Invasive alien aquatic plants (IAAP) species are known to have deleterious effects on the
freshwater ecosystems they invade. This includes both socio-economic and ecologically
important ecosystem goods and services. Thus, IAAP species can be declared a serious threat, second only to habitat modification for causing a loss of aquatic biodiversity. Three control methods have been widely applied to control IAAP species invasion globally; mechanical, chemical and biological control. Both mechanical and chemical control methods are considered short-term and expensive, whereas biological control methods are regarded an effective and long-term solution for the control of IAAP species, mostly at the landscape level. But, little is known of the ecological recovery following the biological control of IAAP species, with mechanical control known to have had mixed success and chemical control to have nontargeted effects on aquatic ecosystems, causing harm to wildlife and human well-being. Biological control practitioners measure the success of biological control based on: (1) the biological control agents establishment and the negative impacts they impose on the targeted weed; and (2) non-native macrophyte species biomass reduction and an increase in native macrophytes species. Arguably, measure of biological control success has been subjective and
variable across the globe. Although some field studies have demonstrate biological control success to have positive socio-economic returns, there is little literature on ecological benefits. Furthermore, there is limited understanding on ecosystem recovery and restoration efforts following the biological control IAAP species, as compared to alien weeds in terrestrial and riparian ecosystems. Thus, this thesis aimed to quantify the ecological recovery e.g. aquatic biodiversity, ecosystem processes and trophic interactions following the management of Salvinia molesta in freshwater ecosystems. The research employed a suite of Before-After Control-Impact mesocosm experiments and field studies to investigate the response of aquatic microalgae, macroinvertebrates and their interactions (food web structure and function) during S. molesta infestation and after mechanical and biological control. The mesocosm experiment (Before invasion, During invasion & After successful biological control) showed that both aquatic microalgae and macroinvertebrate diversity indices were reliable biological indicators to detect S. molesta ecological impacts and recovery following control. The restored treatment (100% S. molesta + biological control agents), demonstrated complete aquatic microalgae and macroinvertebrates recovery following biological control, similar to the reference treatment (open water), where the degraded/impacted treatment (100% S. molesta with no biological control agents) showed a drastic decline in aquatic biodiversity and a complete shift in aquatic biota assemblage structure. Thus, the biological control effort by Cyrtobagous salviniae, the biological control agent for S. molesta assisted in the recovery of aquatic biota following successful biological control. The field study (four field sites, two sites controlled mechanically and two biologically) investigated water quality, aquatic biodiversity and community trophic interactions (aquatic food web) ‘Before and After’ S. molesta control. The study showed a drastic decline in aquatic
biodiversity (with three sites showing no record of aquatic macroinvertebrates, thus no biotic interactions during infestation) and poor water quality due to the shade-effect (abiotic filter) by free-floating S. molesta during the Before S. molesta control phase. However following both mechanical and biological control (After S. molesta control phase), there was a significant shift in abiotic and biotic ecosystem characteristics as compared to the Before S. molesta control phase. Thus, rapid ecosystem recovery was apparent as a result of aquatic microalgae and macroinvertebrates recolonisation. Sites showing a normal functioning ecosystem had improved water quality, increased biodiversity, productivity and trophic interactions, indicative of the return of biologically and functionally important species which were lost during the Before S. molesta phase. However, the Westlake River following the mechanical control of S. molesta demonstrated a series of multiple macrophyte dominated states, moving from a S. molesta infestation to a clear-water state (After S. molesta control), then to a cosmopolitan submerged Ceratophyllum demersum-dominated state, which was later replaced by a floating leaved emergent IAAP Nymphaea mexicana-dominated state. Each state was responsible for a
significant shift in both biotic and abiotic characteristics, affirming macrophyte abilities to
influence aquatic environments structure and functions. Furthermore, this event showed a clear example of a secondary invasion. Thus, a holistic IAAP species management strategy is warranted to restore previously invaded ecosystems and prevent secondary invasion and ecosystem degradation In conclusion, the S. molesta shade-effect (abiotic barrier) like any other free-floating IAAP species, was identified as the main degrading factor and responsible for water quality reduction, loss of aquatic diversity and shift in aquatic biota assemblage structure. Following S. molesta removal (or shade-effect elimination), there was a positive response to aquatic ecosystem species abundance, richness, diversity and community structure. Therefore, in combination, aquatic biota recolonisation rate and increases in biological and functional diversity were instrumental in the recovery of ecosystem structure and functions, following the control of S. molesta. Echoing existing literature, this thesis recommends: (1) IAAP species management to be integrated on broader freshwater conservation and environmental management strategies to aid in ecological restoration, instead of stopping after control; (2) biological control should be used where appropriate to combat free-floating IAAP species in freshwater ecosystems, followed by active introduction of native macrophyte propagules since they are limited by anthropogenic activities; and (3) more freshwater case studies are needed to add to our understanding of IAAP species management and restoration effort incorporating long-term monitoring.