Is habitat amount important for biodiversity in rocky shore systems? A study of South African mussel assemblages

Habitat-forming species on rocky shores are often subject to high levels of exploitation, but the effects of subsequent habitat loss and fragmentation on associated species and the ecosystem as a whole are poorly understood. In this study, the effects of habitat amount on the fauna associated with mussel beds were investigated, testing for the existence of threshold effects at small landscape scales. Specifically, the relationships between mussel or algal habitat amount and: associated biodiversity, associated macrofaunal abundance and density of mussel recruits were studied at three sites (Kidd’s Beach, Kayser’s Beach and Kini Bay) on the southern and south-eastern coasts of South Africa. Samples, including mussel-associated macrofauna, of 10 × 10 cm were taken from areas with 100 % mussel cover (Perna perna or a combination of P. perna and Mytilus galloprovincialis) at each site. The amount of habitat provided by mussels and algae surrounding the sampled areas was thereafter determined at the 4.0 m² scale. A number of significant positive relationships were found between the amount of surrounding mussel habitat and the abundances of several taxa (Anthozoa, Malacostraca and Nemertea). Likewise, there were positive relationships between the amount of surrounding algal habitat and total animal abundance as well as abundance of mussel recruits at one site, Kini Bay. In contrast, abundance of mussel recruits showed a significant negative relationship with the amount of mussel habitat at Kayser’s Beach. Significant negative relationships were also detected between the amount of mussel habitat and species richness and total abundance at Kidd’s Beach, and between amount of mussel habitat and the abundance of many taxa (Bivalvia, Gastropoda, Maxillopoda, Ophiuroidea, Polychaeta and Pycnogonida) at all three sites. No threshold effects were found, nor were significant relationships consistent across the investigated sites. The results indicate that the surrounding landscape is important in shaping the structure of communities associated with these mussel beds, with significant effects of the amount of surrounding habitat per se. The strength and the direction of habitat effects vary, however, between shores and probably with the scale of observation as well as with the studied dependent variables (e.g. diversity, abundance, mussel recruitment, species identity), indicating the complexity of the processes structuring macrofaunal communities on these shores.     

Comparison of mussel bed community between two intertidal mytilids Septifier virgatus and Hormomya mutabilis

Mussel bed community structure of two intertidal mytilids, Septifer virgatus (Wiegmann) and Hormomya mutabilis (Gould), whose beds were contiguous vertically on a rocky intertidal shore (Wakayama Prefecture, Japan), was compared between 1982 and 1983. In the upper S. virgatus bed, crustaceans and bivalves were dominant in terms of both number of individuals and biomass. There were three barnacle species representative of the epizoans, two isopods and one amphipod as mobile fauna, and two bivalves as infauna. The lower H. mutabilis bed supported virtually no epizoans or mobile fauna. Infaunal free-ranging polychaetes and sipunculids were dominant in terms of both number of individuals and biomass. The H. mutabilis bed contained a much greater amount of sediment than did the S. virgatus bed, and the interstices among individual H. mutabilis and among their byssal threads were filled with sediment. The biomass of six of nine species dominant in the S. virgatus bed was negatively correlated with the amount of sediment. Recruitment of these faunal assemblages into artificial mussel clumps was examined in mussel enclosure experiments, and a negative sediment effect in H. mutabilis clumps was detected for one isopod and one limpet species. Factors causing the differences between two mussel bed communities are discussed, focusing on the effects of sediment.     

Habitat structure and complexity as determinants of biodiversity in blue mussel beds on sublittoral rocky shores

Habitat-forming, ecosystem engineer species are common in most marine systems. Still, much uncertainty exists about how individual and population-level traits of these species contribute to ecosystem processes and how engineering species jointly affect biodiversity. In this manipulative field experiment, we examined how biodiversity in marginal blue mussel beds is affected by blue mussel (1) body size, density and patch context and (2) presence of fucoid and algal structures. In the study area, bladder-wrack (Fucus vesiculosus), filamentous algae and blue mussels (Mytilus edulis) coexist at shallow depths in a variety of patch configurations and offer complex habitats with a high variability of resources. We hypothesized that complexity in terms of mussel bed structure and algal presence determines species composition and abundance. Results from the experiment were compared with macrofaunal communities found in natural populations of both engineering species. Results show that the physical structure and blue mussel patch context are important determinants for species composition and abundance. Results further show that the presence of algal structures positively affects diversity in blue mussel habitats due to increased surface availability and complexity that these algae offer. This study shows that blue mussel beds at the very margin of their distribution have an indisputable function for promoting and maintaining biodiversity and suggest that facilitative effects of habitat-modifying species are important on Baltic Sea rocky shores with fundamental importance to community structure.     

