Role of settlement in determining the distribution and abundance of barnacles in a temperate mangrove forest
Introduction
Recent ecological research has demonstrated that variability in larval settlement is also a strong determinant in shaping population structure (Denley and Underwood, 1979, Grosberg, 1982, Yoshioka, 1982, Keough, 1984, Underwood and Denley, 1984, Connell, 1985, Gaines and Roughgarden, 1985, Gaines et al., 1985, Roughgarden et al., 1985, Lewin, 1986, Underwood and Fairweather, 1989, Raimondi, 1990, Sale, 1990, Minchinton and Scheibling, 1991).
Despite increased attention to settlement and recruitment processes, little generality has emerged. Overall levels of settlement have been suggested as a factor that may determine the relative role of settlement, with populations more likely to be settlement-limited at low settlement rates (Connell, 1985). In a similar way, regional productivity, as reflected by regional nutrient levels in the water column, may influence larval production, and hence the role of settlement or recruitment in population dynamics (see, e.g. Menge et al., 1997 for an example linking productivity and community structure).
We see three important obstacles to emergent generality about the role of settlement and/or recruitment: the number of studies focusing on a restricted number of life history stages, the representativeness of the database, and potential scale-dependence of any results.
The first limitation is easily remedied — questions about the importance of recruitment can be answered meaningfully only if they involve information about other life history stages (Keough, 1988). Because recruitment is an anthropocentric, rather than an objective biological term, it is also important to separate settlement from recruitment, to determine whether settlement or post-settlement processes are critical (Keough and Downes, 1982).
Secondly, the database from which to draw any general inferences about invertebrate recruitment is strongly biased towards the intertidal zone of rocky shores. For example, in barnacles, one of the most prominent (and over-represented) groups in the literature, most studies examining the distribution and abundance have been undertaken on rocky intertidal areas (Moore, 1935, Connell, 1961a, Connell, 1961b, Achituv, 1972, Denley and Underwood, 1979, Wethey, 1983, Wethey, 1984, Wethey, 1985a, Wethey, 1985b, Caffey, 1985, Otaiza, 1989, Sutherland, 1990, Raimondi, 1990, and many others). Barnacles are also common elsewhere (e.g. in mangrove forests), but there have been few studies examining the processes that determine the distribution and abundance of barnacles living in these areas (Bayliss, 1993, Coates and McKillup, 1995, Ross and Underwood, 1997).
Thirdly, the relative importance of settlement, early post-settlement, and adult processes may change with the scale under consideration. For example, Keough and Chernoff (1987) attributed large-scale patchiness in the spatial distribution of the bryozoan Bugula neritina to accidents of dispersal, and not to post-settlement processes. In contrast, Keough (1986) found that, at smaller scales, along seagrass leaves, distributional patterns were explained by settlement behaviour and consistent gradients in post-settlement (>1 week after settlement) mortality and growth over scales of 10 to 20 cm.
Here, we examine the importance of settlement in determining the distribution of a barnacle, Elminius covertus, living in temperate mangrove forests of southeastern Australia. We describe the patterns of distribution of Elminius covertus at two spatial scales, horizontally (cross-shore) in a mangrove forest, and, on a smaller scale, vertically from 0 to 15 cm above the sediment surface, on mangrove pneumatophores. We also generated information about barnacle distributions at a third scale, along the shore. By measuring adult distributions, recruitment (using two different temporal windows), and settlement, we determined whether post-settlement mortality influences these patterns of distribution.
Section snippets
Materials and methods
All field work was done in the mangrove forests of Reid Bight (40° 28′ S) and Rhyll Inlet (41° 27′ S) at Rhyll, Phillip Island, in southeastern Australia. These forests are monocultures of Avicennia marina, which is the only mangrove species along this coast. Mangrove forests are very three-dimensional, with trees extending from a seaward fringe, towards the land, and being patchy as one moves along the shoreline. Within the forest, there is a vertical component, including roots and
Horizontal distribution
Overall abundance of E. covertus was influenced strongly by distance along transects and the relationship varied among transects and surveys (Table 1). In general, many more E. covertus were found at the seaward sites and few were present at the landward end of each transect on both forests. The point at which numbers started to change was not the same on all transects (Fig. 1, Fig. 2), although the general pattern was consistent.
Vertical distribution
There were generally more E. covertus on the higher parts of
Discussion
Across these mangrove forests, the density of E. covertus was much less in the landward zone than the seaward zone (Fig. 7). Other studies have also found that there are greater densities of barnacles at the seaward side of mangrove forests (Berry, 1963, Macnae, 1966, Sasekumar, 1974, Achituv, 1984, Hutchings, 1985, Hutchings and Saenger, 1987, Bayliss, 1988, Ross and Underwood, 1997). This pattern was evident at two forest sites and at different locations, although the point in the forest at
Acknowledgements
We would like to thank John Wright, Mike Hollaway, Mark Norman and Somchai Satumanatpan for help in the field. This study was supported by a Melbourne University Postgraduate Scholarship and an Overseas Postgraduate Research Scholarship.
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