Abstract
Salvinia molesta D.S. Mitchell (Salviniaceae) is a damaging free-floating invasive alien macrophyte native to South America. The biological control programme against S. molesta by the weevil Cyrtobagous salviniae Calder and Sands (Erirhinidae) has been successful in controlling S. molesta infestations in the introduced range, however, there is some debate as to how biological control success is measured. This study measured the response of epilithic algae and aquatic macroinvertebrate communities in a S. molesta-dominated state and subsequently where the weed had been cleared by biological control, as a proxy for ecosystem recovery in a before–after control–impact mesocosm experiment. The restored treatment (S. molesta and C. salviniae) demonstrated epilithic algae and aquatic macroinvertebrate recovery during the “after” biological control phase, defined as similar to the control treatment. Comparatively, the impacted treatment (100% S. molesta) showed a drastic decline in biodiversity and shifts in community assemblages. We conclude that the biological control effort by C. salviniae facilitated biodiversity recovery of the impacted treatment. Furthermore, epilithic algae and aquatic macroinvertebrate communities were reliable biological indicators for measuring ecological impacts of invasion and ecosystem recovery following biological control, and thus represent potential tools for evaluating biological control success and ecological restoration.
Highlights
Native submerged macrophytes play a critical role in freshwater ecosystem structure and functioning, and if replaced by floating invasive alien aquatic plants (IAAP) species, ecosystem structure and functioning is altered, influencing ecosystem goods and services [1]
This study showed that the presence of S. molesta negatively affected water quality
dissolved oxygen (DO) concentration, light penetration, and water clarity) and aquatic biodiversity, which is consistent with the findings of Masifwa et al [75], Brendonck et al [76], Midgley et al [77], Chamier et al [78], and Coetzee et al [27] for P. crassipes
Summary
Native submerged macrophytes play a critical role in freshwater ecosystem structure and functioning, and if replaced by floating invasive alien aquatic plants (IAAP) species, ecosystem structure and functioning is altered, influencing ecosystem goods and services [1]. The South American floating-mat-forming IAAP species, Salvinia molesta D.S. Mitchell (Salviniaceae), Pontederia crassipes (≡ Eichhornia crassipes) Mart. Floating IAAP species competitively displace native macrophytes [3], thereby inhibiting light penetration, sequestering nutrients, and reducing dissolved oxygen (DO) concentrations, which have knock-on effects for higher order trophic functioning [4]. Apart from altering ecosystem structure and functioning, the legacy effects of these IAAP species have implications for ecosystem restoration, favouring colonisation by other non-native invaders over native macrophytes, often long after the primary invader has been controlled [5,6].
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