Habitat complexity impacts persistence and species interactions in an intertidal whelk

Although experiments have shown that habitat structure may influence the distribution of species and species interactions, these effects are still not commonly integrated into studies of community dynamics. Since habitat structure often varies within and among communities, this may limit our understanding of how various factors influence communities. Here, we examined how mussel bed complexity (the presence and thickness of mussel layers) influenced the persistence of whelks (Nucella emarginata) and interactions with a top predator (ochre sea stars, Pisaster ochraceus) and prey (mussels, Mytilus californianus). Results from a mark?Crecapture experiment indicate that whelk recapture rates are higher in more complex habitats, and laboratory experiments demonstrate that habitat complexity affects whelk feeding, growth, and nonconsumptive interactions with a keystone predator. Habitat complexity therefore has direct effects on species and also may lead to trade-offs among feeding, refuge, and other factors, potentially influencing the distribution of whelks and the effects of both whelks and sea stars on intertidal communities. These results demonstrate that habitat structure may play an important role in intertidal communities and other habitats and should be further considered in the experimental design of future studies of community dynamics.     

Dramatic declines in mussel bed community diversity: response to climate change?

Mussel beds along the wave-exposed coast of the eastern North Pacific Ocean serve as an important habitat, harboring a high diversity of species. A comparison of California mussel bed community diversity in 2002 to historical data (1960s to 1970s) revealed large declines (mean loss 58.9%), including some declines >141 species (approximately 80% loss). Concurrent work revealed inconsistent changes in mussel populations (biomass and bed thickness) along the California coast, suggesting that diversity declines may be related to large-scale processes rather than local habitat destruction. Potential factors causing declines in mussel community diversity are discussed, with regional climate change associated with the Pacific Decadal Oscillation and climate change induced alterations of ecological interactions and biological processes suggested as likely causes. Although extensive literature has predicted the potential effects of climate change on global diversity, this study is one of the few examples of declines attributed to climate change.     

A fractal approach for detecting spatial hierarchy and structure on mussel beds

Within beds of blue mussel (Mytilus edulis L.), individuals are aggregated into small patches, which in turn are incorporated into bigger patches, revealing a complex hierarchy of spatial structure. The present study was done to find the different scales of variation in the distribution of mussel biomass, and to describe the spatial heterogeneity on these scales. The three approaches compared for this purpose were fractal analysis, spatial autocorrelation and hierarchical (or nested) analysis of variances (ANOVA). The complexity (i.e. patchiness) of mussel aggregations was described with fractal dimension, calculated with the semivariogram method. Three intertidal mussel beds were studied on the west coast of Sweden. The distribution of wet biomass was studied along transects up to 128 m. The average biomasses of blue mussels on the three mussel beds were 1825낚, 179ᆩ and 576ᇖ g per0.1 m², respectively, and the fractal dimensions of the mussel distribution were 1.726ǂ.010, 1.842ǂ.014 and 1.939ǂ.029 on transects 1-3, respectively. Distributions of mussels revealed multiscaling behaviour. The fractal dimension significantly changed twice on different scales on the first bed (thus showing three scaling regions), the second and third beds revealed two and three scaling regions, respectively. High fractal dimension was followed by significant spatial autocorrelation on smaller scales. The fractal analysis detects the multiple scaling regions of spatial variance even when the spatial structure may not be distinguished significantly by conventional statistical inference. The study shows that the fractal analysis, the spatial autocorrelation analysis and the hierarchical ANOVA give complementary information about the spatial variability in mussel populations.     

