as shading provided by neighbors can reduce evaporation and salt accumulation in the soil (30,49), and/or increase survival in areas characterized by intense herbivory due to associational defenses (50). Hence, the mechanism underlying the positive effects of clumping during restoration are likely to be multifaceted and vary from site to site.…”

Crab bioturbation and herbivory reduce pre- and post-germination success of Sarcocornia perennis in bare patches of SW Atlantic salt marshes

“…In seagrass beds and marshes, bioturbation can prevent plant colonization of bare areas, acting mostly during the seedling phase (Dumbauld & Wyllie-Echeverría 2003, Paramor & Hughes 2005. Similarly, in our study system, Sarco-cornia perennis colonizes the mudflat and facilitates the establishment (through seedlings) of the competitively dominant Spartina densiflora by reducing the impact of crab herbivory (Alberti et al 2008). This facilitative succession is essential for the expansion of the Spartina densiflora marsh onto the mudflat, as the asexual expansion is extremely slow due to a combination of both physical stress and crab herbivory (Alberti et al in press).…”

“…These marshes are irregularly flooded (10 to 15 times per month), and they are mainly vegetated by Spartina densiflora, Spartina alterniflora, and Sarcocornia perennis (Isacch et al 2006), and densely populated by the burrowing crab Neohelice granulata (Bortolus & Iribarne 1999, Alberti et al 2007a. At these sites, the first to colonize the mudflat is Sarcocornia Perennis, which grows in circular patches and facilitates the establishment of seedlings of the competitively dominant Spartina densiflora by reducing the impact of crab herbivory (Alberti et al 2008). Each of the areas occupied by these 2 species extends for > 600 m parallel to the shore, and is located on the edge between the Spartina densiflora marsh and the tidal flat.…”

“…Isacch et al 2006). In some of these marshes, the presence of Sarcocornia perennis is essential to the successful establishment of Spartina densiflora (Alberti et al 2008). The burrowing crab Neohelice (Chasmagnathus) granulata is the dominant herbivore in the lowest reaches of these marshes (Bortolus & Iribarne 1999, Alberti et al 2007a).…”

The crab Neohelice (=Chasmagnathus) granulata: an emergent animal model from emergent countries

“…Finally, the role of this crab as an ''ecological ecosystem engineer'' in saltmarshes and estuaries was intensely investigated, discovering that the burrowing activity and subsequent sediment turnover greatly impacts sediment chemistry; enhances both soil drainage and aeration; traps organo-chlorine pesticides; increases S. densiflora productivity; affects the benthic community (e.g., the activity of the crabs Cyrtograpsus angulatus and Uca urugayensis, the razor clam Tagelus plebeius and the polychaete Laeonereis acuta among other invertebrate species), the distribution and abundance of juvenile fishes, the habitat use and migration of shorebirds and even the foraging of rodents (e.g., Alberti et al 2008;Botto et al 2008;Canepuccia et al 2008;Escapa et al 2008;Fanjul et al 2008;Daleo and Iribarne 2009). In fact, most of the publications of Dr. Oscar Iribarne laboratory (Mar del Plata, Argentina) included the words ''effects of …'' in their titles.…”