Understanding the factors influencing species composition at the metacommunity level is key for assessing landscape biodiversity and conservation. This study investigates terrestrial arthropod assemblages in tidal marshes along the southwest Atlantic coast (Argentina), comparing them to assemblages in adjacent terrestrial environments. The research explores how environmental and ecological factors shape these communities. Arthropods were sampled from 4 tidal marshes and neighboring terrestrial environments within the same marine biogeographic province but spanning 3 terrestrial biogeographic provinces. The study measured abundance, richness, alpha and beta diversity (as turnover and nestedness). Akaike's information criterion (AIC)-based multiple regression analysis showed that abundance, richness, and alpha diversity of terrestrial arthropods in marshes increased with marsh area, while richness increased with species richness in terrestrial environments, highlighting that marshes provide habitats for many terrestrial species. Abundance declined with temperature range, and richness and alpha diversity increased with salinity. Beta diversity and turnover decreased with rainfall, while beta diversity increased with temperature range. Nestedness showed the opposite pattern, suggesting that physical factors influence assemblage structure. Ecological traits of species also influenced marsh-terrestrial community composition: higher beta diversity and turnover were associated with lower proportions of generalists, while turnover declined with more good dispersers in terrestrial environments. This study highlights that although the marsh-terrestrial boundary is not impermeable, it is selective. Higher environmental stress correlates with increased beta diversity, turnover, and reduced nestedness. Spatial structure and species traits suggest deterministic processes dominate, emphasizing the importance of landscape connectivity and the need to integrate metacommunity patterns into coastal conservation strategies.

The role of facilitation in shaping natural communities has primarily been studied in the context of plant assemblages, while its relevance for mobile animals remains less understood. Our study investigates whether reciprocal interspecific facilitation may exist between fire ants (Solenopsis richteri) and cavies (Cavia aperea), two mobile animals, in the SW Atlantic coast brackish marshes. Field samples showed a spatial association between ant mounds and cavies, and that ants prefer to use cavy runways for movement within the marsh. Through experiments involving transplanting the dominant plant, cordgrass (Spartina densiflora), and manipulating cavy presence in areas with and without ant mounds, we observed that cavies forage extensively (and defecate more) near ant mounds. The ants actively remove cavy droppings in their mound vicinity. These ant activities and interactions with cavy droppings led to reduced moisture and organic content while increasing nitrate and phosphate levels in marsh sediment. Consequently, this enhanced plant growth, indirectly facilitating the cavies, which preferred consuming vegetation near ant mounds. These cascading indirect effects persisted over time; even four months after cavies left the marshes, transplanted plants near ant mounds remained larger and exhibited more leaf senescence when exposed to cavy herbivory. Therefore, the networks of positive interactions appear to generate simultaneous selection among species (populations), promoting coexistence within the community. Although complex, these reciprocal facilitative effects among mobile animals may be more common than currently believed and should be further studied to gain a better understanding of the underlying mechanisms driving species coexistence in natural communities.

Understanding the determinants of species distribution and richness is key to explaining global ecological patterns. We examined the current knowledge about terrestrial mammals in tidal marshes and evaluated whether species richness increased with the marsh surface area and/or with their proximity to the equator and whether species distribution ranges decreased with latitude. Global. We reviewed the existing literature on terrestrial mammals in tidal marshes. We examined their ecological characteristics (e.g. habitat specialists, native or alien), predicted their variation in species richness and range size along latitude, and explored factors, such as surface area, underlying the global patterns found. We found 962 records, describing 125 mammalian species using tidal marshes worldwide, also including several alien species. Most species (95%) were not marsh specialized, and some (18%) were of conservation concern. There were information gaps in South America, Africa, Australia and Asia, and a lack of information about mammalian ecological roles worldwide. We found that species richness increased with surface area, and showed a bimodal pattern peaked between 40° and 50° latitude in each hemisphere. We found no relationship between latitude and species range size. Our worldwide findings revealed a broader range of tidal marshes inhabited by terrestrial mammals, and higher values of species richness than previously reported. The bimodal pattern of species richness was consistent with the species–area hypothesis, but it also suggested that further studies of species distribution in relation to historical and environmental factors will yield significant insights about variables driving richness in tidal marshes. Despite terrestrial mammal ubiquitous distribution in these ecosystems, there are considerable geographic gaps as regards knowledge about their functional importance and the impact of alien species on tidal marsh functioning. Consequently, extending our research efforts is key to planning the conservation of these coastal ecosystems.

