Crab herbivory regulates re‐colonization of disturbed patches in a southwestern Atlantic salt marsh

Abstract

Recent work exploring the effects of physical stress and herbivory on secondary succession in estuarine plant communities agrees with basic stress models and reveal that herbivory is an important force in brackish and oligohaline marshes but negligible in physically stressful salt marshes. In these systems, herbivores are terrestrial, and thus negatively affected by the same stressful factors that affect marsh plants (i.e. frequent flooding or high salinities). We evaluated the effects of a marine herbivore (i.e. the crab Neohelice granulata) on plant secondary succession in a southwestern Atlantic salt marsh. Field surveys revealed that disturbance‐generated bare patches have harsh physical conditions and that their edges suffer higher herbivore pressure compared to the marsh matrix. A factorial experiment demonstrated that asexual expansion of the surrounding plants is the only possible mechanism to re‐colonize disturbed patches and that crab exclusion can increase this colonization rate by more than 30 times. Our results show that even in highly stressful environments, herbivores strongly impact marsh structure by regulating patch recovery. The synergism of physical stress and herbivory may make plant succession an extremely slow process and lead to the prevalence of bare areas.

Natural enemies govern ecosystem resilience in the face of extreme droughts

“…This process of grazer‐driven expansion of vegetation die‐off along die‐off borders, often driven through the formation and movement of consumer fronts along the edges of remaining plant prey populations, has also been observed in marsh die‐off areas in Argentina (Daleo et al . ), the southeastern USA (Silliman et al . ) and New England (Altieri et al .…”

Biological and physical factors affecting the colonization of vegetation-free patches in a SW Atlantic salt marsh

“…Succession models have been proposed for many salt marshes from the Northern Hemisphere, where the recovery of disturbance-generated bare areas occurs within 3 yr (Pennings & Bertness 2001). In the Southern Hemisphere, however, previous observations revealed that bare patches found on the southwestern (SW) Atlantic Mar Chiquita coastal lagoon (37°44' S, 57°26' W, Argentina) remain almost unchanged for more than 4 yr (Daleo et al 2011). Thus, the aim of our study was to determine which biological and physical factors condition the colonization of bare patches in a SW Atlantic salt marsh.…”

“…As was proposed in our first hypothesis, the dispersal of S. perennis seeds from adult individuals could be a strong environmental filter that limits colonization in these marshes and thereby delays secondary succession. In this context, the existence of a Spartina densiflora matrix might be acting as a natural barrier for seed dispersal, given that bare patches and Sarcocornia perennis plants occur sparsely within this matrix (Daleo et al 2011).…”

“…The secondary succession and patch dynamics in SW Atlantic salt marshes has been previously studied with a focus placed on the interactions between plant species that dominate these marshes, Spartina densiflora and Sarcocornia perennis (Alberti et al 2008), and between these species and the crab Neohelice granulata (Alberti et al 2010a, Daleo et al 2011. These studies showed that after colonizing, Sarcocornia perennis facilitates the establishment of Spartina densiflora through reduced herbivory rates and is finally displaced by the higher competitive ability of S. densiflora.…”

“…For example, poly-chaetes are known for their negative impact on seed availability and seedling survival of marsh pioneer species through herbivory and bioturbation (Paramor & Hughes 2004). In addition, crabs can bury seeds through their burrowing activities and at the same time maintain and expand bare patches by grazing on seedlings and new shoots (Alberti et al 2010a, Daleo et al 2011. Regarding characteristics of the pioneer species, the number of available seeds may play an important role in the colonization of bare areas.…”