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Distribution, scale, and drivers of mass mortality events in Europe's freshwater bivalves
Abstract Mass mortality events (MMEs) are decimating populations and compromising key ecosystem functions around the globe. One taxon particularly vulnerable to MMEs is freshwater bivalve mollusks. This group has important ecosystem engineering capacities and includes highly threatened and highly invasive taxa. Thus, MMEs of freshwater bivalves have important implications for conservation and ecosystems. Despite this, little is known about the magnitude, frequency, duration, distribution, and causes of freshwater bivalve MMEs. Using a questionnaire, we compiled data from 239 reports describing freshwater bivalve MMEs across 22 European countries since 1960. With these data, we analyzed trends in MME timing, location, and magnitude; identified the species affected; and evaluated the suggested causes (including reporter certainty). We found that the frequency of reports of MMEs increased each year, MMEs affected a broad range of species, clear geographical patterns linking certain causes to specific locations were lacking, factors related to drying and habitat destruction predominated suggested causes, and considerable uncertainty surrounded the causes of many MMEs, particularly those associated with potential pollutants and disease agents. Based on our findings, we recommend the standardization of many aspects of MME research (e.g., reporting and recovery assessment protocols), increased surveying for MMEs, further investigation into the causes of MMEs, especially those with significant uncertainty, and immediate actions to improve waterbody management, mitigate the effects of high temperatures, and further protect freshwater bivalves through the development and implementation of appropriate management actions and legislation.
Lower parasite pressure in invasive freshwater bivalves than in sympatric native Unionidae mussels in southern European lakes
AbstractThe Enemy Release Hypothesis (ERH) proposes that the success of bioinvasions is attributable to lower enemy pressure on invasive species compared to native ones, giving a competitive advantage for invaders. In line with the hypothesis, we previously observed in northern European bodies of fresh water that invasive bivalves were subject to lower parasite pressure than sympatric native mussels. Here, we investigated ERH in three southern European lakes, where the native mussels are rapidly declining and being replaced by non-native bivalves. In total, 679 bivalves (n of individuals per species per lake varying from 12 to 187) were collected during 2016–2018. Ten parasite taxa were found. The mean lake-specific number of parasite taxa in the native mussels (Anodonta exulcerata, A. cygnea, A. anatina and Unio elongatulus) was 2.6 times that in the invasive bivalves (Dreissena polymorpha, Corbicula fluminea and Sinanodonta woodiana). Similarly, the mean lake-specific sum of prevalences of infection by different parasite taxa in the native mussels was 3.4 times that in the invasive bivalves. Notable was the complete lack of parasites in C. fluminea. Thus, the results supported the Enemy Release Hypothesis and were in accordance with previous results from northern Europe, suggesting, on average, a lower parasite pressure in invasive bivalves than in sympatric native mussels. As parasites are usually harmful, this may contribute to the observed successful invasion of non-native freshwater bivalves in Europe.
A global meta‐analysis of ecological functions and regulating ecosystem services of freshwater bivalves
AbstractFreshwater bivalves are globally distributed, diverse, and common in benthic communities. Many taxa, particularly in the most species‐rich order, Unionida, are declining due to anthropogenic stressors, while a small number of non‐native species have become increasingly abundant and widespread, commonly replacing native bivalve assemblages. To understand how these global changes may impact ecosystems and people, we conducted a meta‐analysis of existing literature quantifying the ecological functions (= supporting or intermediate ecosystem services) and regulating ecosystem services of freshwater bivalves (hereafter “ecosystem services”). Random effects meta‐analysis modeling across 447 case studies revealed a positive effect on human health, safety, or comfort of freshwater bivalve ecosystem services overall and specifically, via effects on native macrofauna, microorganisms, wastes, and pollutants, and the physico‐chemical condition or quantity of sediments. Generally, effects of native species and species within the orders Unionida and Venerida were more significant and positive than those of other freshwater bivalves. No significant overall effect was found for ecosystem services related to zooplankton, algae, invasive species, and the physico‐chemical condition of ambient water. Moreover, a significant bias toward publication of positive results existed for studies quantifying ecosystem services related to algae. These findings illustrate the global importance of the ecosystem services of freshwater bivalves and highlight the need for large‐scale conservation and restoration efforts for their species and populations globally, including those of common species. Our findings also question common assumptions of strong and ubiquitous effects of freshwater bivalves on algae and water condition, cautioning against extrapolating observations across systems.
Enemy release: loss of parasites in invasive freshwater bivalves Sinanodonta woodiana and Corbicula fluminea
Invasive freshwater bivalves harm native species, ecosystems and biodiversity, and incur economic costs. The enemy release hypothesis posits that invasive species are released from enemies during the invasion process, giving them a competitive advantage in the new environment. We compared parasitism in two invasive freshwater bivalves, Sinanodonta woodiana and Corbicula fluminea between their original range (China) and invaded range (Europe). For S. woodiana , the average sample‐size‐standardized population‐specific parasite taxon richness was 2.1 times as high, and sum of prevalence was 3.0 times as high in the native range (3 populations, 81 individuals studied) as in the invaded range (6 populations, 210 individuals studied). For C. fluminea , the average standardized population‐specific parasite taxon richness was 1.3 and sum of prevalences was 27.5 in the native range (4 populations, 749 individuals studied), whereas all European C. fluminea were free of parasites (7 populations, 418 individuals studied). The results demonstrate loss of parasites as a result of invasion. Previous studies have shown that parasite pressure on S. woodiana and C. fluminea in the invaded range in Europe is, on average, lower than on sympatric native freshwater mussel populations. Together, these results support one aspect of the enemy release hypothesis: invasive bivalves experience reduced parasite loads as a result of invasion, which may contribute to their success, given the costs typically imposed by parasitism.