Invasive plant indirectly affects its self-expansion and native species via bio-geomorphic feedbacks: Implications for salt marsh restoration

Abstract

Invasive engineering species are well-known to have the capacity to alter abiotic and biotic ecosystem characteristics and associated ecological processes. However, it has not been investigated whether and how they can alter reciprocal interactions between biological and physical processes, known as bio-geomorphic feedbacks, which may indirectly influence their invasion and native species. Here, using a tidal channel-salt marsh invaded by Spartina alterniflora in the Yellow River Delta of China as a model system, we first quantified the bio-geomorphic impacts caused by the invasion and subsequently examined the feedbacks of such altered bio-geomorphic impacts on its self-expansion and the growth of native dominants - Suaeda salsa. Field observations and experiments showed that invaded S. alterniflora significantly promoted the sedimentation rate within its expansion belt along tidal channel margins, resulting in marginal depressions with increased inundation conditions. The altered bio-geomorphic processes considerably reduced the survival and growth of natives, while the impacts on S. alterniflora seedlings were threshold-dependent. Seedling establishment, growth and asexual expansion of S. alterniflora were facilitated when the inundation depth of these depressions fell within a threshold (approximately < 15 cm). In contrast, as the inundation depth exceeded the threshold, inundation-related stress occurred and inhibited its growth performance. As such, invasive S. alterniflora could be facilitated to expand laterally into salt marshes while suppressing the growth range of natives for the foreseeable future. Our findings suggest that species invasions contribute substantially to existing bio-geomorphic feedback loops, leading to observed effects on population dynamics that should be integrated into the restoration of salt marshes that have undergone invasion-induced geomorphological metamorphosis.