Study On Waterbird-mediated Dispersal For Aquatic Organisms

More than 100 years ago,Darwin put forward that waterbirds could be vectors of aquatic plants and invertebrates,and participate in the dispersal of different kinds of aquatic organisms.As powerful flyer,waterbirds are able to transport aquatic organism propagules to distant new habitats.In recent years,researchers have conducted persistent study on waterbird-mediated dispersal,indicating that waterbirds can finish this process by external carrying and internal ingestion.Previous studies mainly focused on waterbirds in Europe,but there were few studies on other areas,such as Asian or American flyway.There are still insufficient studies on the way that waterbirds carry,ingest and transport aquatic propagules,and the potential on the dispersal of aquatic invertebrates,especially invasive species by waterbirds.The dispersal of aquatic organisms by waterbirds includes three processes:(1)Aquatic organism propagules attach to body surface or are ingested by waterbirds;(2)Propagules are transported by waterbirds;(3)Propagules establish the population in a new habitat.What are the effects of waterbirds' own factors and environmental factors on the process of waterbird-mediated dispersal for aquatic organisms? For above contents,this study firstly analyzed the process that propagules are ingested by waterbirds through field sampling,then analyzed the transport distance for propagules through feeding experiments,and proved the possibility of propagules colonization by hatching experiment.Finally,this study explored the dispersal of invasive species by waterbirds,showing that waterbirds can promote the dispersal of some invasive species.Main results are as follows.1.Characteristics of waterbird-mediated dispersal for aquatic organismsIn order to study the effects of bird species,sampling time,sample types and dispersal ways on the waterbird-mediated dispersal for aquatic organisms,sampling works for local migratory and resident waterbirds were conducted in Chongming Dongtan,Shanghai in this study.During the spring and fall bird ringing in 2021,external samples of 276 shorebirds from 23 species were collected,including 276 Bill samples and 276 Feather samples.29 phytoplankton propagules from 2 species and 58 invertebrates from 10 taxa were retrieved.The result shows that bird species,sample types and seasons have a significant effect on the abundance of phytoplankton,and seasons have a significant effect on the presence of phytoplankton in external samples.Bird species,sample types,seasons and weight have no significant effect on the abundance and presence of invertebrates in external samples.From autumn 2020 to autumn 2021,a total of 552 samples were collected,including 488 waterbird faeces and64 waterbird regurgitated pellets.A total of 300 propagules from 4 species of phytoplankton were retrieved.The result shows that seasons,bird species and sample types have a significant effect on the abundance of phytoplankton,and seasons have a significant effect on the presence of phytoplankton in internal samples.A total of 1305 invertebrates from 10 taxa were retrieved from internal samples.The result shows that years,seasons and sample types have a significant effect on the abundance of invertebrates,while years,seasons,bird species and sample types have no significant effect on the presence of invertebrates in internal samples.A total of 346 dormant eggs from 13 taxa were retrieved.Bird species and sample types have a significant effect on the abundance of dormant eggs,and sample types have a significant effect on the presence of dormant eggs.The abundance of phytoplankton,invertebrates and dormant eggs in internal samples is significantly higher than external samples,and the richness of phytoplankton and dormant eggs in internal samples is significantly higher than external samples.In this study,waterbird samples in East Asian-Australasian flyway were collected and analyzed.The result shows that waterbirds could disperse aquatic organisms through body surface carrying and internal transport,and bird species,sampling time,sample types and dispersal ways have different effects on the potential in waterbird-mediated dispersal for aquatic organisms.2.Secondary dispersal of aquatic organisms by waterbirdsIn order to analyze the secondary dispersal pathway of aquatic organisms that are dispersed from zoobenthos to waterbirds,main food samples for 3 species of waders were collected and compared with waterbird faeces and regurgitated pellets in Dongtan.In this study,the faeces or pellets of Whimbrel,Grey Heron and Great Knot were collected,as well as 4 species of zoobenthos fed by 3 species of waterbirds.2phytoplankton taxa,7 invertebrate taxa and 6 dormant egg taxa were present in both Whimbrel excretions and zoobenthos samples,2 invertebrate taxa and 1 dormant egg taxon were present in both Grey Heron faeces and zoobenthos samples,and 2phytoplankton species and 2 invertebrate taxa were present in both Great Knot faeces and zoobenthos samples.Further analysis shows that there are significant differences in the abundance of dormant