Native herbivores indirectly facilitate the growth of invasive Spartina in a eutrophic saltmarsh

Abstract

Current theory (e.g., consumer‐controlled theory) predicts that nutrient enrichment typically amplifies herbivory and thereby suppresses the growth and expansion of invasive plants. Herbivores can facilitate plant regrowth in the native community by stimulating complementary growth or ameliorating habitat conditions (e.g., by increasing soil oxygen and nutrient availability), but whether they have similar positive effects on invasive plants, especially under nutrient enrichment, remains unknown. Using a field nitrogen (N)‐enrichment × crab exclusion experiment, we evaluated and compared the effects of both N enrichment and crab herbivory on the growth performance of a global invasive cordgrass, Spartina alterniflora, and a co‐occurring native plant, Phragmites australis. We found that crabs consistently suppressed P. australis by density and aboveground biomass regardless of N enrichment. In contrast, for S. alterniflora, the negative effects of crabs under ambient N were replaced by positive effects under N enrichment, with crabs stimulating complementary increases in density and aboveground biomass. The differing effects between the N treatments were driven by crab burrowing activity, which increased soil N availability, and the nutrient‐use efficiency of S. alterniflora. Our findings revealed that native herbivores can have opposing effects on native and invasive plants, which broadens our understanding of how exotic plants can achieve dominance in a changing world.

Effects of Spartina invasion on the soil organic carbon content in salt marsh and mangrove ecosystems in China

“…Then, all the replaced soil depths were divided into three categories (0–30, 30–60 and 60–100 cm) to test the vertical patterns of Spartina invasions' effects on SOC content according to the root depths of different species. For example, the roots of Suaeda salsa and Scirpus mariqueter are mainly distributed in the top 30 cm, whereas those of S. alterniflora and P. australis can reach 60 cm or even deeper (Liao et al, 2007; Xu, Zhang, et al, 2022).…”

“…Due to its strong ability to tolerate harsh coastal conditions (e.g. hypersalinity and overflooding), high reproductive capability by both seeds and clonal rhizomes, and great competitive ability (Li et al, 2009; Qiu et al, 2020; Xu, Zhang, et al, 2022), S. alterniflora has rapidly invaded coastal China (ranging from 20 to 40 °N) since it was introduced in 1979 and now covers more than 40% of the coastal wetlands in China (Hu et al, 2021; Zhao & Qin, 2021). S. alterniflora has a suite of traits that impact the cycling and accumulation of soil organic C (SOC) content, including an extensive underground system (Darby & Turner, 2008; H. Liu et al, 2021), high photosynthetic rates (Jiang et al, 2009; Liao et al, 2007) and primary productivity (Liu et al, 2020), making the native ranges of S. alterniflora one of the most important blue C sinks across the world (Ouyang & Lee, 2014).…”

Mitigation of nitrogen inputs causes native Phragmites australis recovery following Spartina alterniflora invasion

“…Moreover, S. alterni ora might not resist the recovery of P. australis patches as strongly as invasive plants in terrestrial ecosystems by reinforcing themselves 45,66 due to the limited N mineralization in wetlands (as discussed above). Furthermore, S. alterni ora might not resist the recovery of P. australis patches by pre-emptively vacating soil nutrients as other invasive plants do 53,67 , because P. australis has a lower nutrient requirement than S. alterni ora 23 . and the growth of P. australis was not limited by lowered N availability (Figs.…”

“…In 2021, N was applied to the plots in the enrichment treatment following the method of the SINE experiment 23,65 . Plant survival was recorded separately for each species in a randomly positioned quadrat (50 × 50 cm) established in each plot during the growing season (from March through October) in the same year.…”

“…Over the past century, annual N inputs into the biosphere resulting from human activities (e.g., fossil fuel burning and crop fertilization) have reached 2.1 × 10 8 tons worldwide, creating eutrophic conditions across terrestrial, aquatic, and marine ecosystems 19 . As N is one of the most limiting nutrients for plant growth 20 , invasive plants can often outcompete their native counterparts for N resources, conferring invasive species greater growth capabilities under increased N availability and potentially resulting in the competitive exclusion of native plants [21][22][23] . Therefore, native communities are often more susceptible to plant invasions after N enrichment [24][25][26] .…”