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CHAPTER 6 INITIAL COLONISATION OF DISTURBED SEDIMENTS: THE EFFECTS OF A BIOGENIC SPECIES ON COMMUNITY INTRODUCTION ESTABLISHMENT Zajac and Whitlatch (1982a) defined disturbance as 'any stochastic event initiating species population changes'. Although disturbances have been shown to be important factors influencing marine soft-bottom community structure (Johnson, 1970; Dauer and Simon, 1976a; Woodin, 1976, 1978, 1981; Eckman 1979, 1983; Eckman et al., 1981; Thistle, 1981; Probert, 1984; Hall et al., 1991), their spatial and temporal scales vary considerably (Thrush, 1988; Thrush et al., 1992; Hall et al., 1993). Small-scale disturbances ranging from 1 cm 2 to 1m2 create mosaics of small patches at different stages of recovery (Johnson, 1970; Thrush 1986b). Grassle and Sanders (1973) suggested that the existence of such a mosaic could permit the persistence of species in a community where they were competitively inferior. These small-scale disturbances, which tend to be biologically mediated and usually occur relatively frequently, include pits created by the feeding activities of rays (Levin, 1984a), Walruses (Oliver et al., 1985), flatfish (Hall et al., 1990), shorebirds (Savidge and Taghon, 1988) and crabs (Thrush, 1986b; 1988; Hall et al., 1991) and by sediment modification by macroinfauna (Reise, 1983b; Woodin, 1985; Brey, 1991; Hall et al., 1991). Large-scale disturbances, ranging from metres to kilometres, occur much less frequently and include storms (Ong and Krishnan, 1995), organic and oil pollution (Rosenberg, 1973; Grassle and Grassle, 1974; Pearson and Rosenberg, 1978) and red tides (Dauer and Simon, 1976a, 1976b). The spatial extent of the disturbance can influence both the mode and rate of colonisation by the invertebrate fauna (Levin, 1984a; Gunther, 1992). While colonisation of large-scale patches tends to be dominated by species with planktonic 131
larvae (Santos and Simon, 1980a; Levin, 1984a; Strathmann, 1986), experiments have shown that small-scale patches of disturbed sediments are often initially dominated by species with benthic dispersal (Levin, 1984a; Savidge and Taghon, 1988; Frid, 1989; Smith and Brumsickle, 1989). However, the role of post-larval colonisation of disturbed sediments has also been shown to be important at both large and small scales (Dauer and Simon, 1976a; Thistle, 1981; Zajac and Whitlatch, 1982a; Levin, 1984a; Frid, 1989; Smith and Brumsickle, 1989; Gunther, 1992; Thrush et al., 1992; Turner et al., 1997). Santos and Simon (1980b) suggested however that it is possible to incorrectly infer adult colonisation when sampling intervals are too long and/or sieve mesh sizes are too large to detect larval colonisation. Connell and Slatyer (1977) proposed four generalised models for community assembly during succession. These were the facilitation, inhibition, tolerance and the random colonisation models. Many studies have been performed to elucidate which of these predominate within marine soft-bottom environments (e.g., Woodin 1981; Gallagher et al., 1983; Whitlatch and Zajac, 1985; Trueblood, 1991) and in general, succession proceeds via a mixture of different kinds of interactions rather than one single model prevailing. Whitlatch and Zajac (1985), however, concluded that although biotic interactions between opportunistic organisms were important in controlling successional dynamics, the type of interaction ultimately depended upon the species present, their density and the habitat conditions. Dense aggregations of polychaetes' tubes have been shown to stabilise sediments by altering the characteristics of near-bed flow (Eckman et al., 1981) and have been shown to be particularly important in affecting the early stages of faunal succession (Levin, 1981, 1982; Gallagher et al., 1983; Whitlatch and Zajac, 1985; Noji and Noji, 1991). Therefore, colonisation following disturbances within polychaete tube-beds are likely to have different successional dynamics compared with those outside tube-beds. Most studies investigating community establishment have followed species and community responses to disturbances which have occurred at only one point during the year and/or within only one type of habitat. However, since the colonisation potential of most species is predominantly governed by their larval availability at any one time (Bonsdorff and Osterman, 1984; Levin, 1984a; Ragnarsson, 1996), the 132
- Page 96 and 97: abundance are highly seasonal, were
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- Page 102 and 103: METHODS Study site - The exact posi
- Page 104 and 105: 1 C N W 4----111" 1.5m 2 NW C Contr
- Page 106 and 107: sediment sampling, together with re
- Page 108 and 109: RESULTS Species abundances - The me
- Page 110 and 111: ; 15 35 — 30 — 25 — 10 — 5
- Page 112 and 113: statistical difference from net plo
- Page 114 and 115: Pygospio elegans size distribution
- Page 116 and 117: used, approximately equivalent to t
- Page 118 and 119: artefacts associated with the metho
- Page 120 and 121: present in high numbers around sewa
- Page 122 and 123: lack, hydrogen sulphide-smelling se
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- Page 126 and 127: METHODS Survey design - During late
- Page 128 and 129: The sediments could not be sampled
- Page 130 and 131: RESULTS Species abundances - Table
- Page 132 and 133: 90 — 80 — "-e-' 70 — 60 — 4
- Page 134 and 135: 35 — *** 30 25 — 1.) = .-c‘l
- Page 136 and 137: Pygospio elegans size distributions
- Page 138 and 139: which is difficult to compare with
- Page 140 and 141: eason why some invertebrates showed
- Page 142 and 143: This study did not set out to expli
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- Page 148 and 149: esulting community at any stage of
- Page 150 and 151: ambient sediment had been removed.
