The density and spatial arrangement of the invasive oyster Crassostrea gigas determines its impact on settlement of native oyster larvae

Abstract

Understanding how the density and spatial arrangement of invaders is critical to developing management strategies of pest species. The Pacific oyster, Crassostrea gigas, has been translocated around the world for aquaculture and in many instances has established wild populations. Relative to other species of bivalve, it displays rapid suspension feeding, which may cause mortality of pelagic invertebrate larvae. We compared the effect on settlement of Sydney rock oyster, Saccostrea glomerata, larvae of manipulating the spatial arrangement and density of native S. glomerata, and non-native C. gigas. We hypothesized that while manipulations of dead oysters would reveal the same positive relationship between attachment surface area and S. glomerata settlement between the two species, manipulations of live oysters would reveal differing density-dependent effects between the native and non-native oyster. In the field, whether oysters were live or dead, more larvae settled on C. gigas than S. glomerata when substrate was arranged in monospecific clumps. When, however, the two species were interspersed, there were no differences in larval settlement between them. By contrast, in aquaria simulating a higher effective oyster density, more larvae settled on live S. glomerata than C. gigas. When C. gigas was prevented from suspension feeding, settlement of larvae on C. gigas was enhanced. By contrast, settlement was similar between the two species when dead. While the presently low densities of the invasive oyster C. gigas may enhance S. glomerata larval settlement in east Australian estuaries, future increases in densities could produce negative impacts on native oyster settlement. Synthesis and applications: Our study has shown that both the spatial arrangement and density of invaders can influence their impact. Hence, management strategies aimed at preventing invasive populations reaching damaging sizes should not only consider the threshold density at which impacts exceed some acceptable limit, but also how patch formation modifies this.

Four-year decline in Ostrea chilensis recruits per spawner in Foveaux Strait, New Zealand, suggests a diminishing stock-recruitment relationship

“…Repeat estimates of oyster density (Doonan et al 1992, Michael et al 2001, and of dredge efficiency (the scalar used to estimate absolute abundance, Fu et al 2016) show good consistency in Foveaux Strait. Manipulative studies to investigate the effects of spawner density on recruitment have been undertaken for O. chilensis in Tasman Bay, New Zealand (Brown 2011), and elsewhere for C. gigas and Saccostrea glomerata (Wilkie et al 2013), and for the bay scallop Argopecten irradians (Tettelbach & Wenczel 1993, Peterson et al 1996. The effects of increased spawner density on recruitment were mixed.…”

Population density modifies the ecological impacts of invasive species

“…The ecological impacts of invasive species are complex and context‐dependent (Thomsen et al , Wilkie et al ). Our study adds to a growing body of evidence that impact does not always increase proportionally with invader density (Thiele et al , Elgersma and Ehrenfeld ) and in the case of Kornis et al (), the relationship was counter‐intuitive, with decreased impact at high density.…”

“…A limited number of studies have examined the density‐dependent ecological impacts of invasive species and they detected both linear and non‐linear relationships between population density and ecological impact (Li et al , Kulhanek et al , Thomsen et al , Green and Crowe , Kornis et al , Wilkie et al ). For example, Elgersma and Ehrenfeld () found that the cascading impacts of an invasive shrub (Japanese barberry Berberis thunbergii ) were linear on decomposition rates but non‐linear on microbial community structure.…”

Introduced Pacific oystersCrassostrea gigasin South Africa: demographic change, genetic diversity and body condition

“…• Assessment of the size of the recently established population of Pacific oysters in the Swartkops River estuary, and continued surveillance in estuaries such as those of the Breede and Kaaimans rivers, where the species currently occurs, are management priorities. Manual eradication is recommended for these estuaries, given that early elimination of small populations is effective in preventing invasions (Wilkie et al 2013). The physical destruction of intertidal oysters [using crowbars] has proved successful in Australia (Anon.…”