733 Ecology, 83(3), 2002, pp. 733–742  2002 by the Ecological Society of America POSITIVE PLANT–ANIMAL INTERACTIONS IN THE HIGH MARSH OF AN ARGENTINEAN COASTAL LAGOON ALEJANDRO BORTOLUS, 1 EVANGELINA SCHWINDT, AND OSCAR IRIBARNE Departamento de Biologı ´a, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, 7600 Mar del Plata, Argentina Abstract. Although plant–plant facilitations have been shown to be important struc- turing forces in salt marshes, less attention has been given to the potential role played by plant–animal facilitations in structuring these communities. We used a combination of sampling and field experiments to evaluate the effect of microenvironmental changes pro- duced by plant cover on the distribution of the burrowing crab Chasmagnathus granulata, a dominant macroinvertebrate of high marshes of the southwestern Atlantic coast. Four questions were explored. Is there a relationship between the spatial distribution of C. granulata and the spatial distribution of rooted macrophytes or distance from the marsh edge? How important is plant cover for the establishment and survival of crabs in the high marsh? Does plant cover affect critical physical variables for crab establishment? How important are environmental conditions for the survival of crabs in the high marsh? Surveys of the marsh surface showed that: (1) there was a strong relationship between the presence of plant cover and the spatial distribution of Chasmagnathus granulata in the high marsh, and (2) both mean crab size and burrow density decreased from the marsh edge to high-marsh levels. By shading the substrate, live plants and experimental plant mimics were found to be equally efficient at buffering high temperature, dehydration, and soil hardness in the high marsh. Experimental amelioration of these harsh physical con- ditions led to higher crab densities. Crab burrows also buffered harsh environmental con- ditions, diminishing surface air temperature from 46°C to 23°C. Finally, tethering ex- periments showed that stressful heating in the high marsh is lethal for crabs, and that plant cover is crucial for the establishment and long-term success of crabs in the high marsh. No predation was observed in tethering experiments. Our results suggest that plant cover is largely responsible for determining the spatial distribution of this dominant crab in the high marsh through facilitation. Thus, our work shows that plant–animal facilitations as well as plant–plant facilitations are important struc- turing forces in salt marsh communities. Key words: Argentina; Chasmagnathus granulata; cordgrass; crabs; microenvironment; plant– animal interactions; positive interactions; Salicornia ambigua; salt marsh; Spartina alterniflora; Spar- tina densiflora; stress amelioration. INTRODUCTION Positive interactions (i.e., mutualism, facilitation, in- cluding direct and indirect, trophic and nontrophic re- lationships) are common in most ecosystems. However, until recently they were seldom included in discussions about the organization of natural communities (Bert- ness and Callaway 1994, Callaway 1996, Bertness and Leonard 1997). This was at least partly the result of the strong impact that studies of competitive interac- tions and their importance in structuring communities have had on the scientific community (Kareiva and Bertness 1997). Nevertheless, positive interactions ap- pear to be more important in structuring communities than previously believed (Jones et al. 1997). Direct Manuscript received 24 July 2000; revised 12 February 2001; accepted 10 March 2001; final version received 17 May 2001. 1 Present address: Department of Ecology and Evolution- ary Biology, Brown University, Box G-W 101, 80 Waterman St., Providence, Rhode Island 02912 USA. E-mail: bortolus@brown.edu positive interactions occur when neighbors modify physical or biotic conditions (Bertness and Callaway 1994). For example, the cordgrass Spartina alterniflora has been shown to facilitate the establishment of an entire cobble beach plant community by reducing phys- ical disturbance by waves to tolerable levels (Bruno 2000). Similarly, salt marsh plants may ameliorate harsh and potentially limiting physical microenviron- mental conditions, positively influencing the establish- ment of other organisms and indirectly affecting com- munity structure (Bertness and Hacker 1994, Bertness 1999). Examples of positive interactions between plants and animals are less common in the literature, even though they are likely to be common (e.g., No- mann and Pennings 1998, Pennings and Bertness 2001). In salt marshes, burrowing crabs often inhabit both mud flats and vegetated areas (Chapman 1977, Day et al. 1989, Adam 1993). The environmental conditions in the lower part of the marsh are generally less stress- ful for these organisms, which need water to avoid