Species diversity in subtidal landscapes: maintenance by physical processes and larval recruitment

Ecology, Jan, 1999 by Franz Smith, Jon D. Witman

INTRODUCTION

The distribution and abundance of organisms in natural communities are characterized by heterogeneous spatial patterns, whereby several theories (Janzen 1970, Connell 1971, 1979, Ricklefs 1977, Hubbell 1979) and empirical studies (Connell et al. 1984, Condit et al. 1992) have sought to explain mechanisms that allow for the coexistence of large numbers of species. The realization of the importance of scale in ecological studies (Dayton and Tegner 1984, Wiens 1989, Levin 1992, Pickett and Cadenasso 1995) has stimulated new hypotheses about how species assemblages are maintained (Menge and Olson 1990, Barry and Dayton 1991, Holt et al. 1995). For example, one emerging area of research in community ecology that considers linkages across multiple scales is focused on determining how regional processes contribute to local patterns of species diversity (Ricklefs 1987, Schulter and Ricklefs 1993, Cornell and Karlson 1996).

Since most of the understanding of how communities are organized has been based on studies conducted in small plots or patches of habitat measuring [less than] 10's of square meters in area (Kareiva and Andersen 1988, Jackson 1991), it is uncertain how processes maintaining diversity of these local communities can be extrapolated to explain patterns of diversity over larger spatial scales (Ricklefs 1987, Schulter and Ricklefs 1993, Wiens et al. 1993). The predominance of research conducted within patch types or within landscape elements has focused the attention of ecologists on smaller scale structuring processes, such as competition and predation (Malanson 1993). Consequently, the importance of between-patch processes in maintaining patterns of species diversity has been largely overlooked.

Many factors can account for patterns of spatial heterogeneity observed in sessile marine communities on rocky substrata, including competition (Connell 1961), predation (Paine 1966), larval recruitment (Gaines and Roughgarden 1985, Underwood and Fairweather 1989), and physical or biological disturbances (Dayton 1971, Paine and Levin 1981, Dethier 1984, Sousa 1985, Wethey 1985, Witman 1992). Among these, physical disturbances, such as storm damage (Ayling 1984, Witman 1987), and larger scale biological disturbances, such as severe grazing by large urchin aggregations (Ayling 1981, Witman 1985), have been documented to influence patterns of species diversity over areas of hundreds of meters of continuous habitat. Strong species interactions, such as spatial competition (Sebens 1986), can also influence epifaunal diversity on smaller spatial scales (i.e., meters) but are unlikely to account for patterns observed over hundreds of meters.

Recruitment, regarded as the input of new individuals into a population or community, is another process that can influence community patterns over hundreds of meters of continuous habitat (e.g., Sale 1977, Connell et al. 1984, McGowan and Walker 1985, Condit et al. 1992). Recruitment is also an implicit assumption in several theories of the establishment and maintenance of populations within a disturbance regime (e.g., Connell 1978, Sousa 1984, Connell and Keough 1985, Menge and Sutherland 1987, Petraitis et al. 1989).

Sessile marine species are characterised by mobile dispersal stages, and recruitment of these larvae or propagules serves as a mechanism that can link spatially discrete populations or patch assemblages (Hansson 1991). Although the importance of larval dispersal and recruitment in structuring subtidal epifaunal communities has been demonstrated (Sutherland and Karlson 1977, Keough 1984a, Brault and Bouget 1985, Mullineaux 1988, Hurlbut 1991, Bingham 1992, Farnsworth and Ellison 1996), there is little explicit information relating patterns of species diversity to recruitment in these communities, particularly on scales over hundreds of meters.

In this paper, large-scale patterns of diversity of epifaunal assemblages in a New Zealand fjord were used to test the hypothesis that the patterns of diversity could be maintained by larval recruitment. Patterns of species diversity over hundreds of meters of continuous habitat were quantified along sections of vertical rock wall at three sites separated by 3-6 km. Patterns of epifaunal diversity consisted of localized patches of high diversity bounded by extensive areas of low diversity. These patches were used to determine the relationship between diversity and recruitment. Recruitment was measured at these sites on the same spatial scale in areas of high and low epifaunal diversity on an array of recruitment tiles with predator exclusion treatments. Results demonstrated that areas of high diversity received higher densities and more species of recruits than areas at shallow depths and in adjacent low-diversity areas. Comparisons between caged and uncaged recruitment tiles showed that predation had minimal influence on the observed patterns of recruitment. The depth of a low-salinity surface layer present in the fjords was closely related to the vertical patterns of diversity at each of the three sites. Physical and biological disturbances, including landslides and severe urchin grazing, were related to horizontal patterns of diversity in these assemblages.