Reciprocal facilitation between ants and small mammals in tidal marshes.

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.

Abiotic stress mediates top-down and bottom-up control in a Southwestern Atlantic salt marsh.

Increasing evidence has shown that nutrients and consumers interact to control primary productivity in natural systems, but how abiotic stress affects this interaction is unclear. Moreover, while herbivores can strongly impact zonation patterns in a variety of systems, there are few examples of this in salt marshes. We evaluated the effect of nutrients and herbivores on the productivity and distribution of the cordgrass Spartina densiflora along an intertidal stress gradient, in a Southwestern Atlantic salt marsh. We characterized abiotic stresses (salinity, ammonium concentration, and anoxia) and manipulated nutrients and the presence of the herbivorous crab Neohelice (Chasmagnathus) granulata, at different tidal heights with a factorial experiment. Abiotic stress increased at both ends of the tidal gradient. Salinity and anoxia were highest at the upper and lower edge of the intertidal, respectively. Nutrients and herbivory interacted to control cordgrass biomass, but their relative importance varied with environmental context. Herbivory increased at lower tidal heights to the point that cordgrass transplants onto bare mud substrate were entirely consumed unless crabs were excluded, while nutrients were most important where abiotic stress was reduced. Our results show how the impact of herbivores and nutrients on plant productivity can be dependent on environmental conditions and that the lower intertidal limits of marsh plants can be controlled by herbivory.

Ecosystem engineers activate mycorrhizal mutualism in salt marshes.

Theory predicts that ecosystem engineers should have their most dramatic effects when they enable species, through habitat amelioration, to live in zones where physical and biological conditions would otherwise suppress or limit them. Mutualisms between mycorrhizal fungi and plants are key determinants of productivity and biodiversity in most terrestrial systems, but are thought to be unimportant in wetlands because anoxic sediments exclude fungal symbionts. Our field surveys revealed arbuscular mycorrhizal associations on salt marsh plant roots, but only in the presence of crabs that oxygenate soils as a by-product of burrowing. Field experiments demonstrate that fungal colonization is dependent on crab burrowing and responsible for nearly 35% of plant growth. These results highlight ecosystem engineers as ecological linchpins that can activate and maintain key mutualisms between species. Our findings align salt marshes with other important biogenic habitats whose productivity is reliant on mutualisms between the primary foundation species and micro-organisms.