Ecosystem engineers drive creek formation in salt marshes

Abstract

Ecosystem engineers affect different organisms and processes in multiple ways at different spatial scales. Moreover, similar species may differ in their engineering effects for reasons that are not always clear. We examined the role of four species of burrowing crabs (Sesarma reticulatum, Eurytium limosum, Panopeus herbstii, Uca pugnax) in engineering tidal creek networks in salt marshes experiencing sea level rise. In the field, crab burrows were associated with heads of eroding creeks and the loss of plant (Spartina alterniflora) stems. S. reticulatum was closely associated with creek heads, but densities of the other crab species did not vary across marsh zones. In mesocosm experiments, S. reticulatum excavated the most soil and strongly reduced S. alterniflora biomass. The other three species excavated less and did not affect S. alterniflora. Creek heads with vegetation removed to simulate crab herbivory grew significantly faster than controls. Percolation rates of water into marsh sediments were 10 times faster at creek heads than on the marsh platform. Biomass decomposed two times faster at creek heads than on the marsh platform. Our results indicate that S. reticulatum increases creek growth by excavating sediments and by consuming plants, thereby increasing water flow and erosion at creek heads. Moreover, it is possible that S. reticulatum burrows also increase creek growth by increasing surface and subsurface erosion, and by increasing decomposition of organic matter at creek heads. Our results show that the interaction between crab and plant ecosystem engineers can have both positive and negative effects. At a small scale, in contrast to other marsh crabs, S. reticulatum harms rather than benefits plants, and increases erosion rather than marsh growth. At a large scale, however, S. reticulatum facilitates the drainage efficiency of the marsh through the expansion of tidal creek networks, and promotes marsh health.

Pattern and scale: evaluating generalities in crab distributions and marsh dynamics from small plots to a national scale

“…Our transect locations were selected without consideration of crab dynamics (they were established to track vegetation long before this study was conducted), and they spanned the entire marsh elevation, from landward to seaward edge. Indeed, negative effects at a small scale may scale up to positive effects at the landscape scale: crab burrowing can lead to erosion of creek banks, but this can increase marsh drainage (Vu and Pennings 2017). For instance, Coverdale et al (2012) showed strong negative effects, but average burrow densities were above 100 burrows/m 2 ; we only had such high burrow densities at 2 of 15 sites, and only at a few plots at those two sites.…”

“…He and Silliman (2016) found that the effect of herbivory on coastal vegetation varied with latitude and with nutrient-loading. Vu and Pennings (2017) detected strong effects of herbivory by one crab species in a marsh system, but no effects by three others. Vu and Pennings (2017) detected strong effects of herbivory by one crab species in a marsh system, but no effects by three others.…”

“…Luk and Zajac (2013) found Uca pugnax in the northeastern United States to be most common in low marsh, but not eroding seaward edges. Vu and Pennings (2017) found that the same species had predictable patterns of distribution in the southeastern United States, with highest abundance near creek heads, but three other species showed no clear patterns. Vu and Pennings (2017) found that the same species had predictable patterns of distribution in the southeastern United States, with highest abundance near creek heads, but three other species showed no clear patterns.…”

“…Szura et al (2017) found stronger effects of Sesarma reticulatum with increased inundation. Vu and Pennings (2017) noted that crab burrowing that increases creek size may actually have a positive effect on marsh integrity in the face of SLR, by increasing drainage. Vu and Pennings (2017) noted that crab burrowing that increases creek size may actually have a positive effect on marsh integrity in the face of SLR, by increasing drainage.…”

Predator, prey, and substrate interactions: the role of faunal activity and substrate characteristics

“…This is an example of fluvial zoogeomorphology (Butler ) the process through which animals alter the geomorphology and sedimentology of their environment (Rice et al. , Statzner , Albertson and Allen , Vu and Pennings ). Crayfish are widely recognized zoogeomorphic agents that manipulate coarse sediments and bedload flux (Johnson et al.…”