226 Ecology, 87(1), 2006, pp. 226–234  2006 by the Ecological Society of America CONDITIONAL OUTCOMES OF FACILITATION BY A HABITAT-MODIFYING SUBTIDAL BIVALVE ALF NORKKO, 1 JUDI E. HEWITT,SIMON F. THRUSH, AND GREIG A. FUNNELL National Institute of Water and Atmospheric Research, P.O. Box 11–115, Hamilton, New Zealand Abstract. Facilitation by habitat modifiers is common in ecological communities, but the potential for temporal and spatial variations in environmental conditions to modify the outcome of these interactions and influence the strength of feedbacks is poorly understood. Suspension-feeding bivalves are important habitat modifiers that can facilitate surrounding communities by providing refuge from predation and changing boundary flows and through the production of organically enriched biodeposits. However, numerous studies have high- lighted the problem of finding generalizable patterns. We tested the strength and generality of the relationship between the large suspension-feeding bivalve Atrina zelandica and surrounding macrofauna and hypothesized that facilitation by Atrina is conditional and modulated by site-specific suspended sediment concentration (SSC), which influences the quantity and quality of biodeposit production. We found temporally consistent patterns of higher rates of biodeposition and increased abundance and species richness in close prox- imity to Atrina under low SSC conditions. Facilitation strength decreased with increasing SSC, suggesting that the facilitation effect of Atrina is reduced and reversed along this environmental stress gradient. Key words: Atrina zelandica; facilitation; generality; habitat modification; North Island, New Zealand; soft-sediment macrofauna; suspended sediment concentration; suspension feeder. INTRODUCTION Biogenic habitat generation is an important process with profound influences on the distribution and di- versity of organisms. With habitat degradation of nat- ural ecosystems continuing at an unprecedented rate, developing an understanding of potential positive in- teractions between foundation species and associated communities is imperative for successful conservation and restoration (Jackson et al. 2001, Thrush and Dayton 2002). Foundation species, such as kelps, seagrasses, bivalve reefs, and marsh plants, create habitats that can define entire ecosystems (Dayton 1975, Jones et al. 1994, Lenihan 1999, Bruno et al. 2003). These species often modify their habitats and facilitate conditions for other members of the community by reducing stress or by increasing the flow of resources (Rhoads and Young 1971, Bertness and Callaway 1994, Bruno et al. 2003). The community-level role of positive interactions resulting from habitat modification has recently gained a great deal of interest in terrestrial and aquatic systems (Callaway 1995, Bertness and Leonard 1997, 1999, Menge 2000, Bruno and Bertness 2001, Bruno et al. 2003, Lohrer et al. 2004). Positive feedbacks are po- tentially important in driving rapid evolutionary chang- es and influencing the dynamics of populations and communities (see Crespi (2004) for a review). Empir- Manuscript received 1 February 2005; revised 1 June 2005; accepted 6 June 2005. Corresponding Editor: S. G. Morgan. 1 Present address: Finnish Institute of Marine Research, PB2, FI-00561 Helsinki, Finland. E-mail: alf.norkko@fimr.fi ical evidence suggests that the balance between posi- tive and negative interactions can shift along environ- mental gradients, with facilitation being successively more important in physically harsh environments (e.g., Bruno et al. 2003; but see Tielbo ¨rger and Kadmon 2000). Spatial variation in process strength and inter- action is likely to be pervasive within landscapes of large habitat-forming species (Micheli and Peterson 1999), and as processes interact with environmental drivers, the relative importance of facilitation or in- hibition is likely to be scale-dependent (Hewitt et al. 2002). Thus understanding the manner in which vari- ations in environmental conditions in time and space modify the balance between facilitation and inhibition is important both to advance general ecological theory and to apply this knowledge wisely in conservation and resource management. In aquatic environments, suspension-feeding bi- valves are important habitat modifiers, with a plethora of direct and indirect effects on surrounding commu- nities at local to regional scales (Dame 1993, Gili and Coma 1998, Lenihan 1999, Gutie ´rrez et al. 2003). Bio- genic bivalve reefs can influence surrounding macro- faunal communities by providing refuge from preda- tion, reducing physical stress, and enhancing the de- position of pelagic larvae and food (Lenihan 1999, Bru- no and Bertness 2001, Gutie ´rrez et al. 2003, Commito et al. 2005). But despite the potential role of suspen- sion-feeding bivalves in facilitating surrounding mac- rofaunal communities, results from numerous surveys and small-scale field experiments have proved incon-