Positive effects of the introduced green alga, Codium fragile ssp. tomentosoides, on recruitment and survival of mussels

The green macroalga, Codium fragile ssp. tomentosoides, is an important component of sheltered low-shore assemblages on breakwaters along sandy shores in the northern Adriatic Sea. Macroscopic thalli of C. fragile are not perennial, but develop from propagules and/or undifferentiated forms in early spring, when the dominant native space-occupier, the mussel Mytilus galloprovincialis, recruits. By mid-summer, rapid growth of C. fragile leads to the formation of a dense canopy. We investigated the effects of juvenile and adult thalli of C. fragile on recruitment, survival and growth of mussels. Two experiments tested the hypotheses: (1) that recruitment of mussels is greater within patches of juvenile thalli (primordia) of C. fragile than on adjacent bare surfaces; (2) that the presence of a canopy of C. fragile affects the survival and growth of mussel recruits. The number of recruits of mussels was significantly larger within clumps of primordia of C. fragile than on bare surfaces. The removal of the canopy of C. fragile affected negatively the density of mussels after 2 months from the start of the experiment, but there were no effects on the mean size of individuals, nor on the size–frequency distribution. The same trend persisted after 4 months from the start of the experiment. These results show that re-colonisation of space by mussels is enhanced by C. fragile. Given that mussels, in turn, have the potential to reduce recruitment rates of C. fragile, quick recovery of mussel beds after disturbances could be crucial for controlling the abundance of this alga on breakwaters. Results also suggest that the effects of introduced species on native assemblages can be explained only through studies encompassing different life-stages of interacting organisms.     

Meta-ecosystem engineering: nutrient fluxes reveal intraspecific and interspecific feedbacks in fragmented mussel beds

Ecologists still have to elucidate the complex feedback interactions operating among biodiversity and ecosystem processes in engineered systems. To address this, a field experiment was conducted to mimic natural mussel bed meta-ecosystems (Mytilus spp.) of the lower St. Lawrence Estuary (Quebec, Canada) and partition the effects of their biotic and abiotic properties and spatial structure on ecosystem processes and community dynamics of associated macro-invertebrates. We found positive intraspecific feedbacks between mussels and their recruits, and negative interspecific feedbacks between mussels and their associated ecosystem. These feedbacks were associated with mussel bed ecosystem processes (fluxes of ammonium and oxygen). In addition, we showed that proximity between mussel patches increased within-patch nutrient fluxes. Our study revealed the potential for meta-ecosystem engineering to drive feedback interactions between community and ecosystem functioning in marine fragmented systems. It also shows the relevance of meta-ecosystem theories as a conceptual framework to elucidate biotic and abiotic processes controlling ecosystem and community structure. Such framework could contribute to ecosystem-based management of spatially structured systems such as reserve networks and fragmented ecosystems.     

Diversity and cover of a sessile animal assemblage does not predict its associated mobile fauna

Habitat-forming organisms often determine the structural properties and food resources available to a wide diversity of associated mobile species. Sessile invertebrate assemblages on marine hard substrates support an abundant fauna of mobile invertebrates whose associations with traits of their host assemblages are poorly known. To assess how changes to habitat-forming species are likely to affect their associated mobile fauna, the relationships between abundance, diversity and composition of mobile invertebrates and the diversity, cover and composition of the sessile assemblages they use as habitat were quantified in Sydney Harbour, Australia (33°50′S, 151°16′E). Similar compositions of sessile species were more likely to share a similar composition of mobile species, but univariate measures of the habitat (percent cover, species and functional diversity, prevalence of non-indigenous species) did not predict variation in associated mobile assemblages. These results demonstrate that in this habitat it is difficult to predict the diversity of marine assemblages based on common surrogate measures of biodiversity.     

Hydrodynamic stress and habitat partitioning between indigenous (Perna perna) and invasive (Mytilus galloprovincialis) mussels: constraints of an evolutionary strategy

The ability of a mussel to withstand wave-generated hydrodynamic stress depends mainly on its byssal attachment strength. This study investigated causes and consequences of different attachment strengths of the two dominant mussels species on the South African south coast, the invasive Mytilus galloprovincialis and the indigenous Perna perna, which dominate the upper and the lower areas of the lower balanoid zone, respectively and co-exist in the middle area. Attachment strength of P. perna was significantly higher than that of M. galloprovincialis. Likewise solitary mussels were more strongly attached than mussels living within mussel beds (bed mussels), and in both cases this can be explained by more and thicker byssal threads. Having a wider shell, M. galloprovincialis is also subjected to higher hydrodynamic loads than P. perna. Attachment strength of both species increased from higher to lower shore, in response to a gradient of stronger wave action. The morphological features of the invasive species and its higher mortality rates during winter storms help to explain the exclusion of M. galloprovincialis from the low shore. The results are discussed in the context of the evolutionary strategy of the alien mussel, which directs most of its energy to fast growth and high reproductive output, apparently at the cost of reduced attachment strength. This raises the prediction that its invasive impact will be more pronounced at sites subject to strong but not extreme wave action.     