Individual traits such as age-class can have profound effects on space utilisation by individual birds, with juvenile birds more often exhibiting dispersive movements. We studied the origin of juvenile Olrog's Gulls Larus atlanticus, a threatened species, using stable isotopes ratios of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S). We analysed feathers from individuals captured in the wintering season at Mar Chiquita Coastal Lagoon, Argentina, and the potential trophic sources of their breeding sites at Bahía Blanca and San Blas. Mean δ13C was similar for trophic sources and mean δ15N was higher for Bahía Blanca. The δ34S showed a greater variation among prey species than among sites. The mean contributions of the main colonies resulted in assigning 37.5% individuals as from Bahía Blanca, 16.7% from San Blas, and 45.8% undetermined. Results indicate that Mar Chiquita coastal lagoon attracts juvenile Olrog's Gulls dispersing from the two main breeding grounds of the species. This work is one of the first approaches to study the origin of dispersive juvenile larids through the application of stable isotope analysis. This isotopic approach allows obtaining trophic and movement information when it is not possible to use other tools.

AimUnderstanding the determinants of species distribution and richness is key to explaining global ecological patterns. We examined the current knowledge about terrestrial mammals in tidal marshes and evaluated whether species richness increased with the marsh surface area and/or with their proximity to the equator and whether species distribution ranges decreased with latitude. LocationGlobal. MethodsWe reviewed the existing literature on terrestrial mammals in tidal marshes. We examined their ecological characteristics (e.g. habitat specialists, native or alien), predicted their variation in species richness and range size along latitude, and explored factors, such as surface area, underlying the global patterns found. ResultsWe found 962 records, describing 125 mammalian species using tidal marshes worldwide, also including several alien species. Most species (95%) were not marsh specialized, and some (18%) were of conservation concern. There were information gaps in South America, Africa, Australia and Asia, and a lack of information about mammalian ecological roles worldwide. We found that species richness increased with surface area, and showed a bimodal pattern peaked between 40 degrees and 50 degrees latitude in each hemisphere. We found no relationship between latitude and species range size. Main conclusionsOur worldwide findings revealed a broader range of tidal marshes inhabited by terrestrial mammals, and higher values of species richness than previously reported. The bimodal pattern of species richness was consistent with the species-area hypothesis, but it also suggested that further studies of species distribution in relation to historical and environmental factors will yield significant insights about variables driving richness in tidal marshes. Despite terrestrial mammal ubiquitous distribution in these ecosystems, there are considerable geographic gaps as regards knowledge about their functional importance and the impact of alien species on tidal marsh functioning. Consequently, extending our research efforts is key to planning the conservation of these coastal ecosystems.

