Facilitating your replacement? Ecosystem engineer legacy affects establishment success of an expanding competitor

Abstract

Interactions with resident species can affect the rate that expanding species invade novel areas. These interactions can be antagonistic (biotic resistance), where resident species hinder invasive establishment, or facilitative (biotic assistance), where residents promote invasive establishment. The predominance of resistance or assistance could vary with the abiotic context. We examined how the effects of a resident ecosystem engineer interact with abiotic stress to resist or assist the establishment of an expanding competitor. In Florida salt marshes, native cordgrass, Spartina alterniflora, is an influential ecosystem engineer that, when dead, exerts a legacy effect by forming persistent wrack patches. We examined how the legacy effect of Spartina wrack varies with spatial context and abiotic conditions to influence establishment of the northward-expanding black mangrove, Avicennia germinans. Field surveys documented that Spartina wrack and Avicennia propagules co-occur in the high intertidal zone, and we conducted two outdoor mesocosm experiments to investigate this association. Wrack positively affected propagule establishment when propagules were beneath wrack, but negatively affected establishment when propagules were above wrack. The abiotic tidal regime influences the magnitude of wrack effects by controlling ambient moisture, and the positive and negative effects of wrack were stronger in low moisture conditions that simulated desiccation stress during harsh neap tides. Thus, the same resident engineer can either resist or assist an expanding competitor and the magnitude of these effects depends on abiotic conditions. We propose that under harsh conditions, there is greater scope for an engineer's mediating influence to affect associated species, both positively and negatively.

Jump‐starting coastal wetland restoration: a comparison of marsh and mangrove foundation species

“…Other disturbances may have additional, and possibly non‐symmetrical, impacts on mangrove and salt marsh foundation species depending on their severity and frequency (e.g. wrack deposition, storm events, herbivory, fire) (Nyman & Chabreck ; Guo et al ; Castorani et al ; Smith et al ). By planting Spartina , rapid vegetative restoration can be attained, limiting the risk of die‐off from freeze events, and providing the needed structure for long‐term restoration of key ecosystem functions such as basal carbon food web support, habitat provisioning, soil stabilization, and nutrient cycling.…”

Dead litter of resident species first facilitates and then inhibits sequential life stages of range‐expanding species

“…After A. germinans propagules successfully root, they transition into seedlings when they sprout true leaves, which occurs within 2–4 weeks after rooting (Smith et al., 2018). We predicted that wrack could negatively affect seedling survival via two potential mechanisms.…”

“…Tides and currents deposit the buoyant wrack at high marsh elevations, where it smothers vegetation (Bertness & Ellison, 1987), alters moisture, salinity and nutrient conditions (Pennings & Richards, 1998), and provides refuge for intertidal species (Smith et al., 2019). In the high marsh, A. germinans propagules are more abundant on and under wrack piles compared to adjacent vegetation (Smith et al., 2018). This pattern suggests that propagules either raft in with wrack (Minchinton, 2006) or are trapped by existing wrack.…”

“…Spartina alterniflora wrack also changes the local moisture regime. In mesocosm experiments, wrack facilitated propagules placed underneath it by increasing moisture, but caused severe desiccation of propagules placed above it (Smith et al., 2018). Wrack could also potentially buffer freezing temperatures and create a more favourable microhabitat for mangrove early life stages that are vulnerable to freezes (Coldren & Proffitt, 2017; Osland et al., 2015).…”

Freeze tolerance of poleward‐spreading mangrove species weakened by soil properties of resident salt marsh competitor

“…Mangroves provide an opportunity to examine the relative importance of positive and negative plant-soil interactions in the context of a climate-driven range expansion. As temperatures warm, mangroves are expanding poleward and replacing salt marshes between subtropical and warm temperate climatic zones world-wide (Guo, Zhang, Lan, & Pennings, 2013;Saintilan, Wilson, Rogers, Rajkaran, & Krauss, 2014;Smith, Blaze, Osborne, & Byers, 2018). The northern distribution of mangroves in the southeastern United States is controlled primarily by the frequency of extreme cold events (Cavanaugh et al, 2014;Osland, Enwright, Day, & Doyle, 2013).…”