The effects of experimental bait collection and trampling on a<Emphasis Type="Italic"> Mytilus californianus</Emphasis> mussel bed in southern California

Rocky shores in southern California are heavily visited by humans. At sites used by recreational fishers, the effects of foot traffic combined with the collection of mussels for bait may reduce mussel cover and create mussel-free gaps. To test this hypothesis, the effects of trampling and bait-removal on mussel populations were experimentally examined. Plots in a mussel bed were subjected to monthly combinations of trampling (0, 150, or 300 steps) and simulated bait-removal (0 or 2 removed mussels). Although the experiment was done during a period of high natural disturbance associated with the 1997–1998 ENSO, plots receiving treatments experienced significantly greater reductions in mussel cover, mass, and density than controls. These results indicate that visitor foot traffic and bait-removal by fishers can significantly reduce mussel cover, density, biomass, and sizes.Communicated by P.W. Sammarco, Chauvin     

Tidal height,rather than habitat selection for conspecifics,controls settlement in mussels

Settlement is a major determinant of intertidal populations. However, the energy costs of lost larvae are very high. Accordingly, arrival and attachment on suitable substrata are essential requirements for species’ survival. On the intertidal, the presence of cues left by adult or juvenile conspecifics could be vital for the successful establishment of larvae arriving on the shore. Two mussel species, the indigenous Perna perna and the invasive Mytilus galloprovincialis, co-occur on the lower eulittoral zone on the south coast of South Africa. P. perna dominates the low and M. galloprovincialis the high mussel zones, with co-existence in the mid mussel zone. This study tested the hypothesis of settlement selectivity for conspecifics in these two mussel species, to understand whether the final adult distribution of mussels on the shores is determined by active behavioural and chemical mechanisms. Preferential selection by larvae for conspecifics was tested in the field during the peak settlement period in 2004 in natural mussel beds across zones and through manipulative experiments in the mid-zone where the species co-exist. On natural beds, settlement was determined by counts of settlers attached over 48 h onto artificial collectors. Collectors were placed on beds of P. perna and M. galloprovincialis present at both high- and low-adult densities, as well as in mixed beds. On such natural beds, settlers of both species consistently favored low-zone P. perna beds. Settlement patterns over 24 h onto experimentally created mussel patches consisting of P. perna, M. galloprovincialis or the two species combined beds, set in the mixed zone, did not conform with the results of the natural beds study: settlers of both species settled with no discrimination among different patches. The results indicate that mussels, which are sedentary, lack attraction to conspecifics at settlement. This highlights the importance of tidal height in setting settlement rates, and of post-settlement events in shaping populations of these broadcast spawners.     

Community structure in Florida Escarpment seep and Snake Pit (Mid-Atlantic Ridge) vent mussel beds

Comparisons between invertebrate communities hosted by similar foundation species under different environmental conditions permit identification of patterns of species distributions that might be characteristic of the different ecosystems. Similarities and differences in community structure between two major types of chemosynthetic ecosystems were assessed by analyzing samples of invertebrates associated with Bathymodiolus heckerae Gustafson et al. mussel beds at the Florida Escarpment seep (Gulf of Mexico, 26°01.8N; 84°54.9W; October 2000) and B. puteoserpentis von Cosel et al. mussel beds at the Snake Pit vent (Mid-Atlantic Ridge, 23°22.1N; 44°56.9W; July 2001). Macrofaunal species richness was nearly twice as high in the seep mussel bed compared to the vent mussel bed, and only a single morphospecies, the ophiuroid Ophioctenella acies Tyler et al., was shared between the sites. Similarities between the two faunas at higher taxonomic levels (genus and family) were evident for only a small percentage of the total number of taxa, suggesting that evolutionary histories of many of these seep and vent macrofaunal taxa are not shared. The taxonomic distinctiveness of the seep and vent mussel-bed macrofaunal communities supports the hypothesis that environmental and oceanographic barriers prevent most taxa from occupying both types of habitats. Macrofaunal community heterogeneity among samples was similar in seep and vent mussel beds, indicating that spatial scales of processes regulating community variability may be similar in the two types of ecosystems. Suspension feeders were not represented in the macrofauna of seep or vent mussel beds. Primary consumers (deposit feeders and grazers) contributed more to the total abundance of macrofauna of seep mussel beds than vent mussel beds; secondary consumers (polychaetes and shrimp) were more abundant in the vent mussel beds.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00227-004-1304-z.Communicated by J.P. Grassle, New Brunswick     