The effects of mound building ants on soil and vegetation have been described worldwide; however, few studies have explored their effects in salt marsh communities. Here, we studied the effects of the black fire ant Solenopsis richteri on the cordgrass Spartina densiflora in a southwestern Atlantic salt marsh. We found that S. richteri nests are more abundant in the high marsh than in the low marsh and in the terrestrial environment. Sediment characteristics were examined in the above-ground and below-ground portions of nests, and in the surrounding soil apart from them. Nests' sediment had lower organic matter content, lower bulk density and higher pH values than the non-nest soil. These differences were in general more marked in the above-ground portion of nests. Grain size distribution was biased towards coarser grain particles in nests, with the smallest particles being more abundant in the non-nest soil. Ammonium content was higher in the nests than in the non-nest soil, despite nitrate and dissolved inorganic nitrogen were not different. Phosphate content was higher in the below-ground portion than in the above-ground portion of nests. Samplings and transplant experiments showed that plants growing in contact with mounds grew longer, had wider stems and higher root biomass than plants apart from mounds. These results indicate that by affecting sediment characteristics, ants indirectly and positively affect plants condition, with potential cascading up effects on community structure and dynamics. Because mounds are relocated every ~3.5 months, bioturbation impacts may extend over time through the whole high marsh, highlighting the role that black fire ants may have in the ecology of southwestern Atlantic marshes.

A new crambid moth, Haimbachia spartina Solis and Canepuccia, new species is described from Argentina and images of the adults and their genitalia are provided. The larvae were discovered feeding on species of saltmarsh cordgrass or Spartina Schreb. (Spartinaceae). This is the first description and illustration of a Haimbachia Dyar larva. Twelve species have been described in the Western Hemisphere, but only Haimbachia maroniella Dyar and Heinrich, has been previously described from South America. The new species is compared to H. maroniella and images are provided of its type specimen, labels, and male genitalia.

Studies on bidirectional benefactor-beneficiary interactions between organisms have generally neglected the importance of this feedback in trophic levels other than plants. The burrowing crab Neohelice granulata aids the development of larvae of the stem-boring moth Haimbachia sp. nov. within the stems of Spartina alterniflora. In our research, we evaluated whether the stem-boring moth subsequently influences crab feeding on these marsh plants. Surveys and experiments in a tidal marsh of the SW Atlantic coast (36 degrees 22' S) showed that at the beginning of the stem-boring moth attack there was no difference in crab herbivory between plants with or without larvae of the stem-boring moth. However, after 3.5 mo, crabs foraged more on plants without larvae than on those with larvae. Plant tissue analyses showed a decrease in leaf tissue carbon concentrations in plants with larvae. This change in the nutritional quality of leaves, caused by construction of the stem-boring moth galleries, could explain the segregation in plant use between both herbivores. Unlike an allelochemical response, the non-specificity of the induced nutritional change could impair a wide variety of herbivores regardless of their feeding modes or taxonomic proximity. These effects could propagate bottom-up through the food-web, leading to more diffuse interspecific effects. Thus, here we show how the benefactor-beneficiary feedback between herbivores can be important for the maintenance of species coexistence and the functioning of communities.

Large amounts of tidally accumulated detritus (i.e., wrack) are an important source of disturbance affecting different abiotic and biotic characteristics in salt marshes, which could in turn affect the macrofauna assemblage. The purpose of this study was to evaluate the importance of wrack disturbance in Southwest Atlantic (SWA) salt marshes and its effects on the surface-active arthropod assemblage under different environmental contexts. By sampling the most important SWA salt marshes (from 36° 19′ S to 41° 01′ S), we found that wrack is a widespread disturbance in this region, present in all the salt marshes and periods sampled. However, the biomass and type of wrack ( Spartina alterniflora vs. S. densiflora ) vary according to the species that dominates each salt marsh. At two of these sites (Bahia Blanca (BB), 38° 59′ S and San Clemente (SC), 36° 19′ S), chosen because they represent the two salt marsh types in the SWA region (dominated by Spartina alterniflora or S. densiflora ), we performed a field experiment by manipulating the presence and absence of wrack and conducting field samplings of sediment organic matter content and water content. We found that wrack affects surface arthropod assemblage but that this effect was not consistent for the different salt marshes: in BB, it changed the surface-active arthropod assemblage (shifted towards more detritivorous taxa) and increased the number of total individuals but had no effect on the number of species or diversity. At SC, wrack had no effect on any of the parameters evaluated. We suggest that the type of wrack in each salt marsh modulates the amount of organic matter content in the sediment: BB had wrack of better nutritional quality (dominated by S. alterniflora ) and in turn had greater organic matter content in the sediment of wrack zones than in no-wrack zones, while in SC (dominated by S. densiflora ), there is no differences between the two zones. We also suggest that depending on the original surface-active arthropod assemblage, those modifications will either favor (BB) or not favor (SC) wrack colonization by the surface-active arthropod assemblage. Moreover, considering that SWA wrack has different compositions and that the biomass differs among the different salt marshes, we expect wrack effects in the SWA, and probably in other regions, to be site-specific.

