Reply to Wilson et al.: Feedbacks between geomorphology and fauna engineers are key to predicting coastal response to rising seas.

In their letter to the editor, Wilson et al. (1) highlight their historical work demonstrating that the burrowing and grazing activities of Sesarma reticulatum at tidal creekheads helps drive erosion and headward elongation of creeks in US Atlantic salt marshes. Combined with our previous work mechanistically linking sea level rise, geomorphology, sediment dynamics, and the emerging keystone grazer S. reticulatum (2), Wilson emphasizes that future research should evaluate feedbacks between Sesarma, other biota, and geomorphology in the context of sea-level rise. We agree.

We further propose that future study of vegetated coastal ecosystem response to climate change broadly requires more deliberate and nuanced inclusion of fauna in order to remain relevant and accurate. We acknowledge that vegetated coastal systems are built and structurally defined by habitat-forming plants. However, ecosystem-engineering infauna and epifauna—organisms whose physical structures and/or activities alter nutrient cycling, habitat complexity, sediment transport, and other functions—often powerfully modulate ecosystem structure and stability at landscape scales (3). Despite the demonstrated importance of faunal engineers, models of accretion, migration, and creek formation widely ignore their effects. Instead, the role of sea-level rise in controlling coastal wetland vertical persistence is considered solely in the context of vegetation–geomorphology feedbacks (4).

Recent work demonstrates that fauna modify both vegetation and geomorphology—and that climate change amplifies these impacts, elevating these organisms to keystone roles (2) (Fig. 1). Examples are evident across ecosystems; herbivorous crabs, fungal-farming snails, and other consumers strongly decrease plant biomass directly through grazing activities, and/or indirectly through the spread of disease (Fig. 2). Effects of these consumers can drive large-scale vegetation dieback or rapid geomorphological evolution (6, 7). Likewise, trampling and grazing by larger-bodied herbivores and consumers can compact vegetation and landforms at landscape scales (8) (Fig. 2).

Fig. 1.

Historical and projected roles of faunal engineers. (A) Conceptual models of keystone species, originating in Power et al. (5), map the impact of community members as well as keystone, foundation, dominant, and rare species as a function of proportional biomass. Those species with disproportionate effects are termed keystone species (or foundation species if they are also numerically dominant) and fall above the 1:1 line. (B) Historical roles of marsh flora and fauna. (C) In the context of sea-level rise, however, the effects of S. reticulatum on marsh ecology and geomorphology are far greater, or disproportionate, to their population size, elevating them to keystone status.

Fig. 2.

Fauna engineering effects on ecogeomorphology of vegetated coastal ecosystems [adapted from Fagherazzi et al. with permission from John Wiley and Sons (4)]. Conceptual model depicting the mechanisms through which (A) grazers, (B) livestock, (C) biodepositers, and (D) bioturbators alter ecogeomorphology, with indirect (blue) and direct + indirect (green) effects highlighted.

In contrast, deposition by suspension- and filter-feeding organisms, such as bivalves and sponges, and resuspension of sediment by bioturbators, such as burrowing crabs and worms, can directly increase landform elevation and significantly alter rates of sediment import and export (9). Simultaneously, through deposition of nutrient-rich material, oxygenation of soil, and enhancement of plant growth, these faunal engineers also indirectly enhance sediment capture by plant leaves or soil accumulation via root production (9) (Fig. 2 ). Finally, as warming drives poleward range expansions, many of these coastal systems will also be experiencing shifts in faunal communities, further complicating community and biogeochemical dynamics (10).

In many cases, the role of fauna in mediating the structure and function of biogenic landforms is dictated by their spatial distribution—a feature that is, ironically, often predictably controlled by underlying geomorphology and associated hydrology—processes that control larval and food delivery, and spatial and temporal gradients in physical and biological stress (11). We thus concur that deeper evaluation of the relationship between fauna and geomorphology is an urgent scientific need. We further highlight that the key to this investigation will be understanding the bidirectional feedbacks among geomorphology, vegetation, and spatial patterning of fauna, and the response of such interactive effects to rising seas.