Differences in the structures of fish assemblages inThalassia testudinum beds in Guadeloupe,French West Indies,and their ecological significance

The structures of fish assemblages in twoThalassia testudinum beds in Guadeloupe, French West Indies, one adjacent to mangroves and the other adjacent to coral reefs, were compared between January 1983 and May 1984. The aim of the study was to compare the influences of mangroves and coral reefs on the utilization of seagrass beds by fishes through examination of species composition, catch rate, size of fishes and temporal changes. The two fish assemblages were similar in terms of the number of species they had in common (nearly 44% of the total number of species collected) and the great abundance of juveniles. They both comprised species that usually inhabit other habitats, i.e., estuaries, open waters or coral reefs. Estuary-associated species (e.g. Gerreidae) were the most abundant species in the seagrass bed near the mangroves, while small pelagic species (e.g. Clupeidae) were the most abundant species in the seagrass bed near the coral reefs. The seagrass bed near the mangroves was preferentially utilized as a nursery area by small juveniles of various species (e.g. Clupeidae, Sparidae, Gerreidae, and at least one coral reef species,Ocyurus chrysurus). The abundance of these species varied frequently, suggesting successive arrivals and departures of juveniles over time. The seagrass bed near the coral reefs was characteristically utilized by fishes that are more able to avoid predation, i.e., fishes that forage over seagrass beds at night and shelter in or near the coral reefs during the day (large juveniles of coral reef species and adults of schooling pelagic species, respectively). The constant migrations of these fishes between the coral reefs and seagrass beds explained the relative stability of the structure of the fish assemblage in the seagrass bed over time. Thus, the two seagrass beds were not equivalent habitats for fishes. The distinct ecological influences of the mangroves (as a nursery for small juveniles) and coral reefs (as a shelter for larger fishes) on the nearby seagrass beds was clearly reflected by the distinct utilizations of these seagrass beds by fishes.     

Influence of wave exposure on South African mussel beds and their associated infaunal communities

This study investigates effects of wave exposure on beds of the mussel Mytilus galloprovincialis and the infaunal communities associated with them. Nine sites of varying wave exposure were sampled near Groenrivier, on the west coast of South Africa. Wave exposure was measured using a Palumbi device, and exposure ranged from 7.17×10³ to 18.49×10³ N m^–2. Percentage mussel cover at each site was measured using 50×50 cm quadrats, and three 10×10 cm samples were removed from the mussel beds in the mid-intertidal zones of each site for examination of infauna. Percentage mussel cover, mean mussel length, mussel biomass and mussel bed depth all peaked at intermediate exposures, declining towards both the most sheltered and most exposed sites. Infaunal species diversity and richness both showed the reverse trend, peaking at the most sheltered and most exposed sites, and declining at intermediate exposures. Neither infaunal abundance, nor biomass, was significantly correlated to wave exposure, although abundance was highest at sheltered sites and biomass greatest at the most exposed site, which was dominated by limpets and large robust polychaetes.Communicated by J.P. Thorpe, Port Erin     

High seagrass diversity and canopy-height increase associated fish diversity and abundance

Seagrass species function as typical foundation species that unifies most ecosystem processes. This ecosystem role depends largely on the morphological characteristics and structural complexity of seagrass beds, including their ecological importance for fish species. This study examined relationships between seagrass bed characteristics and associated fish communities in mixed seagrass beds. Correspondence analysis (CA) and canonical correlation analysis (CCoA) were performed to estimate relationships for individual seagrass bed characteristics. The CCoA results revealed that species richness and three-dimensional structure of seagrass had great effect on the biomass and richness of the associated fish community. The CA results revealed that the relative importance of seagrass bed characteristics differed among fish functional groups including fishes appearing on the surface of, inside, and on the bottom of seagrass beds. The fishes found on the surface of the beds preferred beds with low seagrass biomass and high three-dimensional structure, those inside the beds preferred beds with high seagrass biomass and high three-dimensional structure, and those on the bottom of the beds preferred locations with low seagrass biomass and low three-dimensional structure. The results of this study provide compelling evidence that seagrass beds with high species diversity and high three-dimensional structure, but intermediate biomass, may provide the great benefit to the associated fish community. Such niche complementarity among fishes may be a process facilitated by seagrass diversity for secondary production as an ecosystem functioning.     

