Mauricio Escapa

CRAB HERBIVORY REGULATES PLANT FACILITATIVE AND COMPETITIVE PROCESSES IN ARGENTINEAN MARSHES

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.

Effects of the intertidal burrowing crab Chasmagnathus granulatus on infaunal zonation patterns, tidal behavior, and risk of mortality

Rhythmic movements in response to tidal cycles are characteristic of infaunal inhabitant of intertidal soft-bottoms, allowing them to remain in the area with best living conditions. The effect of bioturbators as modifier of local environmental conditions and thus of gradients in intertidal habitats, has not been investigated yet. The Atlantic estuarine intertidal areas are dominated by the burrowing crabChasmagnathus granulatus that generates strong environmental heterogeneity by affecting the physical-chemical characteristics of the sediment. The comparison between intertidal areas with and without crab shows that sediments in the crab beds remain more humid, softer, and homogeneous across the intertidal and along the tidal cycle than areas without crabs. The densities of infauna were higher at high intertidal zones in crab beds than in similar areas without crabs. Infaunal organisms performed vertical movements into the sediment following the tidal cycle that were always of higher magnitude in habitats without crabs. Infaunal species tend to spend most of the time buried into the sediment in the crab bed. Migratory shorebirds use the Atlantic estuarine environments as stopover or wintering sites. They feed (mainly on polychaetes) in the low intertidal zones of both habitats (with and without crabs), but they also feed in the upper intertidal of the crab bed; polychaete per capita mortality rate is higher in the upper part of the crab bed. Environmental heterogeneity produced by crab disturbance has an effect on the infaunal behavior, risk of mortality, and the zonation pattern. This is another example of the ecosystem engineering ability of a burrowing intertidal species.

Does the presence of the SW Atlantic burrowing crab Chasmagnathus granulatus Dana affect predator–prey interactions between shorebirds and polychaetes?

The burrowing crab Chasmagnathus granulatus is an important bioturbator that generates dense burrow assemblages (crab beds) characteristic of intertidal habitats of SW Atlantic estuaries. Crab bioturbation affects the topography and hydrodynamics of the sediment, increasing sediment water and organic matter content, decreasing sediment hardness and changing the grain size frequency distribution. In this study, we found that burrowing crabs can decrease the impact of predation by shorebirds on polychaetes. The polychaete Laeonereis acuta Treadwell has U-shaped burrows outside crab beds, which are associated with surface deposit-feeding while their burrows are mainly I-shaped inside which is associated with subsurface deposit feeding behavior. This pattern is likely the result of larger vertical sediment mixing inside crab beds due to crab burrowing. As a result of their feeding strategy, polychaetes appear on the surface more often outside crab beds, which increases their availability for shorebirds. In addition, shorebird species differentially use crab beds. The White-rumped Sandpiper, Calidris fuscicollis Vieillot, preferentially forage outside crab beds, meanwhile the Two-banded Plover Charadrius falklandicus Latham forage more frequently inside crab beds. However, experiments excluding shorebirds inside and outside crab beds showed negative effects of shorebirds only outside crab beds. Thus, our results show that the SW Atlantic burrowing crab C. granulatus affects the strength of the predator–prey interaction between shorebirds and polychaetes.

Can a Single Species Challenge Paradigms of Salt Marsh Functioning

Over the history of ecology, well-established generalizations were refined or even changed after the appearance or consideration of new evidence. Here, we review results obtained in Southwestern Atlantic salt marshes (between southern Brazil −32° 1′ S- and the Argentinean Patagonia −53° 48′ S-). Most of these salt marshes are inhabited by the intertidal burrowing crab Neohelice granulata, a species that influences many ecological processes through bioturbation and herbivory. The experimental evaluation of these processes shows that in some cases, the results were not consistent with generalizations and models of salt marsh ecological functioning. However, this does not imply that the generalizations grounded mainly on the results from North American sites are not valid. In turn, we suggest that these apparently conflicting results emerged because two major processes, herbivory and bioturbation, have been overlooked until recently. Thus, their relative contribution has not been included in the models of salt marsh functioning. In conclusion, we believe that there is a need for performing parallel and simultaneous experiments comparing distant sites with varying environmental (i.e., abiotic and biotic) conditions to be able to uncover common processes and causes of contingencies. Particularly, Southwestern Atlantic salt marshes could lead the way in providing information to better incorporate herbivory and bioturbation into current models or paradigms about how salt marshes work.