eggs of rotifers,cladocerans,Chiromantes dehaani and others between waterbird faeces,pellets and zoobenthos samples in Whimbrel group.In Grey Heron group,there is no significant difference in the abundance of dormant eggs of rotifers,copepods,cladocerans and others between faeces and zoobenthos samples.The abundance of dormant eggs in different samples shows a consistent trend,and the abundance of eggs in faeces samples is higher except for others.The analysis for community structure shows that there are significant differences in community composition between waterbird samples and zoobenthos samples,and these differences could be explained by feeding habits of waterbirds.Through collection and analysis of waterbird and zoobenthos samples,the study shows that aquatic organism propagules are dispersed by secondary dispersal from zoobenthos to waterbirds,and zoobenthos can promote the dispersal of local aquatic plants and invertebrates by waterbirds feeding on them.3.Effects of digestive tract status on dispersal of aquatic organisms by waterbirdsIn order to analyze the effects of waterbird gut status changing on potential dispersal possibility and distance of propagules,3 species of zooplankton dormant eggs of Artemia sp.,Brachionus plicatilis and Daphnia pulex were collected and fed to mallards Anas platyrhynchos.The migration of migratory birds was simulated.8 male mallards were selected for two feeding experiments.In the first feeding experiment,when each mallard was fed 200 eggs for every species,the recovery rate of Artemia and rotifer eggs in No fasting group is significantly higher than Fasting group.In the second feeding experiment,each mallard was fed 400 eggs for every species,and the average recovery rate of Artemia and rotifer eggs in Fasting group is higher than No fasting group.In two experiments,the average gut retention time of dormant eggs in Fasting group is higher than No fasting group,but there is no significant difference between groups.In the first hatching experiment,the hatching rate of 3 species of eggs in Control group is significantly higher than Ingestion group.In the second hatching experiment,the hatching rate of Artemia and rotifer eggs in Control group is significantly higher than Ingestion group.In this study,ingestion will significantly reduce the viability of dormant eggs,and fasting before migration will lead to a lower number of dispersed dormant eggs and a longer dispersal distance.4.Effects of waterbirds on aquatic biological invasionBoth of golden apple snail Pomacea canaliculata and crayfish Procambarus clarkii are the most destructive invasive species around the world.In this study,the potential of waterbirds in dispersing these species was evaluated,including the viability of eggs in waterbird faeces and regurgitated pellets.A total of 30,400 eggs were ingested by 8 male mallards in 19 feeding trials.46 intact eggs were recovered from mallard faeces in 14 trials,and 684 intact eggs were recovered from regurgitated pellets in 14 trials.Most eggs were defecated between 2-4 h after feeding,or regurgitated between 0-1 h after feeding.2 snail eggs recovered from faeces and 74 eggs recovered from regurgitated pellets successfully hatched.These results demonstrate that golden apple snail eggs can survive through gut passage of mallards.Similarly,every mallard was fed 1600 intact P.clarkii eggs,regurgitation event didn't occur,and no egg was recovered from faeces.The study shows that mallard can contribute to biological invasion by endozoochory,and this way is selective.In this study,we conducted field sampling in Chongming Dongtan and feeding experiments for invertebrate propagules.The study shows waterbird-mediated dispersal could be affected by waterbirds' own factors and environmental factors.When propagules are ingested,transported by waterbirds and colonize,zoobenthos ingested by waterbirds can contribute to the dispersal of local aquatic organisms,and waterfowl gut status will affect dispersal possibility and distance for aquatic organisms.Moreover,waterbirds can promote biological invasion by endozoochory.The potential of waterbirds in dispersing aquatic organisms was quantified in this study,and field sampling and laboratory experiments were designed for three stages of dispersal,but there are still some shortcomings in our study.For example,the field work was only conducted on a single site in East Asian-Australasian flyway,and only a few species of waterbirds and aquatic organisms were used for field and laboratory studies.In future studies,for fieldwork,new sampling sites in East Asian-Australasian flyway could be added to study waterbird-mediated dispersal between different locations,meanwhile analyzing the effects of latitude,salinity gradient and waterfowl taxa.For laboratory work,species of waterbirds and propagules in feeding experiment could be added,especially various invertebrate dormant eggs and invasive species seeds or eggs.Through comparing laboratory and field data,we can analyze the effects of ingestion by waterbirds on viability of different aquatic propagules,dispersal distance and biological invasion.