Despite long-standing interest in the hypothesis that the strength of plant-herbivore interactions decreases at higher latitude, supporting evidence is scarce and the data are conflicting. We conducted a field survey and 2 experiments to examine this hypothesis, focusing on herbivory by a stem-borer moth (Haimbachia sp. nov.) on the dominant SW Atlantic marsh grasses (Spartina aiterniflora and S. densiflora). Field surveys indicated that herbivore abundances and damage, although tending to decrease simultaneously, are unrelated to latitude. Herbivore abundances were related to latitude-dependent variables such as day length and temperature, and also to latitude-independent variables such as precipitation, salinity, and tide amplitude. Abundances were indirectly related to the effects of these variables and sediment characteristics on plant traits like density, height, and tissue composition. After 33 mo, herbivore abundances and damage to high-latitude plants transplanted to low latitudes were 50 times greater than in plants transplanted from low- to high-latitude sites. In a common garden experiment (38 degrees 56' 5) without herbivore pressure, differences persisted in plant traits from high and low latitudes, suggesting a lack of herbivore-induced effects on these plant traits. The persisting conspecific differences in plant traits translocated along latitude suggest that these variations are under genetic control. Thus, our results provide evidence that although plant-herbivore interactions are more important at lower latitudes, many additional and contingent variables unrelated to latitude can interrupt this geographic pattern.

Understanding how wetland organisms interact with neighbor habitats along environmental gradients is important to recognize wetland integrity and its connectivity at landscape-level. We evaluated whether assemblage characteristics (e.g. α-diversity) of marsh-ants are associated with geographic changes in environmental conditions, and if these factors are associated with marsh-upland dissimilarity in ant species composition (β-diversity). Ant-samples were collected both in the marsh and in the neighboring upland habitat at 5-sites along the South-west Atlantic (SWA) coastline (36°S to 40°S), encompassing two-distinct biogeographic regions. Generalized Linear Models showed that at the marsh scale, ant occurrence increased with maximum temperature and Spartina densiflora cover, but decreased with total-plant cover. Ant richness increased with salinity, S. densiflora cover and marsh area; and ant α-diversity increased with S. densiflora cover and decreased with total marsh plant cover and plant height. Composition of ant assemblages differed between the marsh and the upland habitat depending on the site, and β-diversity decreased with precipitation, salinity, tidal amplitude and α-diversity of the herbaceous stratum. Then, the abundance and α-diversity of ants varied along SWA marshes in relation to changes in local environmental factors and the regional landscape. Moreover, changes in species characteristics across coastal-landscape seem to interact with environmental gradients, resulting in reduced β-diversity values with increasing environmental harshness. Thus, our results suggest that the link of geographic changes in the physical environment with the changes in species traits drives the variation in marsh-upland dissimilarity across the space.