Despotic, high-impact species and the subcontinental scale control of avian assemblage structure

Some species have disproportionate influence on assemblage structure, given their numbers or biomass. Most examples of such "strong interactors" come from small-scale experiments or from observations of the effects of invasive species. There is evidence that entire avian assemblages in open woodlands can be influenced strongly by individual species over very large areas in eastern Australia, with small-bodied species (< 50 g) being adversely affected. We used data from repeated surveys in 371 sites in seven districts across a region from Victoria to Queensland (> 2000 km). A series of linked Bayesian models was used to identify large-bodied (> or = 50 g) bird species that were associated with changes in occurrence and abundance of small-bodied species. One native species, the Noisy Miner (Manorina melanocephala; family Meliphagidae), was objectively identified as the sole large-bodied species having similar detrimental effects in all districts, depressing occurrence of 57 of 71 small-bodied species. Adverse effects on abundances of small-bodied species were profound when the Noisy Miner occurred with mean site abundances > or = 1.6 birds/2 ha. The Noisy Miner may be the first species to have been shown to influence whole-of-avifauna assemblage structure through despotic aggressiveness over subcontinental scales. These substantial shifts in occurrence rates and abundances of small-bodied species flow on to alter species abundance distributions of entire assemblages over much of eastern Australia.     

Surf-zone fish assemblages in south-western Australia: do adjacent nearshore habitats and the warm Leeuwin Current influence the characteristics of the fish fauna?

Nearshore fish faunas from 32 sites along 1500 km coastline of temperate south-western Australia were sampled by seine net between 1991 and 1992 to examine the species composition in sandy surf zones along this region, and to determine whether it is influenced by adjacent nearshore habitats and the warm southward flowing Leeuwin Current. Although the ichthyofauna was diverse, with 95 species from 47 families recorded, it was numerically dominated by only a few species. Species of Atherinidae, Mugilidae, Tetraodontidae, Clupeidae and Pomatomidae, such as Atherinomorus ogilbyi, Leptatherina presbyteroides, Mugil cephalus, Aldrichetta forsteri, Torquigener pleurogramma, Hyperlophus vittatus, Spratelloides robustus and Pomatomus saltatrix, were often common to these surf zones. When the species composition of the surf zones was compared with that found in adjacent nearshore habitats, 38 and 42% of the species were shared with reefs and seagrass beds, respectively, and 22% were present in all three habitats. Classification and ordination demonstrated that the faunal composition on the west coast was distinct from that on the south coast, and within each of these regions there were discrete assemblages. There was a marked decline in the number of species on the south coast, with 20 to 66 species reported from the six west-coast assemblages and 11 to 16 species collected from the four south-coast assemblages. A high proportion of resident species was found in the surf zones on both coasts; however, there was a smaller contribution of transient species on the south coast than on the west coast. This decline in transient species was associated with the absence of tropical species on the latter coast. Benthic invertevores were dominant on both coasts, while trophic diversity decreased and the proportion of zooplanktivores increased on the south coast. These differences in the characteristics of the fish fauna between the two coasts can be related to the presence of seagrass beds and limestone patch reef adjacent to sandy surf-zone areas on the west coast which provide more microhabitats for fish. The presence of inshore limestone reefs along the west coast moderates wave energy, producing more sheltered and temporally stable surf zones. The lower number of species on the south coast can also be attributed to the reduced influence of the Leeuwin Current. This southward flowing current acts as a mechanism for the dispersal of tropical species which display no regular association with the surf zones on the lower west coast.     

Density-dependent biodiversity effects on physical habitat modification by freshwater bivalves

Several decades of research have shown that biodiversity affects ecosystem processes associated with resource capture and the production of biomass within trophic levels. Although there are good reasons to expect that biodiversity influences non-trophic ecosystem processes, such as the physical creation or modification of habitat, studies investigating the role of biodiversity on physical processes are scarce. Here we report the results of a study using artificial streams to test the influence of freshwater mussel biodiversity on gravel erosion during high flows while manipulating mussel abundance. Mussel species vary in traits that should influence their effects on erosion, such as size, shell morphology, and burrowing behavior. We found that mussel species richness was associated with an increase in erosion at both low and high densities. Planned contrasts showed that the erosion observed in species mixtures was purely additive at low density, indicating that erosion in a species polyculture could routinely be predicted by the performance of monocultures. However, at high density certain combinations of species showed nonadditive effects on erosion, suggesting that organism abundance can fundamentally alter biodiversity effects. Although this may have been a result of altered species interactions at high density, our study design cannot confirm this.