- Page 152 and 153: emoved since they were the only tax
- Page 154 and 155: All statistics were performed using
- Page 156 and 157: RESULTS Univariate analysis of spec
- Page 158 and 159: 3.5 3 5 2 11 5 1 0.5 0 40 35 Ca 30
- Page 160 and 161: of non-patch areas (Figure 6.3(vi))
- Page 162 and 163: the individuals colonising patch az
- Page 164 and 165: Multivariate analysis of community
- Page 166 and 167: Month Sample statistic (Global R) N
- Page 168 and 169: 2NP 3NP 4NP .•,, 6NP 5NP 6P 1NP i
- Page 170 and 171: Figure 6.8: Two-dimensional MDS ord
- Page 172 and 173: - - 5P ... 4P . 6P • .‘2NP 1NP
- Page 174 and 175: I 50. 1 60. 70. 80. 90. 100. BRAY-C
- Page 176 and 177: 'P2-AZ P3-AZ N2-AZ .- - - " .„ ..
- Page 178 and 179: o • o -o + 350 — 300 = 250 7 g
- Page 180 and 181: The importance of the ambient commu
- Page 182 and 183: In April, when P. elegans larval av
- Page 184 and 185: not only for errant polychaetes, bu
- Page 186 and 187: observed in this study. How crucial
- Page 188 and 189: Micro-scale spatial patterns of mac
- Page 190 and 191: METHODS Experimental design - A pre
- Page 192 and 193: study. These individuals would not
- Page 194 and 195: RESULTS Pilot survey - The pilot su
CHAPTER 6<br />
INITIAL COLONISATION OF DISTURBED SEDIMENTS: THE<br />
EFFECTS OF A BIOGENIC SPECIES ON COMMUNITY<br />
INTRODUCTION<br />
ESTABLISHMENT<br />
Zajac and Whitlatch (1982a) defined disturbance as 'any stochastic event initiating<br />
species population changes'. Although disturbances have been shown to be important<br />
factors influencing marine soft-bottom community structure (Johnson, 1970; Dauer<br />
and Simon, 1976a; Woodin, 1976, 1978, 1981; Eckman 1979, 1983; Eckman et al.,<br />
1981; Thistle, 1981; Probert, 1984; Hall et al., 1991), their spatial and temporal scales<br />
vary considerably (Thrush, 1988; Thrush et al., 1992; Hall et al., 1993). Small-scale<br />
disturbances ranging from 1 cm 2 to 1m2 create mosaics of small patches at different<br />
stages of recovery (Johnson, 1970; Thrush 1986b). Grassle and Sanders (1973)<br />
suggested that the existence of such a mosaic could permit the persistence of species<br />
in a community where they were competitively inferior. These small-scale<br />
disturbances, which tend to be biologically mediated and usually occur relatively<br />
frequently, include pits created by the feeding activities of rays (Levin, 1984a),<br />
Walruses (Oliver et al., 1985), flatfish (Hall et al., 1990), shorebirds (Savidge and<br />
Taghon, 1988) and crabs (Thrush, 1986b; 1988; Hall et al., 1991) and by sediment<br />
modification by macroinfauna (Reise, 1983b; Woodin, 1985; Brey, 1991; Hall et al.,<br />
1991). Large-scale disturbances, ranging from metres to kilometres, occur much less<br />
frequently and include storms (Ong and Krishnan, 1995), organic and oil pollution<br />
(Rosenberg, 1973; Grassle and Grassle, 1974; Pearson and Rosenberg, 1978) and red<br />
tides (Dauer and Simon, 1976a, 1976b).<br />
The spatial extent of the disturbance can influence both the mode and rate of<br />
colonisation by the invertebrate fauna (Levin, 1984a; Gunther, 1992). While<br />
colonisation of large-scale patches tends to be dominated by species with planktonic<br />
131
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