Changes in rainfall patterns caused by anthropogenic global climate change or planetary-scale events, such as the El Nino Southern Oscillation, can significantly affect the abundance and distribution of organisms. Despite the evidence of such effects on marine and terrestrial systems, ecological consequences of rainfall fluctuations in coastal marine ecosystems remain poorly understood. Here we evaluate the effects of rainfall intensity on the interaction between the cordgrass Spartina densiflora and Azara's grass mouse Akodon azarae in a southwestern Atlantic salt marsh (Mar Chiquita coastal lagoon, Argentina). Field surveys showed that the abundance of A. azarae increased during rainy summers (i.e. El Nino 2005 and 2007) and had lowest values during dry summers (i.e. La Nina 2008). Salt content in sediment and plant tissue were negatively related with rainfall. In addition, field experiments showed that increased sediment salinity resulted in increased salt content in plant tissues. Elevated soil salinity also increased the proportion of senescent S. densiflora tissues and reduced plant growth. The consumption of S. densiflora leaves by A. azarae also decreased with increased soil salinity. The proportion of S. densiflora in feces collected during the driest summer was very small. Therefore, changes in the abundance of A. azarae could be mediated by plant-rodent trophic interaction or by plant cover changes. In conclusion, rainfall fluctuations changed the abiotic environment (i.e. salinity), decreasing primary production and indirectly modifying habitat use by the omnivore A. azarae and its trophic interaction with S. densiflora. The present study provides evidence that rainfall can modify ecological processes that affect the structure and dynamics of coastal marine ecosystems.

Tidal marshes are narrow wetlands distributed worldwide between the ocean and a variety of inland-habitats. The high diversity of interacting terrestrial habitats may influence terrestrial species composition and abundance in marshes. We investigated if characteristics of small mammal assemblages inhabiting the South-West-Atlantic (SWA) marshes are influenced by marsh characteristics, or if they vary in relation to changes in the inland biogeographic context. Sampling at five-Spartina marshes across the SWA-coast showed that plant cover, richness and height (ANOVA analysis) and small mammal assemblages (PERMANOVA analysis) differed between habitats (marsh or inland) depending on the site. Generalized Linear Models (GLMs) showed that abundance of small mammals in marshes is related to vegetation cover and diversity. However, its richness was related to the richness of small mammal at the inland habitats. In fact, species present in each marsh were also recorded in adjacent inland habitats. Species composition differed among similar marshes surrounded by different landscapes, while those in the same landscape did not. Consequently, regional abundance of small mammals in SWA-marshes is consistent with ecological sorting of abundance ranges along environmental gradients (e.g., plant-cover and richness). In contrast, landscape composition and configuration strongly affect marsh small mammal species assemblages, even if the marsh-habitats are quite similar.

Disturbance can generate heterogeneous environments and profoundly influence plant diversity by creating patches at different successional stages. Herbivores, in turn, can govern plant succession dynamics by determining the rate of species replacement, ultimately affecting plant community structure. In a south-western Atlantic salt marsh, we experimentally evaluated the role of herbivory in the recovery following disturbance of the plant community and assessed whether herbivory affects the relative importance of sexual and clonal reproduction on these dynamics. Our results show that herbivory strongly affects salt marsh secondary succession by suppressing seedlings and limiting clonal colonization of the dominant marsh grass, allowing subordinate species to dominate disturbed patches. These results demonstrate that herbivores can have an important role in salt marsh community structure and function, and can be a key force during succession dynamics.

Salt marsh zonation patterns generate different abiotic and biotic conditions that can accentuate species inherent differences in primary production and biomass. In South West Atlantic marshes, there are two Spartina species: Spartina alterniflora in the low intertidal and Spartina densiflora in the high intertidal. These two species are generally found in all marshes but with different dominance: In some marshes, the S. densiflora zone occupies higher extents, and in others, the S. alterniflora zone is the one that prevails. We found through field sampling that, in six studied marshes, there is greater S. densiflora live and total (i.e., dead+live) aboveground biomass (g m−2) in the marshes dominated by S. densiflora than in the ones dominated by S. alterniflora. Spartina alterniflora had similar aboveground biomass in the six marshes, regardless of the dominance of each species. When comparing the two Spartina species within each marsh, S. densiflora had greater live and total biomass in the marshes it dominates. In the marshes dominated by S. alterniflora, both species had similar live and total biomass. In all marshes, there was greater dead S. densiflora biomass. A multivariate analysis using selected abiotic factors (i.e., salinity, latitude, and tidal amplitude) showed that S. alterniflora aboveground biomass patterns are mainly correlated with salinity, while S. densiflora live biomass is mainly correlated with salinity and latitude, dead biomass with salinity and tidal amplitude, and total biomass with salinity alone. We conclude that in S. densiflora dominated marshes, the main processes of that species zone (i.e., nutrient accumulation) will be accentuated because of its higher biomass. We also conclude that climatic conditions, in combination with specific Spartina biotic and ambient abiotic parameters, can affect marsh ecological functions.

How species similarity changes between habitats along environmental gradients is still a central challenge in ecological studies. We assessed whether marsh plant characteristics are associated with geographic changes in environmental conditions and whether there are environmental factors associated with marsh-inland dissimilarity in species composition. Field samples of vegetation were collected at 6 sites along the SW Atlantic to determine plant characteristics (cover, tallness, richness and a-diversity), and marsh-inland dissimilarity (beta-diversity) in species composition was calculated. PERMANOVA analysis showed that plant assemblage changes among sites. Forward stepwise multiple regression analysis showed that in lower marsh, plant cover increased in association with tidal range and decreased in association with salinity. In the high marsh, plant cover decreased in association with tidal range, salinity and with minimum temperatures. Plant richness increased in association with tidal range and with marsh area, while alpha-diversity decreased in association with precipitation and increased with salinity. Beta-diversity, estimated by SIMPER analysis, increased in association with precipitation and decreased with salinity and daily thermal amplitude. We present evidence that there is an increase in alpha-diversity but a decrease in beta-diversity with environmental severity among co-specific marshes distributed along the SW Atlantic coast. Thus, communities developing in more benign conditions, regardless of their low local diversity, may increase biodiversity at a landscape scale by decreasing their similarities.

Questions Do current models that predict shifting effects of herbivores on plant diversity with varying nutrient conditions apply to stressful systems like salt marshes? Do herbivores affect different components of the diversity as nutrient availability varies? Location Salt marsh-salty steppe transition zone at the SW Atlantic Mar Chiquita coastal lagoon (37 degrees 44'52 ' S, 57 degrees 26'6 ' W), Argentina. Methods We experimentally evaluated the separate and interactive effect of nutrients and rodent (Cavia aperea) herbivory, using exclosures and applying fertilizer (mostly nitrogen), following a factorial design in 50 cm x 50 cm plots. Results We found a negative effect of herbivory on diversity in the resource-poor scenario (due to a reduction in species richness), but a positive effect when nutrients were added, by reducing the abundance of the dominant plant (and hence increasing evenness). Conclusions Our experimental results contribute to the limited factorial evidence evaluating the role of nutrients and herbivory on the diversity of terrestrial plant communities, even in highly stressful environments like salt marsh-salty steppe transition zones. Our results also support the model that predicts negative effects of herbivores on plant diversity in low-nutrient conditions and positive effects in nutrient-enriched scenarios, and also support the mechanism assumed to act in these situations.

Boundary habitats are frequently hot spots for the production and flow of organic matter (OM) and exert strong effects on ecological processes in the habitats which they link. Salt marshes, which are boundary habitats occurring between the land and the sea, are important sources of OM for coastal habitats. Primary productivity and tides are among the main causes of OM production and export from salt marshes. By field sampling and experiments we found that the stem-boring moth Haimbachia sp. nov. substantially increases the production of detritus in salt marshes along the SW Atlantic coastline. The larvae of this moth enhance the natural mortality of Spartina alterniflora and S. densiflora by feeding inside the basal and middle portions of the stem tissue. The attacks of the moth larvae produce dead and debilitated stems that are more easily broken and transported by the tides than non-attacked stems. Because the moth-attack frequencies will vary geographically in response to variation in the physical environment, the amount of OM flow between habitats will also vary, resulting in a positive relationship between moth-attack frequencies and OM production on a regional scale. Our field and experimental results show that herbivory by this moth and tidal transport could be the main determinants of the production of Spartina macrodetritus in these marshes. A key finding based on this previously undescribed interaction is that biological interactions (i.e. the effects of herbivores) can change the permeability of boundary habitats by altering the OM flow between terrestrial and aquatic ecosystems.

After intense debate it is now accepted that nutrients (a bottom-up process) and herbivores (a top-down process) are both important controls of plant productivity in many systems. Besides their direct effects, herbivores may also have profound positive or negative indirect effects that can be modulated by nutrients and time. The interactive relationships between time, nutrient availability and herbivore impacts (direct and indirect) on plant growth dynamics are an emerging research topic that merits further effort. Here we did several experiments in a SW Atlantic marsh to contribute towards that gap by focusing on the dominant plant, Spartina densiflora, and one of the dominant herbivores, the crab Neohelice (Chasmagnathus) granulata, in the marsh. Herbivory by the crab was highly seasonal, with most of the consumption occurring in fall. Even though crabs preferred nutrient enriched leaves, nitrogen content was not the driver of these seasonal variations. Crab herbivory had markedly indirect negative impacts on S. densiflora leaves, reducing their growth rates and increasing their senescence. These deleterious impacts may partially explain the seasonal decline in leaf growth and a net loss in leaf biomass observed in the fall. Fertilization did not seem to alter these processes. Adding nutrients increased leaf growth in the spring, where ambient herbivory was low, but it also increased herbivory in the fall, resulting in similar patterns as the ones observed under non-fertilized conditions. Herbivory by the crab also greatly affected the dynamics of S. densiflora stems. Increases in stem density in relation to initial conditions were larger in non-grazed than in grazed plots regardless of whether nutrients were added or not. Together, these results indicate that, in Southwestern marshes populated by S. densiflora and N. granulata, herbivory by the crab represents an important direct and indirect control of plant growth. Our results also emphasize the importance of considering impacts on growth rates and not only on biomass because not considering reduced growth after herbivory may lead to improper calculations of nutrient cycling or detritus production.

Hemisphere scale events such as El Niño-Southern Oscillation (ENSO) can alter rainfall regimes worldwide, with important effects on species abundance and distribution. The evidence of ENSO effects on terrestrial communities is, however, restricted to a few ecosystem types. We explored the effects of ENSO episodes on plant/terrestrial–herbivore interactions through changes in the rainfall regime in a southwestern Atlantic salt marsh (Mar Chiquita coastal lagoon, Argentina. 37° 40′S, 57° 23′W). Surveys showed a positive relationship between winter rainfall and the abundance of the wild guinea pig Cavia aperea. The highest salt marsh abundances of C. aperea were associated with rainy periods during El Niño episodes, and the lowest ones were associated with the driest La Niña episodes. Rainfall was negatively associated with marsh sediment salinity, and experiments revealed that increased salinity reduces growth and increases mortality of cordgrass (Spartina densiflora). Salt increase also causes the highest percentage of dry area in S. densiflora leaves and reduced carbon content, and more salt content and secretion in S. densiflora stems. A factorial experiment in which we manipulated C. aperea presence and salinity along the edges of S. densiflora patches showed that plants can asexually invade unvegetated areas when salinity is reduced and C. aperea is excluded. Conversely, S. densiflora edges retracted when salinity was increased or there was C. aperea herbivory. Changes in nutritional quality of S. densiflora could explain the low herbivory of (and lack of impacts from) C. aperea in plots with high salinity. Thus, plant distribution responds directly to climate oscillations through changes in salt stress, and indirectly, through changes in plant–herbivore interactions. Herbivores respond indirectly to climate oscillations through changes in plant food quality, which suggests that top-down effects increase when bottom-up stressors are relaxed. ENSO events have direct and indirect effects on marsh communities that modulate the relative importance of top-down and bottom-up effects and have a considerable effect on the primary productivity of S. densiflora marshes.